U.S. patent application number 15/672670 was filed with the patent office on 2017-11-23 for variable current limit for power tool having a brushless motor.
The applicant listed for this patent is Black & Decker Inc.. Invention is credited to Michael K. Forster, Bhanuprasad V. Gorti, Tal Gottesman, John C. Vanko.
Application Number | 20170338753 15/672670 |
Document ID | / |
Family ID | 54018408 |
Filed Date | 2017-11-23 |
United States Patent
Application |
20170338753 |
Kind Code |
A1 |
Forster; Michael K. ; et
al. |
November 23, 2017 |
VARIABLE CURRENT LIMIT FOR POWER TOOL HAVING A BRUSHLESS MOTOR
Abstract
A power tool is provided including a brushless electric motor, a
switching arrangement having motor switches and interposed between
the electric motor and a power supply, and a controller configured
to control a switching operation of the motor switches for driving
the electric motor and enforce a current limit on the current
delivered to the electric motor. The controller receives a measure
of current passing from the power supply to the switching
arrangement and takes corrective action to reduce current delivered
to the electric motor if the measured current exceeds the current
limit. The controller further receives a temperature signal
indicative of temperature of the power tool and adjusts the current
limit in accordance with the temperature signal. Additional or
alternatively, the controller adjusts the current limit at tool
start-up.
Inventors: |
Forster; Michael K.; (White
Hall, MD) ; Vanko; John C.; (Timonium, MD) ;
Gorti; Bhanuprasad V.; (Perry Hall, MD) ; Gottesman;
Tal; (Lutherville, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Black & Decker Inc. |
New Britain |
CT |
US |
|
|
Family ID: |
54018408 |
Appl. No.: |
15/672670 |
Filed: |
August 9, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14715079 |
May 18, 2015 |
9762153 |
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15672670 |
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14057003 |
Oct 18, 2013 |
9314900 |
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14715079 |
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62000307 |
May 19, 2014 |
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61994953 |
May 18, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02P 6/15 20160201; H02P
6/14 20130101; H02P 2006/045 20130101; H02P 6/28 20160201; H02K
7/145 20130101; B24B 47/12 20130101; B25F 5/008 20130101; B24B
23/028 20130101 |
International
Class: |
H02P 6/08 20060101
H02P006/08; H02P 6/28 20060101 H02P006/28; H02P 6/14 20060101
H02P006/14; B24B 47/12 20060101 B24B047/12; B24B 23/02 20060101
B24B023/02; B25F 5/00 20060101 B25F005/00; H02P 6/15 20060101
H02P006/15 |
Claims
1. A power tool, comprising: a housing; a brushless electric motor
drivably connected to an output shaft to impart rotary motion
thereto; a switching arrangement having a plurality of motor
switches and interposed between the electric motor and a power
supply; and a controller configured to control a switching
operation of the plurality of motor switches for driving the
electric motor and enforce a current limit on the current delivered
to the electric motor, the controller configured to receive a
measure of current passing from the power supply to the switching
arrangement and take corrective action to reduce current delivered
to the electric motor if the measured current exceeds the current
limit, wherein the controller is further configured to receive a
temperature signal indicative of temperature of the power tool and
adjust the current limit in accordance with the temperature
signal.
2. The power tool of claim 1, further comprising a temperature
sensor configured to output the temperature signal.
3. The power tool of claim 2, wherein the temperature sensor is
disposed in or proximate the motor.
4. The power tool of claim 1, wherein the controller is configured
to set the current limit to a fixed value while the temperature
signal is below a temperature threshold, and decrease the current
limit from the fixed value when the temperature signal exceeds the
temperature threshold.
5. The power tool of claim 1, wherein the controller operates over
periodic time intervals to enforce the current limit on the current
delivered to the electric motor, wherein the controller is
configured to enforce the current limit by receiving a measure of
instantaneous current passing to the switching arrangement within a
present time interval of the periodic time intervals, comparing the
measure of instantaneous current to the current limit, and, in
response to the measure of instantaneous current exceeding the
current limit, turning off the motor switches for a remainder of
the present time interval and thereby interrupting current flowing
to the electric motor.
6. The power tool of claim 5, wherein a duration of each time
interval of the period time intervals is fixed as a function of the
given frequency at which the plurality of motor switches is
controlled by the controller.
7. The power tool of claim 5, wherein the controller is configured
to turn on select motor switches at the end of the present time
interval and thereby resume current flow to the electric motor.
8. The power tool of claim 5, wherein the duration of each time
interval is approximately ten times an inverse of the given
frequency at which the plurality of motor switches is controlled by
the controller.
9. The power tool of claim 5, wherein the electric motor is
controlled by pulse width modulated (PWM) signals and the duration
of the each time interval corresponds to a period associated with
the PWM signals.
10. The power tool of claim 1, wherein the brushless electric motor
is further defined as a three-phase DC motor and the switching
arrangement is comprised of six motor switches, each phase of the
DC motor is coupled to a high-side switch and a low-side switch,
wherein the controller enforces the current limit using the three
low-side switches.
11. A power tool, comprising: a housing; a brushless electric motor
drivably connected to an output shaft to impart rotary motion
thereto; a switching arrangement having a plurality of motor
switches and interposed between the electric motor and a power
supply; and a controller configured to control a switching
operation of the plurality of motor switches for driving the
electric motor and enforce a current limit on the current delivered
to the electric motor, the controller configured to receive a
measure of current passing from the power supply to the switching
arrangement and take corrective action to reduce current delivered
to the electric motor if the measured current exceeds the current
limit, wherein the controller is further configured to enforce the
current limit at a first value at tool start-up and enforce the
current limit at a second value greater than the first value
thereafter.
12. The power tool of claim 11, wherein the controller is
configured to set the current limit to the first value if a
rotational speed of the motor is smaller than a predefined
threshold and, if set the current limit to the second value if the
rotational speed of the motor exceeds the predefined threshold.
13. The power tool of claim 12, wherein, after the rotational speed
of the motor exceeds the predefined threshold, the controller does
not revert the current limit back to the first value unless the
rotational speed of the motor is less than a second predefined
threshold smaller than the predefined threshold.
14. The power tool of claim 11, wherein the second value is double
the first value.
15. The power tool of claim 11, wherein the controller operates
over periodic time intervals to enforce the current limit on the
current delivered to the electric motor, wherein the controller is
configured to enforce the current limit by receiving a measure of
instantaneous current passing to the switching arrangement within a
present time interval of the periodic time intervals, comparing the
measure of instantaneous current to the current limit, and, in
response to the measure of instantaneous current exceeding the
current limit, turning off the motor switches for a remainder of
the present time interval and thereby interrupting current flowing
to the electric motor.
16. The power tool of claim 15, wherein a duration of each time
interval of the period time intervals is fixed as a function of the
given frequency at which the plurality of motor switches is
controlled by the controller.
17. The power tool of claim 15, wherein the controller is
configured to turn on select motor switches at the end of the
present time interval and thereby resume current flow to the
electric motor.
18. The power tool of claim 15, wherein the duration of each time
interval is approximately ten times an inverse of the given
frequency at which the plurality of motor switches is controlled by
the controller.
19. The power tool of claim 15, wherein the electric motor is
controlled by pulse width modulated (PWM) signals and the duration
of the each time interval corresponds to a period associated with
the PWM signals.
20. The power tool of claim 11, wherein the brushless electric
motor is further defined as a three-phase DC motor and the
switching arrangement is comprised of six motor switches, each
phase of the DC motor is coupled to a high-side switch and a
low-side switch, wherein the controller enforces the current limit
using the three low-side switches.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
application Ser. No. 14/715,079 filed May 18, 2015, which is a
continuation-in-part of U.S. patent application Ser. No. 14/057,003
filed on Oct. 18, 2013, and also claims the benefit of U.S.
