U.S. patent application number 10/415298 was filed with the patent office on 2004-07-08 for position transducer.
Invention is credited to Stridsberg, Lennart.
Application Number | 20040129869 10/415298 |
Document ID | / |
Family ID | 27555295 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040129869 |
Kind Code |
A1 |
Stridsberg, Lennart |
July 8, 2004 |
Position transducer
Abstract
An encoder for detecting the angular position of a shaft of an
electric motor comprises an encoder disc (101) having tracks (515)
sensed by detectors in reading heads (106a, 106b). Analog position
signals from the detectors are sampled in sampling circuits
(1701-1704) at times given by sampling pulses issued by a device
such as a counter (506). The sampled analog values are converted to
digital values by a converter (1706) and provided to a local
processor (1707) calculating position values. The local processor
sends commands to the device (506) to issue sampling pulses at
accurately defined times, normally regularly spaced in time. The
local processor also receives, from a central processor, read
command signals commanding to it provide position values. These
read command signals normally arrive rather regularly but are not
synchronised. In order to provide positions values at suitable
times after receiving the read command signals, the local processor
can send its commands earlier or later, in relation to the normal
regular rate.
Inventors: |
Stridsberg, Lennart;
(Stockholm, SE) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
27555295 |
Appl. No.: |
10/415298 |
Filed: |
July 28, 2003 |
PCT Filed: |
October 22, 2001 |
PCT NO: |
PCT/SE01/02313 |
Current U.S.
Class: |
250/231.13 |
Current CPC
Class: |
G01D 5/2451 20130101;
G01D 5/24447 20130101 |
Class at
Publication: |
250/231.13 |
International
Class: |
G01D 005/34 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 2000 |
SE |
0003866-1 |
Oct 25, 2000 |
SE |
0003954-5 |
Oct 26, 2000 |
SE |
0003955-2 |
Dec 17, 2000 |
SE |
004717-5 |
Feb 28, 2001 |
SE |
0100715-2 |
Apr 3, 2001 |
SE |
0101222-8 |
Claims
1. A method of controlling an electric motor having a shaft,
comprising the steps of: detecting the angular position of the
shaft and providing based on the detected angular position an
output analog signal, deriving, at commanded times, from the output
signal digital values of the angular position and using them in the
controlling, providing request pulses for requesting a derived
value, the commanded times appearing with a constant time interval
between them except when the time period from the time of deriving
a value to a time when receiving a request pulse is not within a
predetermined range.
2. A method according to claim 1, characterized in that in the case
where the time period from the time of deriving a value to a time
when receiving a request pulse is shorter than a lower limit of the
predetermined range, a next commanded time is made to occur later,
and that in the case where the time period from the time of
deriving a value to a time when receiving a request pulse is longer
than an upper limit of the predetermined range, a next commanded
time is made to occur earlier.
3. A method according to claim 1 for controlling at least two
electrical motors, characterized in that for each motor values of
angular position of their respective shaft are derived and that the
derived values are sent in a time multiplexed way on a single bus
cable for the controlling.
4. An encoder for detecting the angular position of a shaft,
comprising: an encoder disc rigidly attached to the shaft and
having at least one first encoding track, at least one detector for
detecting variations in the first encoding track and providing an
analog position signal, characterized by a sample-and-hold device
connected to sample the analog position signal to provide sampled
analog values, a sampling signal device issuing sampling pulses to
the sample-and-hold device commanding it to make a sample, an
analog-to-digital converter connected to the sample-and-hold device
for converting the sampled analog values to digital values, a local
processor connected to the converter for receiving the digital
values and therefrom calculate and provide position values and
further connected to the sampling signal device for sending to it
commands to issue sampling pulses at following, accurately defined
times, the local processor arranged to receive read command signals
commanding to it provide position values and to delay the command
to issue a sampling pulse in the case where the interval between
the previously issued sampling pulse and a thereafter received read
command signal is too small or to issue it earlier in the case
where the interval is too large.
5. An encoder according to claim 4, characterized in that the local
microprocessor is connected to or includes a clock circuit issuing
a clock signal including clock pulses, and that the sampling signal
device comprises a counter counting the clock pulses.
6. An encoder according to claim 5, characterized in that the
sampling signal device comprises a programmable divide by n
counter, n being a positive integer.