Provisional Application No. 61/994,953, filed on May 18, 2014 and
U.S. Provisional Application No. 62/000,307, filed on May 19, 2014.
The entire disclosure of each of the above applications are
incorporated herein by reference.
FIELD
[0002] The present disclosure relates to a powered apparatus and
more generally to a power tool having a brushless electric
motor.
BACKGROUND
[0003] Handheld angle grinders are commonly used for cutting,
grinding, sanding, and polishing workpieces. Due to the large
diameter of the grinding disk and/or the large bias applied by the
tool operator during certain tasks, the current demand by the tool
from an AC power outlet can exceed the rating of the circuit
breaker associated with the power outlet, thereby causing the
breaker to trip. Consequently, there is a need to increase the
power output of such grinders and other handheld power tools within
the limits of the AC power source and without resorting to
complicated and expensive power conversion circuits. Various
techniques are set forth below for increasing power output by
handheld power tools, especially ones employing a brushless
electric motor.
[0004] This section provides background information related to the
present disclosure which is not necessarily prior art.
SUMMARY
[0005] This section provides a general summary of the disclosure,
and is not a comprehensive disclosure of its full scope or all of
its features.
[0006] A handheld AC power tool is provided. The power tool is
comprised generally of: a brushless electric motor; a power cord
connectable to an AC power socket; a converter circuit configured
to receive input power from the power cord and operable to output a
DC bus voltage, a switching arrangement having a plurality of motor
switches and interposed between the electric motor and the
converter circuit; a motor drive circuit interfaced with the motor
switches to control switching operation of the motor switches; and
a power switch electrically connected between the converter circuit
and the motor drive circuit and operable by a user to selectively
energize the motor drive circuit and thereby power on the tool. The
converter circuit includes a rectifier and a capacitor electrically
coupled across the rectifier, such that the capacitor has
capacitance sized to produce a DC bus voltage whose magnitude from
an AC power source is substantially same as magnitude of voltage
from a DC power source, where voltage rating of the AC power source
is same as voltage rating of the DC power source.
[0007] In one embodiment, the capacitor has a nominal capacitance
in the range of 200-400 micro Farads for tool having a current
rating of approximately 10 Amps.
[0008] In another embodiment, the capacitor has a nominal
capacitance on the order of 350 micro Farads for tool having a
current rating of approximately 15 Amps.
[0009] In yet another embodiment, the capacitor has a nominal
capacitance on the order of 500 micro Farads for tool having a
current rating of approximately 20 Amps.
[0010] The power tool may further include a controller configured
to receive a measure of instantaneous current passing from the
rectifier to the switching arrangement and operate over periodic
time intervals to enforce a limit on current delivered to the
electric motor. The controller enforces the current limit by
comparing instantaneous current measures to the current limit and,
in response to an instantaneous current measure exceeding the
current limit, turning off the motor switches for remainder of a
present time interval and thereby interrupt current flowing to the
electric motor. The controller further operates to turn on select
motor switches at end of the present time interval and thereby
resumes current flow to the electric motor.
[0011] In another aspect of this disclosure, a handheld grinder is
provided. The grinder is comprised of: an elongated housing having
a grip portion that is shaped to be grasped by a user; a brushless
electric motor drivably connected to an output shaft to impart
rotary motion thereto; a grinding disk connected to one end of the
output shaft; a rectifier configured to receive power from an
alternating current (AC) power source and operable to convert an
alternating current to a direct current; a switching arrangement
having a plurality of motor switches and interposed between the
electric motor and the rectifier; a capacitor electrically coupled
across the rectifier and interposed between the rectifier and the
switching arrangement; a motor drive circuit interfaced with the
motor switches and operates at a given frequency to control
switching operation of the motor switches; and a controller
configured to receive a measure of instantaneous current passing
from the rectifier to the switching arrangement and operates over
periodic time intervals to enforce a current limit on the current
delivered to the electric motor.
[0012] In some embodiments, the controller enforces the current
limit by comparing instantaneous current measures to the current
limit and, in response to an instantaneous current measure
exceeding the current limit, turning off the motor switches for
remainder of present time interval and thereby interrupt current
flowing to the electric motor, where duration of each time interval
is fixed as a function of the given frequency at which the electric
motor is controlled by the controller. Additionally, the controller
turns on select motor switches at the end of the present time
interval and thereby resumes current flow to the electric motor.
The duration of each time interval is approximately ten times an
inverse of the given frequency at which the electric motor is
controlled by the controller. More specifically, the duration of
each time interval is on the order to 100 microseconds.
[0013] In one embodiment, the electric motor is controlled by pulse
width modulated (PWM) signals received from the motor drive circuit
and duration of the each time interval equals period of the PWM
signals.
[0014] In some embodiments, the brushless electric motor is further
defined as a three-phase DC motor and the switching arrangement is
comprised of six motor switches, each phase of the DC motor is
coupled to a high-side switch and a low-side switch. In this case,
the controller controls motors motor operation using the three
high-side switches and enforces the current limit using the three
low-side switches.
[0015] The capacitor has a capacitance in range of 0-200 micro
Farads and preferably in the range of 5-20 micro Farads. In an
example embodiment, the capacitor is implemented by two capacitors
arranged in parallel and collectively having a capacitance on the
order of 9.4 micro Farads.
[0016] In another aspect of this disclosure, a method is provided
for detecting stall condition of an output shaft driven by an
electric motor of a power tool. The method includes: measuring
current delivered to the electric motor of the power tool at least
once during consecutive time intervals; determining whether a
measured current exceeds a current limit; incrementing an event
counter by one in response to a determination that the measure
current exceeds the current limit; resetting the event counter to
zero in response to a determination that the measured current does
not exceed the current limit; determining whether value of the
event counter exceeds an event threshold; and initiating a
protective operation in response to a determination that the event
counter exceeds an event threshold.
[0017] In response to a determination that the measure current
exceeds the current limit, current flow to the electric motor is
interrupted for the remainder of a given time interval and resuming
current flow to the electric motor at an end of the given time
interval.
[0018] The method can also include: detecting beginning of a new
time interval; incrementing an interval counter by one in response
to detecting the beginning of a new time interval; determining
whether value of the interval counter exceeds an interval
threshold; and determining whether value of the event counter
exceeds an event threshold in response to a determination that the
value of the interval counter exceeds the interval threshold.
[0019] In some embodiments, the method can include: monitoring
rotational speed of the electric motor; determining whether the
rotational speed is less than a speed threshold; and initiating the
protective operation in response to a determination that the event
counter exceeds an event threshold and a determination that the
rotational speed is less than the speed threshold.
[0020] Further areas of applicability will become apparent from the
description provided herein. The description and specific examples
in this summary are intended for purposes of illustration only and
are not intended to limit the scope of the present disclosure.
DRAWINGS
[0021] The drawings described herein are for illustrative purposes
only of selected embodiments and not all possible implementations,
and are not intended to limit the scope of the present
disclosure.
[0022] FIG. 1 is a perspective view of a handheld grinder;
[0023] FIG. 2A is a block diagram of an example embodiment of a
motor control scheme for use in the grinder;
[0024] FIG. 2B is a block diagram of an alternative embodiment of a
motor control scheme which may be used in the grinder;
[0025] FIG. 2C is a block diagram of another embodiment of a motor
control scheme which may be used in a grinder;
[0026] FIG. 3 is a partial schematic depicting an example switching
arrangement for use in the grinder;
[0027] FIG. 4A is a diagram illustrating the DC bus voltage
obtained from a fully-rectified AC input voltage with no load;
[0028] FIG. 4B is a diagram illustrating the DC bus voltage
obtained from a fully-rectified AC input voltage at a fixed
load;
[0029] FIG. 4C is a diagram illustrating an idealized AC current
drawn from an AC input voltage with a relatively large
capacitance;
[0030] FIG. 4D is a diagram illustrating an idealized AC current
drawn from an AC input voltage with a relatively small
capacitance;
[0031] FIG. 5 is a graph illustrating the relationship between
power output, average DC bus voltage and bus capacitance in an
example power tool;
[0032] FIG. 6 is a graph illustrating implementation of a 20 amp
cycle-by-cycle current limit;
[0033] FIG. 7 is a block diagram depicting a portion of the motor
control scheme modified to support cycle-by-cycle current
limits;
[0034] FIG. 8 is a flowchart illustrating an example method for
implementing cycle-by-cycle current limits;
[0035] FIG. 9 is a graph depicting how the cycle-by-cycle current
limit can be adjusted during tool start-up conditions;
[0036] FIG. 10 is a flowchart illustrating an example method for
detecting a stall condition using cycle-by-cycle current limit;
[0037] FIG. 11 is a flowchart illustrating an example method for
detecting a stall condition using cycle-by-cycle current limit;
and
[0038] FIG. 12 is a graph depicting how the cycle-by-cycle current
limit can be adjusted in accordance with motor temperature.
[0039] Corresponding reference numerals indicate corresponding
parts throughout the several views of the drawings.
DETAILED DESCRIPTION
[0040] Example embodiments will now be described more fully with
reference to the accompanying drawings.
[0041] FIG. 1 depicts an example of a handheld grinder 10. In this
example embodiment, the grinder 10 is comprised of a housing 12
having an elongated shape. A user can grasp the grinder 10 by
placing the palm of the user's hand over and around the housing 12.
An output member 18 is positioned at one end 12-1 of the housing 12
and comprises a right angle gear set 20 that drives a rotating disk
22. In this example embodiment, the rotating disk 22 comprises a
grinder disk. The rotating disk 22 may be removed and replaced with
a new rotating disk. For example, a user of the grinder 10 may
replace the existing rotating disk 22 with a new rotating disk
after the existing rotating disk 22 wears out. An adjustable guard
24 may cover at least a portion of the rotating disk 22 to obstruct
sparks and debris generated during operation of the power tool
10.
[0042] While the present description is provided with reference to
a grinder, it is readily understood that the broader aspects of the
present disclosure are applicable to other types of power tools,
including but not limited to sander, drill, impact driver, tapper,
fastener driver, and saw. For example, the power tool 10 may
include a chuck that is configured to receive a drill bit or a
screw driver bit, thereby allowing the power tool 10 to be used as
a power drill or a power screw driver. In another example
embodiment, the output member 18 may be removed and replaced with
another output member that may be more suitable for a drill, a
screw driver, or any other power tool, thus creating a multipurpose
power tool by virtue of a plurality of output members 18.
[0043] The housing 12 has a first portion 14 and a second portion
16. The first portion 14 and second portion 16 may be secured
together with screws 26, illustratively six, and enclose an
electric motor 28 and electronic circuit components, as further
described below, that drive the output member 18. The first portion
14 further includes a power on/off switch 32 and a spindle lock
switch 34. Putting the power on/off switch 32 in on and off
positions turns on and off the electric motor 28, respectively.
Pressing and holding the spindle lock switch 34 enables the user to
change the rotating disk 22. A plurality of narrow slot openings 36
of the first 14 and second 16 portions allow for venting of the
electric motor 28 and the electronic circuit components. The one
end 12-1 of the housing 12 also includes a threaded opening 38 for
selectively attaching a side-handle (not shown) to enable
two-handed operation.
[0044] A power cord 30 is connectable to an AC power socket and is
positioned at an opposite end 12-2 of the housing 12. The power
cord 30 provides power to the electric motor 28 and the electronic
circuit components of the power tool 10. Additionally, the power
tool 10 is configured to receive a detachable battery pack 42.
Specifically, the housing 12 includes a battery mounting portion to
which one or more battery packs 42 releasably couple thereto. The
battery pack 42 provides DC power to the electric motor 28 and the
other electronic components of the power tool. Different attachment
mechanisms for battery packs are readily known in the art and may
be employed in this application.
[0045] In some embodiments, it is envisioned that the tool may be
configured to work with battery packs having different nominal
voltage ratings. For example, the tool may be configured to work
with a single state low voltage pack (e.g., 20V) or a convertible
low/medium voltage pack (e.g., 20V/40V or 20V/60V). In other
examples, the tool may be configured to work with two convertible
medium voltage packs to yield a high output voltage (e.g., 120V or
230V). It is understood that number and arrangement of battery
cells in a pack as well as the cell chemistry may vary. Further
information regarding power tool systems and convertible battery
packs may be found in U.S. Provisional Patent Application Nos.
61/994,953 filed on May 18, 2014 and 62/000,112 filed on May 19,
2014 which are incorporated herein in their entirety.
[0046] FIG. 2A depicts a schematic that illustrates an embodiment
of a motor control system 80 that may be employed by the power tool
10. The motor control system 80 is comprised generally of the
controller 78, a switching arrangement 82 and a driver circuit 84.
The motor control system 80 operates to drive an electric motor 28
which may be further defined as a brushless electric motor. The
brushless motor may be a three-phase permanent magnet synchronous
motor including a rotor having permanent magnets and a wound stator
that is commutated electronically as described below. The stator
windings are designated herein as U, V, and W windings
corresponding to the three phases of the motor 28. It must be
understood, however, that other types of brushless motors, such as
switched reluctance motors and induction motors, are within the
scope of this disclosure. It must also be understood that the
brushless motor 28 may include fewer than or more than three
phases. While many of the concepts presented herein are
particularly applicable to brushless motors, some of the concepts
can be applied to other types of motors as well.
[0047] The motor control system 80 may further include position
sensors 86, 88, 90 that are configured to detect rotational motion
of the electric motor 28 and generate a signal indicative of the
rotational motion. The signal may have a periodic waveform whose
magnitude may vary in accordance with the rotational position of
the electric motor 28. The controller 78 is configured to receive
signals output by the position sensors 86, 88, 90. In other
embodiments the position sensors 86, 88, 90 may be interfaced with
the driver circuit 84.
[0048] In the example embodiment, the power tool 10 is configured
to work with either an AC power supply 92 or a DC power supply 91.
The AC power supply 92 delivers an alternating current to the
rectifier 44, for example via the power cord 30. The rectifier 44
converts the alternating current into a direct current.
[0049] Likewise, a DC input signal from the DC power supply 92
passes through the rectifier 44. When powered from the DC power
supply 92, the rectifier 44 simply ensures that the DC supply is
connected to the switching arrangement 82 and driver circuit 84
with the correct polarity. In one embodiment, the DC power supply
is one or more rechargeable battery packs that detachably couple to
the housing 12 of the power tool 10. In other embodiments, the
power tool 10 may be configured to receive a DC input via the power
cord 30 from other types of sources, such as a generator, a
portable welder or a DC outlet in certain types of electrical
installations.
[0050] The AC power supply 92 and the DC power supply 91 must not
be providing power to the tool at the same time to prevent damage
to one or both of the two power supplies. Accordingly, an
electrical and/or mechanical interlock is used to ensure that only
one of the power supplies is providing power to the tool. Various
forms of interlocks are readily known in the art and may be
employed in this context. It is envisioned that the interlock is
configured to generate a signal indicative of whether a battery
pack is coupled to the tool. In the case the tool can be powered by
battery packs having different nominal voltages, the signal
generated by the interlock may further specify the type and/or
nominal voltage of the battery packs coupled to the tool.