7. An encoder according to claim 5, characterized by a register for
storing the content of the counter on receiving the read command
signal.
8. An encoder according to claim 4 connected to the shaft/rotor of
an electric motor, characterized in that the encoder comprises a
low resolution track and a low resolution detector used to provide
signals for coarse positioning in a low power mode of the motor
and/or for commutation during initial stages of a high power
mode.
9. An electric motor system including: a central processor, at
least one electric motor, the electric motor having a shaft and
including: an encoder for detecting the angular position of the
shaft, comprising: an encoder disc rigidly attached to the shaft
and having at least one first encoding track, at least one detector
for detecting variations in the first encoding track and providing
an analog position signal, the encoder further characterized by a
sample-and-hold device connected to sample the analog position
signal to provide sampled analog values, a sampling signal device
issuing sampling pulses to the sample-and-hold device commanding it
to make a sample, an analog-to-digital converter connected to the
sample-and-hold device for converting the sampled analog values to
digital values, a local processor connected to the converter for
receiving the digital values and therefrom calculate and provide
position values and further connected to the sampling signal device
for sending to it commands to issue sampling pulses at following,
accurately defined times, the local processor connected to the
central processor to receive therefrom read command signals
commanding to it provide position values and to delay the command
to issue a sampling pulse in the case where the interval between
the previously issued sampling pulse and a thereafter received read
command signal is too small or to issue it earlier in the case
where the interval is too large.
10. An electric motor system according to claim 9, characterized by
at least two electric motors, each including an encoder, local
processors of the electric motors connected to the central
processor through a single bus line for receiving read command
signals on the bus line.
Description
TECHNICAL FIELD
[0001] The present invention is concerned with devices such as
angular transducers or encoders for detecting the position of a
rotating shaft relative to a stationary part such as a chassis, in
particular for detecting the angular position of a rotor or shaft
of an electric motor/generator relative to the stator of the
motor/generator.
BACKGROUND AND PRIOR ART
[0002] Important examples of high resolution encoders for motors
are the EnDat series from the company Dr Johannes Heidenhain GmbH,
83301 Traunreut, Germany and SinCos systems from the company Max
Stegmann GmbH, 78 156 Donaueshingen, Germany. Both permit a
resolution of some million positions per turn by sending
incremental analog position data in analog form while also
providing the possibility to read absolute information in a serial
digital format. The transmission of the analog signals used
requires separate cabling between each encoder and the computer
that controls the current sent to the motor. Such a cabling and the
use of analog signals makes it simple to obtain a synchronisation
between the time when the motor position is read and the time when
the calculations for the next current controlling switch setting is
performed. The analog position data are typically sampled in the
same unit as calculates and sets the timing for the switches
controlling the currents.
[0003] Presently used encoders for motor applications can be
classified in two groups depending on if they have bearing system
of their own or not. Encoders having an own bearing system can
achieve very high resolutions at the expense of higher costs and a
greater length in the axial direction of the motor. To avoid that
the bearing system of the encoder creates large forces conflicting
with the bearing system of the motor, the encoder chassis or rotor
must be assembled against the motor using a flexible coupling. This
normally makes the total system even longer and introduces possible
resonances in the system.
[0004] Encoders without own bearings are limited to resolutions in
the order of 8000 positions per turn against some millions for the
high resolution devices. The company Renco and Danaher Controls,
Gurnee, Ill., U.S.A., sells such encoders with the added benefit of
integrated devices to adjust the position of the encoder disc
relative to the encoder chassis during assembly. Such encoders
normally transmit their information as digital incremental
signals.
[0005] Angular transducers and encoders of a kind similar to those
described herein are e.g. disclosed in the published International
patent application WO 99/54683 and U.S. Pat. Nos. 5,998,783,
6,084,234 and 6,191,415 which are incorporated by reference
herein.
SUMMARY
[0006] An object of the invention is to provide a high resolution
encoder with a low cost for cable assembly.
[0007] Another object of the invention is to provide a high
resolution encoder that can be connected through a cable bus system
while permitting a good synchronisation between the time when the
motor position is read and the time when the calculations for the
next current controlling switch setting are performed.
[0008] Yet another object of the invention is to provide a high
resolution encoder that can be connected through a cable bus
system.
[0009] Yet another object of the invention is to provide a high
position resolution encoder having a high resolution also in
measuring speed by ensuring a precise and known time interval
between position sampling events.