[0051] One example embodiment of an interlock is the
double-pole-double-throw (DPDT) switch 93 which connects the AC
power supply 92 to the rectifier 44 when it is energized. The
switch is in the form of a relay, the coil of said relay energized
by the closure of a second small switch that closes only when the
AC power supply line is connected to the power tool. Thus,
connection of the AC power supply line to the power tool closes the
small switch which in turn energizes the relay which drives the
DPDT switch 93 in such a way as to connect AC power supply 92 to
rectifier 44. Conversely, without the AC power supply line the
small switch is not closed, the relay is not energized, and the
DPDT switch 93 remains in its unenergized position connecting the
DC power supply 91 to the rectifier 44.
[0052] Another example embodiment of an interlock to cause the DPDT
switch 93 to connect the AC power supply 92 to the rectifier 44 is
to slide a mechanical shutter, said shutter covering either the
opening which allows connection of the AC power supply line, or
covering the other opening which allows connection of the DC power
supply 91. In some embodiments, it is envisioned that no interlock
is required because the AC power supply and DC power supply have
respective protection circuitry such that in the event of
simultaneous connection, neither power supply is damaged.
[0053] One or more DC bus capacitors 50 are electrically connected
in parallel with the rectifier 44. In one aspect of this
disclosure, the DC bus capacitors 50 are sized to ensure that the
tool delivers comparable output power from either the AC power
supply or the DC power supply 91 without exceeding breaker limits
as will be further described below.
[0054] In the example embodiment, the switching arrangement 82 is
electrically connected with the DC bus capacitors 50 and may
receive a pure DC signal or substantially pure DC signal from the
DC bus capacitors 50. The switching arrangement 82 includes a
plurality of motor switches that, when switched on, deliver the DC
current to the electric motor 28. Example motor switches include
field effect transistors (FETs), insulated-gate bipolar transistors
(IGBTs), etc. In the example embodiment, the switching arrangement
82 may be further defined as a three-phase inverter bridge as shown
in FIG. 3. As shown, the three-phase bridge circuit includes three
high-side and three low-side FETs. The gates of the high-side FETs
driven via signals UH, VH, and WH, and the gates of the low-side
FETs are driven via motor control signals UL, VL, and WL. In an
embodiment, the sources of the high-side FETs are coupled to the
drains of the low-side FETs to output power signals PU, PV, and PW
for driving the electric motor 28. Other switching arrangements are
also contemplated by this disclosure.
[0055] The driver circuit 84 is interfaced with the motor switches
of switching arrangement 82. The driver circuit 84 controls the
state of the motor switches, for example using pulse width
modulated (PWM) control signals. In this embodiment, the driver
circuit 84 is shown as being separate from the switching
arrangement 82. In other embodiments, the driver circuit 84 and the
switching arrangement 82 may be a single integrated circuit which
may be commercially available from various manufactures. For
example, the switching arrangement 82 and the driver circuit 84 may
be a part of an integrated power module.
[0056] The controller 78 manages the overall operation of the tool.
For example, the controller controls switching operation of the
motor switches in the switching arrangement. In one embodiment, the
controller is implemented by a microcontroller. Controller may also
refer to an electronic circuit, an application specific integrated
circuit (ASIC), a processor (shared, dedicated, or group) and/or
memory (shared dedicated, or group) that execute one or more
software or firmware programs, a combinational logic circuit,
and/or other suitable components that provide the described
functionality.
[0057] In this embodiment, the controller 78 receives power from a
driver power supply 94. The driver power supply 94 is electrically
connected in series with the rectifier 44 and operates to power the
driver circuit 84 via the power on/off switch 32. In an example
embodiment, driver power supply 94 includes a buck converter and/or
a linear regulator to reduce the power voltage, for example, to 15V
for powering the driver circuit 84 and to 3.2V for powering the
controller 78. In an alternate embodiment, the controller 78 may
receive power directly from the rectifier 44.
[0058] The power on/off switch 32 is positioned between the driver
power supply 94 and the driver circuit 84. In an example
embodiment, the switch contact of the power on/off switch 32 is
positioned between the driver power supply 94 and the driver
circuit 84. In other embodiments, the power switch 32 may be
implemented by a variable speed actuator.
[0059] When the power on/off switch 32 is switched to the on
position, the driver circuit 84 receives power from the driver
power supply 94. When the driver circuit 84 receives power, the
driver circuit 84 is able to control the state of the motor
switches and the electric motor 28 is on.
[0060] Conversely, when the power on/off switch 32 is switched to
the off position, the driver circuit 84 does not receive power from
the driver power supply 94. When the driver circuit 84 does not
receive power, the driver circuit 84 is not able to control the
state of the motor switches and the electric motor 28 is off.
[0061] As illustrated, the power on/off switch 32 is electrically
connected between the rectifier 44 and the driver circuit 84. The
power on/off switch 32 is positioned such that the power, conveyed
from either power supply through the switching arrangement 82, does
not pass through the power on/off switch 32. Furthermore, the
current being drawn by the electric motor 28 does not pass through
the power on/off switch 32. The current passing through the power
on/off switch 32 is the current being drawn by the driver circuit
84 and the current being drawn by the driver circuit 84 is lower
than the current being drawn by the electric motor 28.
[0062] The power on/off switch 32 has a current rating that is
approximately equal to the lower current being drawn by the driver
circuit 84 and not the higher current being drawn by the electric
motor 28. Similarly, the power on/off switch 32 has a voltage
rating that is approximately equal to the lower voltage at which
the driver circuit 84 operates and not the higher voltage at which
the electric motor 28 operates. The power on/off switch 32 is a low
current and low voltage switch. Advantageously, the power on/off
switch 32 has smaller physical dimensions and generates less heat
than a switch that would be required to withstand the higher
current and higher voltage at which the electric motor 28
operates.
[0063] FIG. 2B depicts an alternative embodiment of a motor control
system 80 that may be employed by the power tool 10. In this
embodiment, the output from the DC power supply 91 bypasses the
rectifier 44 and is injected directly across the DC bus capacitors
50. By bypassing the voltage drop across the diodes of the
rectifier 44, this approach increases the efficiency for a DC power
supply. DC polarity is critical in this embodiment as the opposite
polarity would damage the switching arrangement. It is also noted
that the interlock is positioned before the rectifier 44 but could
also be placed after the rectifier 44 as well. Except with respect
to the differences discussed herein, this embodiment is
substantially the same as the one described above in relation to
FIG. 2A.
[0064] In some embodiments, the handheld grinder 10 may be
configured to receive power only from an AC power source. That is,
the grinder includes a power cord for connecting to an AC outlet
but does not include a mounting portion for a battery pack. FIG. 2C
depicts a motor control system 80 for such a power tool. In this
variant, the rectifier 44 is configured to receive an alternating
current directly from an AC power supply without the need for an
interlock. Except with respect to the differences discussed herein,
this motor control system is substantially the same as the one
described above in relation to FIGS. 2A and 2B.
[0065] Brushless DC power tools have been adapted to AC
applications by simply rectifying the AC power into DC power. This
can be accomplished, for example by means of a full-wave bridge
rectifier followed by a suitable capacitor, and the use of higher
voltage electronic components. Higher power output in brushless AC
power tools also requires increases in the steady-state current
carrying capacity of the electronics and the ability to remove
additional waste heat. With these changes, brushless DC motors can
be adapted to AC power sources.
[0066] Because brushless DC power tools are most often operated
from a rechargeable battery, their power source is a relatively
constant DC voltage, namely the battery voltage. With the advent to
brushless AC power tools, the rectifier and capacitor were sized
accordingly to produce a relatively constant DC voltage from which
the brushless DC motor could operate as shown in FIG. 4A. The
full-wave rectified AC waveform as measured across the capacitor 50
is referred to as the "DC bus voltage". It is noted that in this
diagram, it is assumed that the tool is operating under a maximum
heavy load that the tool is rated to handle.
[0067] Constant, or relatively constant, voltage is typically
achieved with large values of capacitance following the full-wave
bridge rectifier. For discussion purposes, the comparatively small
voltage drops across the individual diodes in the full-wave bridge
rectifier are ignored. The numerical value of the capacitor will be
determined by the current demands of the motor under load, and the
allowable voltage ripple on the capacitor. Voltage ripple on the
capacitor means there is ripple current in the capacitor, and
because all real capacitors have some (but not zero) equivalent
series resistance (ESR), this ripple current will cause heating
inside the capacitor. Small amounts of heat are acceptable but
large quantities of heat generated inside the capacitor are not
acceptable and may cause damage.
[0068] For a given load, the larger the numerical value of
capacitance, the lower the ripple voltage as seen in FIG. 4B. The
ripple current is equal to the value of capacitance times the time
derivative of the ripple voltage across the capacitor, so long as
the equivalent series resistance is very small compared to the
reactance. A larger capacitor can store more charge and deliver
more current, with less voltage ripple and lower equivalent series
resistance, than a smaller capacitor. Thus, large values of
capacitance, resulting in DC voltages with very low ripple, were
the first choice when making a brushless AC power tool with a
brushless DC motor. They most resemble the batteries for which
brushless motors were designed.
[0069] There are two important consequences of a large capacitor--a
smaller power factor for the power tool and less work that can be
performed with an AC circuit breaker of fixed root-mean-square
(RMS) current limit. Because the capacitor is large in value and
the voltage ripple is low, the DC voltage across it is considered
"stiff". As the power tool draws current from the DC Bus, it will
come from the charge stored on the capacitor, if the instantaneous
magnitude of the AC voltage is not sufficient in the moment to
supply it. The capacitor's charge will then be replenished from the
AC power source through the full-wave bridge rectifier when the
magnitude of the AC voltage later rises slightly above the voltage
across the capacitor, and the DC Bus voltage will then follow the
AC voltage up to its maximum as shown in FIG. 4B. Said differently,
when the instantaneous magnitude of the AC voltage is lower than
the DC Bus voltage, the capacitor will supply the current required
by the motor. The DC Bus voltage will decrease accordingly, so long
as the instantaneous magnitude of the AC voltage is lower. At the
point where the DC Bus voltage equals the instantaneous magnitude
of the AC voltage, the DC Bus voltage will cease decreasing and
begin to follow the AC voltage upward. At this point, the current
demands of the motor are supplied directly by the AC power
source.
[0070] With large values of capacitance, the motor current is
usually supplied by the capacitor. In the short period where the
instantaneous magnitude of the AC voltage is higher than the DC Bus
voltage, current will rush from the AC source to replenish the
charge on the capacitor as well as to power the motor. Thus, large
spikes of current are drawn from the AC source near the maxima and
minima of the AC voltage as seen in FIG. 4C.
[0071] The spiky nature of the AC current is undesirable for two
reasons. First, it means that the power factor of the power tool is
low and the harmonic content of the AC current is high. The second
undesirable result is that for a high value of output power
delivered by the power tool the RMS value of the AC input current
will be high. The practical result is that an unnecessarily large
AC circuit breaker is required with such spiky AC currents for a
given amount of work.
[0072] Many AC circuits are protected, for example by 15 amp RMS
circuit breakers. If the AC current is extremely spiky, as it is
with a large value of capacitor, then relatively little work can be
performed. If the AC current were lower in value and more spread
out in time then much more work could be performed from the same 15
Amp circuit.
[0073] One technique for spreading out the AC current drawn from
the AC source is to use a relatively small value of capacitance for
the DC bus capacitors 50. For example, a high power output tool
(e.g., 1.7 kilowatts) with a brushless motor will have its nominal
capacitance for the DC bus capacitors 50 in the range of 15-20
.mu.F. For low power output tools, the nominal capacitance for the
DC bus capacitors may be set lower. For example, a nominal
capacitance of 10 .mu.F may be suitable for a tool having a power
output of 0.85 kilowatts. Thus, the capacitance value is dependent
upon the output power requirements of the tool. It is envisioned
that the nominal capacitance is in the range of 0-200 .mu.F and
preferably in the range of 5-20 .mu.F.
[0074] In an example embodiment, the DC bus capacitors 50 are
implemented using film capacitors. Film capacitors can be made from
polymer plastics metalized on both sides and may be rolled with
additional suitable insulators. The DC bus capacitors 50 may be
implemented by a single capacitor or a series of capacitors (e.g.,
two 4.7 .mu.F capacitors in parallel to achieve a nominal
capacitance of 9.4 .mu.F or four 4.7 .mu.F capacitors in parallel
to achieve a nominal capacitance of 18.8 .mu.F). In comparison to
traditional electrolytic capacitors, film capacitors have smaller
physical dimensions and less equivalent series resistance per unit
capacitance. By reducing the nominal capacitance of the DC bus
capacitors 50, the time which the AC voltage replenishes charge on
the capacitor is greatly increased. The result is that the power
factor increases, the harmonic content of the AC current goes down,
and the amount of work that can be performed through a 15 Amp AC
circuit breaker increases as shown in FIG. 4D.
[0075] It is envisioned that the power cord of the power tool is
connectable to a DC power source, e.g., a DC generator such as a
welder having a DC output power line, having a DC output voltage of
120V. With a small capacitor 50 having a capacitance value of
approximately 0-50 .mu.F, power tool 10 may provide a higher max
power out from a DC power source having a nominal voltage of 120
VDC, than it would from a 120V AC mains power source. Specifically,
using a small capacitor of 0-50 microF, the DC bus voltage
resulting from a 120V AC mains power source has a nominal value of
approximately 108V.
[0076] An exemplary power tool may provide a maximum cold power
output of approximately 1600 W from the 108V DC bus voltage. By
comparison, the same power tool provides a maximum cold power
output of more than 2200 W from the DC bus when power is being
supplied by the 120V DC power source. This improvement represents a
ratio of 2200/1600=1.37 (which corresponds to the voltage ratio 3,
i.e., (120/108).sup.3).
[0077] In another aspect of this disclosure, comparable power
outputs from the AC and DC power sources can be achieved by
adjusting the capacitance value of the capacitor 50. FIG. 5 depicts
power output and average DC bus voltage in relation to capacitance
values for capacitor 50. The x axis in this diagram depicts varying
capacitance values from 0 to 1000 uF; whereas, the Y axes
respectively represent the maximum power watts-out (W) of the power
tool ranging from 0-2500 W as well as the average DC bus voltage
(V) ranging from 100-180V. The three RMS current values represent
the rated RMS current of the AC power supply. For example, in the
US, the wall socket may be protected by a 15 A RMS current circuit
breaker. In this example, it is assumed that the power tool is
operating under heavy load close to its maximum current rating.
[0078] As shown in this diagram, for a power tool configured to be
powered by a 10 A RMS current power supply (i.e., the tool having a
current rating of approximately 10 A RMS current, or a power supply
having a current rating of 10 A RMS current), the average DC bus
voltage under heavy load is in the range of approximately 108-118V
for the capacitor range of 0-200 uF; approximately 118-133V for
capacitor range of 200 to 400 uF; approximately 133-144V for
capacitor range of 400-600 uF, etc.
[0079] Similarly, for a power tool configured to be powered by a 15
A RMS current power supply (i.e., the tool having a current rating
of approximately 15 A RMS current, or a power supply having a
current rating of 15 A RMS current), the average DC bus voltage
under heavy load is in the range of approximately 108-112V for the
capacitor range of 0-200 uF; approximately 112-123V for capacitor
range of 200 to 400 uF; approximately 123-133V for capacitor range
of 400-600 uF, etc.
[0080] Similarly, for a power tool configured to be powered by a 20
A RMS current power supply (i.e., the tool having a current rating
of approximately 20 A RMS current, or a power supply having a
current rating of 20 A RMS current), the average DC bus voltage
under heavy load is in the range of approximately 108-110V for the
capacitor range of 0-200 uF; approximately 110-117V for capacitor
range of 200 to 400 uF; approximately 117-124V for capacitor range
of 400-600 uF, etc.
[0081] In one embodiment, in order to provide an average DC bus
voltage from the AC mains power source (e.g., a 108V nominal RMS
voltage) that is comparable to the nominal voltage received from
the DC power source (120 VDC), the capacitor value may be sized
based on the current rating of the power tool and the target DC bus
voltage. For example, a capacitor value of approximately 230 uF may
be used for a tool powered by a 10 A RMS current power supply
(i.e., the tool having a current rating of approximately 10 A RMS
current, or configured to be powered by a power supply having a
current rating of 10 A RMS current) to provide an average DC bus
voltage of approximately 120V from the AC mains. This allows for
the power tool to provide substantially similar output levels for
120V AC power source as it would from a 120V DC power source.
[0082] Likewise, a capacitor value of approximately 350 uF may be
used for a tool powered by a 15 A RMS current power supply (i.e.,
the tool having a current rating of approximately 15 A RMS current,
or configured to be powered by a power supply having a current
rating of 15 A RMS current) to provide an average DC bus voltage of
approximately 120V from the AC mains. More generally, capacitor may
have a value in the range of 290-410 uF for a tool powered by a 15
A RMS current power supply to provide an average voltage
substantially close to 120V on the DC bus from the AC mains. This
allows for the power tool to provide substantially similar output
levels for 120V AC power source as it would from a 120V DC power
source.
[0083] Finally, a capacitor value of approximately 500 uF may be
used for a tool powered by a 20 A RMS current power supply (i.e.,
the tool having a current rating of approximately 20 A RMS current,
or configured to be powered by a power supply having a current
rating of 20 A RMS current) to provide an average DC bus voltage of
approximately 120V from the AC mains. More generally, the capacitor
may have a value in the range of 430-570 uF for a tool powered by a
20 A RMS current power supply to provide an average voltage
substantially close to 120V on the DC bus from the AC mains. This
allows for the power tool to provide substantially similar output
levels for 120V AC power source as it would from a 120V DC power
source.
[0084] In another aspect of this disclosure, a cycle-by-cycle
current limit is also implemented in the power tool 10. When the
instantaneous bus current in a given cycle exceeds a prescribed
current limit, the select drive signals to the switches in the
switching arrangement 82 are turned off from the remainder of the
cycle. At the beginning of the next cycle, these drive signals are
restored. For each cycle, the instantaneous current continues to be
evaluated in a similar manner. This principle is illustrated in
FIG. 6, where the thinner line indicates the instantaneous current
without a limit and the thicker line indicates the instantaneous
current with an enforced 20 Amp limit. Cycle-by-cycle current limit
enables the power tool to achieve similar performance across
different types of power sources and under varying operating
conditions as will be further described below.
[0085] Cycle-by-cycle current limiting can be implemented by
modifying the motor control system 80 as shown in FIG. 7.
Specifically, a current sensor 94 is configured to sense the
current through the DC bus and provide a signal indicative of the
sensed current to the controller 78. In an exemplary embodiment,
the current sensor 94 is implemented using a shunt resistor
disposed in series between the capacitor 50 and the switching
arrangement 82. Although not limited thereto, the shunt resistor is
preferably positioned on the low voltage side of the DC bus. In
another embodiment, a shunt may be placed in each lower leg of the
switching arrangement 82. In this case, current for each phase is
sensed and reported to the controller 78. It is also envisioned
that the shunts may be positioned on the input side of the
rectifier as well. In any case, the controller 78 is able to detect
the instantaneous current being delivered to the motor.
[0086] The controller 78 is configured to receive a measure of
instantaneous current passing from the rectifier to the switching
arrangement operates over periodic time intervals (i.e.,
cycle-by-cycle) to enforce a current limit. With reference to FIG.
8, the controller enforces the current limit by measuring current
periodically (e.g., every 5 microseconds) at 801 and comparing
instantaneous current measures to the current limit at 802. If the
instantaneous current measure exceeds the current limit, the
controller 78 cooperatively operates with the driver circuit 84 to
turn off the motor switches for remainder of present time interval
at 803 and thereby interrupt current flowing to the electric motor.
If the instantaneous current measure is less than or equal to the
current limit, the controller 78 continues to compare the
instantaneous current measures to the current limit periodically
for the remainder of the present time interval as indicated at 804.
Such comparisons preferably occur numerous times during each time
interval (i.e. cycle). When the end of the present time interval is
reached, the controller 78 cooperatively operates with the driver
circuit 84 to turn on the motor switches at 805 and thereby resume
current flow to the motor for the next cycle. In one embodiment,
the duration of each time interval is fixed as a function of the
given frequency at which the electric motor is controlled by the
controller. For example, the duration of each time interval is set
at approximately ten times an inverse of the frequency at which the
electric motor is controlled by the controller. In the case the
motor is controlled at a frequency of 10 kilo-Hertz, the time
interval is set at 100 microseconds. In other embodiments, the
duration of each time interval may have a fixed value and no
correlation with the frequency at which the electric motor is
controlled by the controller.
[0087] In the example embodiment, the electric motor is controlled
by pulse width modulated (PWM) signals received from the motor
drive circuit and duration of the each time interval equals period
of the PWM signals. In a fixed speed tool under a no load
condition, the duty cycle of the PWM control signals is set, for
example at 60%. Under load, the controller 78 operates to maintain
a fixed speed by increasing the duty cycle. If the current through
the DC voltage bus increase above the current limit, the controller
78 interrupts current flow as described above which in effect
reduces the duty cycle of the PWM signals. For a variable speed
tool under a no load condition, the duty cycle of the PWM control
signals ranges for example from 15% to 60%, in accordance with user
controlled input, such as a speed dial or a trigger switch. The
controller 78 can increase or decrease the duty cycle of the PWM
signals during a load condition or an over current limit condition
in the same manner as described above. In one embodiment, speed
control and current limiting may be implemented independently from
each other by using three upper motor switches for speed control
and the three lower switches for current limiting. It is envisioned
that the two functions may be swapped between the upper and lower
switches or combined together into one set of switches.
[0088] In the examples set forth above, the time interval remained
fixed. When this period (time interval) remains fixed, then the
electronic noise generated by this switching will have a
well-defined fundamental frequency as well as harmonics thereof.
For certain frequencies, the peak value of noise may be
undesirable. By modulating the period over time, the noise is
distributed more evenly across the frequency spectrum, thereby
diminishing the noise amplitude at any one frequency. In some
embodiment, it is envisioned that the time interval may be
modulated (i.e., varied) over time to help distribute any noise
over a broader frequency range.
[0089] In a variant, the controller 78 enforces the cycle-by-cycle
current limit by setting or adjusting the duty cycle of the PWM
drive signals output from the gate driver circuit 84 to the power
switch circuit 82. In an embodiment, the duty cycle of the PWM
drive signals may be adjusted in this manner following the instant
current cycle (i.e., at the beginning of the next cycle). In a
fixed speed tool, the controller 78 will initially set the duty
cycle of the drive signals to a fixed value (e.g., duty cycle of
75%). The duty cycle of the drive signals will remain fixed so long
as the current through the DC bus remains below the cycle-by-cycle
current limit. The controller 78 will independently monitor the
current through the DC bus and adjust the duty cycle of the motor
drive signals if the current through the DC bus exceeds the
cycle-by-cycle current limit. For example, the controller 78 may
lower the duty cycle to 27% to enforce the 20 amp current limit. In
one embodiment, the duty cycle value may be correlated to a
particular current limit by way of a look-up table although other
methods for deriving the duty cycle value are contemplated by this
disclosure. For variable speed tool, the controller 78 controls the
duty cycle of the motor drive signals in a conventional manner in
accordance with the variable-speed signal from the variable-speed
actuator. The cycle-by-cycle current limit is enforced
independently by the controller 78. That is, the controller will
independently monitor the current through the DC bus and adjust the
duty cycle of the drive signals only if the current through the DC
bus exceeds the cycle-by-cycle current limit as described
above.
[0090] In some embodiments, the cycle-by-cycle current limit may be
dependent upon the type and/or nominal voltage of the power supply.
For a tool configured to receive power from either a DC or AC power
supply, the controller 78 may receive a signal from the interlock
mechanism 96, where the signal indicates whether the tool is
coupled to a DC supply (e.g., 120V) from a battery pack or an AC
supply (e.g. 120V) from an AC socket. In lieu of or in addition to
the DC supply, the tool may be further configured to receive power
from an AC source having different nominal voltages (e.g., 120 v or
230V). Likewise, the tool may be configured to receive power from
battery packs having different nominal voltages (e.g., 60V or
120V). In these cases, the controller 78 also receives a signal
indicative of the amplitude of the input power signal. In other
embodiments, the controller may be configured to sense directly the
nominal voltage being supplied by the power supply. Other methods
for determining the type and/or nominal voltage of the power supply
are also contemplated by this disclosure.
[0091] Given the type and/or nominal voltage of the power supply,
the controller 78 selects a current limit to enforce during
operation of the power tool. In one embodiment, the current limit
is retrieved by the controller 78 from a look-up table. An example
look-up table is as follows:
TABLE-US-00001 Source type Nominal voltage Current limit AC 120 V
40 A AC 230 V 20 A DC 120 V 35 A DC 108 V 40 A DC 60 V 70 A DC 54 V
80 A
[0092] That is, the controller 78 will enforce a 40 Amp current
limit when the tool is coupled to a 120V AC power supply but will
enforce a 20 Amp current limit when the tool is coupled to a 230V
AC power supply. As a result, the effective output power of the
tool is substantially the same. Because the average voltage
supplied from by a 120V AC source is approximately 108 volts DC,
the controller 78 will also enforce a slightly lower current limit
(e.g., 35 amps) when the tool is coupled to a 120V DC power supply.
This lower current limit results in output power so that it is
substantially the same as when the tool is coupled to 120V AC power
supply. Similarly, the controller 78 will enforce a higher current
limit (e.g., 70 amps) when the tool is coupled to a 60V DC power
supply. In this example, output power of the tool has been
normalized to the case of a 120V AC power supply. Tool performance
could also be normalized in relation to one of the other types of
power supplies. Moreover, it is understood that the current limits
set forth above are approximate values and more precise values may
be used in practice to achieve similar performance from the tool
across different power supplies.
[0093] During motor rotation, the electronic switches in the
switching arrangement 82 transition back and forth between current
conduction and no current conduction. In their conducting state
they are not lossless, but generate some heat which, if not
removed, will raise the temperature of the switch to the point of
failure. Limiting the amount of current through the switch will
limit the amount of heat generated within it. Often, extreme
temperature is the reason for failure in electronic switches.
Maintaining a reasonable temperature allows the electronic switch
to continue to operate. Conventionally, this is accomplished by
cooling the switches with airflow and a heat sink, and also by
limiting the current through the switches. Because electronic
switches in an electronically commutated three-phase motor
typically conduct current approximately one-third of the time, it
becomes possible to raise the level of current during the one-third
of the time the switches are conducting because there is no
conduction in the other two-thirds of the time.
[0094] When the electronically commutated motor is first started,
it begins rotating from a stationary position and the appropriate
upper and lower electronic switches in the switching arrangement
are conducting continuously. Only after the motor achieves some
significant rotational speed does each electronic switch experience
the one-third duty cycle sufficient to average out the heat
generation across the cycle.
[0095] To prevent the electronic switches from overheating during
start-up, cycle-by-cycle current limiting may be used to limit
current at start-up. Rather than implementing a single current
limit, the tool 10 is configured to apply a lower current during
start-up as shown in FIG. 9. In this example, a current limit of 10
amps is enforced by the controller 78 when the shaft speed is less
than a predefined threshold (e.g., 12,000 rpm). After a sufficient
shaft speed has been achieved, the cycle-by-cycle current limit is
raised to a high limit of 20 amps. Should the shaft speed decrease,
the current limit may revert back to the lower limit of 10 Amps.
This feature may be implemented with hysteresis so that the current
limit does not revert back to the lower value until the shaft speed
is less than a second predefined threshold which has a value lower
than the first threshold (e.g. 7,000 rpm). Current limits and speed
threshold values are merely illustrative and may vary depending on
switch conduction time and other factors. It is envisioned that the
shaft speed may be determined by the controller 78, for example
from input from position sensors.
[0096] In another aspect of this disclosure, cycle-by-cycle current
limiting can be used to detect a pinch event or stall condition and
possibly protect a user from losing control of the tool. A pinch
event or stall condition is understood to mean an event that very
quickly decelerates the rotation of the tool and causes an
immediate rise in current. In the case of a grinder, a pinch event
may occur when using the grinder as a cut-off tool and the
workpiece moves in such a way as to pinch the cut-off accessory.
Causes for a pinch event vary with the type of tool and the type of
accessory.
[0097] An example method for detecting a pinch event using
cycle-by-cycle current limit is further described in relation to
FIG. 10. For each cycle, the instantaneous bus current is measured
periodically (e.g., every 1 microsecond) during the cycle as
indicated at 112. The measured current is compared at 113 to the
current limit presently being enforced by the controller. When the
current limit is exceeded, select drive signals to the switches in
the switching arrangement 82 are turned off for the remainder of
the cycle as indicated at 114. Additionally, an event counter is
incremented by one at 115 and an over limit flag is set true at
116.
[0098] Before checking the current limit, the over limit flag is
checked at step 111. Measurement of the current and its comparison
to the current limit will continue for the entire cycle so long as
the over limit flag is set false. Conversely, these steps are
skipped for the reminder of the cycle once the over the limit flag
is set to true. In this way, the event counter is incremented (if
applicable) only once per cycle. Thus, the check of the current
limit will be repeated until either the current limit is exceeded
or a new cycle begins.
[0099] At the beginning of the next cycle, the drive signals to the
switches are restored by the controller. Additionally, a cycle
counter is incremented by one at 117 and the over limit flag is
reset to false at 118. Moreover, a determination is made as to
whether a pinch event has occurred once a sufficient sample size
has accrued. For example, a determination about a pinch event can
be made every 10 milliseconds. Assuming a cycle of 100
microseconds, the determination about a pinch event would be made
when the cycle counter reaches a count of 100. To do so, the cycle
counter is compared at 119 to a predefined cycle threshold whose
value correlates to the determination period (e.g., 100 in the
example above).
[0100] When the cycle count reaches the threshold value, the event
counter is then compared at 120 to an event threshold. In one
embodiment, a pinch event is deemed to have occurred when the event
count exceeds the event threshold (e.g., 20). In this case, the
controller initiates a protective operation at 122 to protect to
the tool operator. For example, the controller may cut power to the
motor and/or apply a brake to the output shaft of the tool coupled
thereto. Suspending drive signals effectively makes the motor
coast. In a pinch event, momentum is transferred to the tool and
thus the user. With braking, this momentum is reduced to lower
levels than experienced during coasting, which helps to protect the
user. Other types of protective operations are also contemplated by
this disclosure.
[0101] In other embodiments, when the event count reaches the event
threshold, the present 10 millisecond time interval is counted as
an over-current interval, and a pinch event is deemed to have
occurred, when a fixed number (e.g., 10) of over-current intervals
occur consecutively. When the event count does not exceed the event
threshold, the event counter and the cycle counter are reset at 121
and processing of the remainder of the cycle continues as indicated
at 112. Likewise, when the cycle count does not exceed the cycle
threshold, processing of the remainder of the cycle continues.
Values set forth above have been found to be reasonable for a wide
variety of conditions but are not considered limiting. Different
values can be used to accommodate specific type of pinch conditions
in different types of tools. It is to be understood that only the
relevant steps of the methodology are discussed in relation to FIG.
10, but that other software-implemented instructions may be needed
to control and manage the overall operation of the system.
[0102] The method set forth above may also be used to detect stall
conditions as well as other tool conditions which may develop at a
slower rate than a pinch event. For example, when a tool operates
under heavy loads, the current levels are high and the motor speed
is low such that airflow is low and the tool is at risk of
overheating. With reference to FIG. 11, the methodology is
substantially similar to that described above in relation to FIG.
10. A secondary tool parameter is used to detect a stall condition.
In addition to checking the event count, the rotational speed of
the shaft is determined at 223 and compared to a speed threshold at
224. A stall condition is deemed to have occurred when the event
count exceeds the event threshold and the rotational speed of the
shaft is less than a speed threshold (e.g., 4,000 rpm). In this
scenario, the controller initiates a protective operation at 222 to
prevent damage to the tool. The remaining steps and variants
thereof are described above in relation to FIG. 10 and omitted here
for brevity. While reference is made to rotational shaft speed as
the secondary tool parameter, other tool parameters may also be
used in conjunction with over current events to detect stall
conditions. One of those other tool parameters may be time since
the last signal from any position indicator. If that time, as
expressed as a count of a timer, exceeds some limit, and the over
current event counter also exceeds its limit, then motor operation
is suspended as described above.
[0103] There are also other techniques for testing the
instantaneous current and the methods described above are not meant
to limit the scope of this disclosure. In the examples above the
current is measured at discrete intervals. But analog comparators
can also be used to signal an instantaneous over current limit and
these signals can then be used in similar fashion to effect the
same result as described above. It is again to be understood that
only the relevant steps of the methodology are discussed in
relation to FIG. 11, but that other software-implemented
instructions may be needed to control and manage the overall
operation of the system.
[0104] Adjustments to the cycle-by-cycle current limit may also be
used to prevent overheating conditions from occurring in the tool.
Variable speed grinders present a special challenge--preventing
burn up under heavy loads at low speeds. Variable speed of a
variable speed grinder may be set, for example by a rotary
thumbwheel. Ranging from high speed selection to low speed
selections, the thumbwheel position selects the nominal, unloaded
speed of the grinder. The thumbwheel is commonly referred to as a
speed dial, and the position of the speed dial is the speed setting
for the tool.
[0105] Certain applications for grinders are best performed at low
speed dial settings. In these applications, as the user may apply
more and more force, thereby placing the grinder under increasingly
heavier loads. At some point, which may occur relatively quickly at
lower speed dial settings in comparison with higher speed dial
settings, the motor will begin to slow down as it is simply not
capable of delivering that level of power at the requested speed
dial setting. As the motor speed decreases, the cooling fan
attached to the motor shaft also slows down. The airflow generated
by the fan drops faster than the speed. As a result, the cooling of
the motor, the gear case, and the electronics, becomes less and
less effective. Eventually temperatures may build up to the point
of failure.
[0106] One technique for preventing such overheating conditions is
through the use of a temperature sensor disposed inside the housing
of the tool. Although not limited hereto, the temperature sensor
may be placed in or on a temperature sensitive component of the
motor (e.g., a coil winding). In one embodiment, the controller
sets and enforces the cycle-by-cycle current limit in the manner
set forth above. The controller may also be configured to receive a
signal indicative of temperature from the temperature sensor.
During operation, the controller monitors the temperature and
further operates to adjust the cycle-by-cycle current limit in
accordance with the measured temperature as shown for example in
FIG. 12. In this example, the controller enforces a 20 Amp current
limit so long as the temperature remains below 100 Celsius. Once
the motor temperature exceeds 100 Celsius, the controller lowers
the cycle-by-cycle current limit and thereby lowers the power that
can be delivered by the tool. In practice, the user will experience
the reduction in tool performance and back off the force being
applied to the tool which in turn allows the motor speed to
increase. As motor speed increase, airflow increases and thereby
lowers the motor temperature. Conversely, as motor temperature
decreases, the controller increases the cycle-by-cycle current
limit until it returns to the initial 20 Amp limit. Adjustments to
the current limit may be correlated with temperature linearly as
shown or in some other manner such as exponentially. In this way
the power tool user learns the maximum operating point of the
grinder at any particular speed dial setting and can avoid
overheating the tool.
[0107] Some of the techniques described herein may be implemented
by one or more computer programs executed by one or more processors
residing, for example on a power tool. The computer programs
include processor-executable instructions that are stored on a
non-transitory tangible computer readable medium. The computer
programs may also include stored data. Non-limiting examples of the
non-transitory tangible computer readable medium are nonvolatile
memory, magnetic storage, and optical storage.
[0108] Some portions of the above description present the
techniques described herein in terms of algorithms and symbolic
representations of operations on information. These algorithmic
descriptions and representations are the means used by those
skilled in the data processing arts to most effectively convey the
substance of their work to others skilled in the art. These
operations, while described functionally or logically, are
understood to be implemented by computer programs. Furthermore, it
has also proven convenient at times to refer to these arrangements
of operations as modules or by functional names, without loss of
generality.
[0109] Unless specifically stated otherwise as apparent from the
above discussion, it is appreciated that throughout the
description, discussions utilizing terms such as "processing" or
"computing" or "calculating" or "determining" or "displaying" or
the like, refer to the action and processes of a computer system,
or similar electronic computing device, that manipulates and
transforms data represented as physical (electronic) quantities
within the computer system memories or registers or other such
information storage, transmission or display devices.
[0110] Certain aspects of the described techniques include process
steps and instructions described herein in the form of an
algorithm. It should be noted that the described process steps and
instructions could be embodied in software, firmware or hardware,
and when embodied in software, could be downloaded to reside on and
be operated from different platforms used by real time network
operating systems.
[0111] The foregoing description of the embodiments has been
provided for purposes of illustration and description. It is not
intended to be exhaustive or to limit the disclosure. Individual
elements or features of a particular embodiment are generally not
limited to that particular embodiment, but, where applicable, are
interchangeable and can be used in a selected embodiment, even if
not specifically shown or described. The same may also be varied in
many ways. Such variations are not to be regarded as a departure
from the disclosure, and all such modifications are intended to be
included within the scope of the disclosure.
[0112] Example embodiments are provided so that this disclosure
will be thorough, and will fully convey the scope to those who are
skilled in the art. Numerous specific details are set forth such as
examples of specific components, devices, and methods, to provide a
thorough understanding of embodiments of the present disclosure. It
will be apparent to those skilled in the art that specific details
need not be employed, that example embodiments may be embodied in
many different forms and that neither should be construed to limit
the scope of the disclosure. In some example embodiments,
well-known processes, well-known device structures, and well-known
technologies are not described in detail.
[0113] The terminology used herein is for the purpose of describing
particular example embodiments only and is not intended to be
limiting. As used herein, the singular forms "a," "an," and "the"
may be intended to include the plural forms as well, unless the
context clearly indicates otherwise. The terms "comprises,"
"comprising," "including," and "having," are inclusive and
therefore specify the presence of stated features, integers, steps,
operations, elements, and/or components, but do not preclude the
presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof. The
method steps, processes, and operations described herein are not to
be construed as necessarily requiring their performance in the
particular order discussed or illustrated, unless specifically
identified as an order of performance. It is also to be understood
that additional or alternative steps may be employed.
* * * * *