[0010] Yet another object of the invention is to provide a high
resolution encoder having a low production and assembly cost.
[0011] Yet another object of the invention is to provide a high
resolution encoder having a high noise immunity inherent in digital
information transfer.
[0012] Yet another object of the invention is to provide a high
resolution encoder having a short total length.
[0013] Yet another object of the invention is to provide a short
encoder having a multiturn registration ability.
[0014] The objects above are achieved by the invention, the
characteristics of which appear from the appended claims.
[0015] Thus generally, an encoder for detecting the angular
position of a shaft of an electric motor comprises an encoder disc
having one or more encoding tracks having variations of some
detectable physically detectable quantity along the periphery of
the disc. The variations are sensed by one or more detectors in one
or more reading heads. Analog position signals from the detectors
are sampled in sampling-and-hold devices to give analog values, the
sampling times given by sampling pulses issued by a sampling pulse
device, e.g. a counter. The sampled analog values are converted to
digital values, e.g. by first being time multiplexed and then
passing an analog-to-digital converter, and then provided to a
local processor, a microprocessor calculating position values. The
local processor sends commands to the sampling pulse device to
issue sampling pulses at accurately defined times, which are
normally regularly spaced in time. The local processor also
receives, from a central processor, read command signals commanding
to it provide position values. These read command signals normally
arrive rather regularly but are not synchronised to the sampling
pulses. In order to provide positions values at suitable times
after receiving the read command signals, the local processor is
arranged to send its commands earlier or later, in relation to the
normal regular rate.
[0016] This can be made by calculating the interval between the
time of receiving a read command signal and the time when the last
command for issuing a sampling pulse has been given, comparing the
interval to a set value dependent on the processing times in the
various components that the detected analog signals has to pass to
give the calculated position values and taking the appropriate
action when the calculated interval differs too much from the set
value.
[0017] Furthermore generally, when controlling an electric motor
having a shaft, the following steps are performed:
[0018] 1. The angular position of the shaft is detected, e.g.
optically or magnetically, such as generally by detecting physical
variations in an encoding track, and based on the detected angular
position an output analog signal is provided.
[0019] 2. At commanded times, which are set to basically appear
regularly, i.e. with a constant time interval between, starts the
deriving, from the output signal, digital values of the angular
position at those commanded times, and after finishing the
deriving, the digital values are used in the controlling, e.g. are
immediately sent to a central processor.
[0020] 3. Request pulses, each for requesting a derived digital
value, are issued, the request pulses generally appearing regularly
in time but sometimes having deviations therefrom and not
synchronised with the commanded times and not with the constant
time interval between the commanded times.
[0021] 4. The time interval between the commanded times is made to
deviate from the constant time interval when the time period from
the time of starting of deriving a value to a time when next
receiving a request pulse is not within a predetermined range.
After the deviation having been made, the constant time interval is
again used for setting the next commanded time.
[0022] Obviously then, in the case where the time period from the
time of starting to derive a value to the time when receiving a
next request pulse is shorter than a lower limit of the
predetermined range, a next commanded time is made to occur later.
In the corresponding way, in the case where the time period from
the time of deriving a value to a time when receiving a next
request pulse is longer than an upper limit of the predetermined
range, a next commanded time is made to occur earlier.
[0023] In the case where at least two electrical motors are
controlled, values of the angular position of the shaft of each
motor are derived and then, the derived values are sent in a time
multiplexed way on a single bus cable for controlling the
motors.
[0024] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realised and obtained by means of the methods, processes,
instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] While the novel features of the invention are set forth with
particularly in the appended claims, a complete understanding of
the invention, both as to organisation and content, and of the
above and other features thereof may be gained from and the
invention will be better appreciated from a consideration of the
following detailed description of non-limiting embodiments
presented hereinbelow with reference to the accompanying drawings,
in which:
[0026] FIG. 1a is a partly sectional view along the axial direction
of a motor having two position reading heads included in an optical
encoder,
[0027] FIG. 1b is a front view as seen from the end of the motor of
FIG. 1a showing the setting of the angular position of the two
position reading heads,
[0028] FIG. 1c is a partly sectional view similar to that of FIG.
1a along the axial direction but in a plane perpendicular to that
of FIG. 1a showing electrical connections between light sensors,
light sources and signal processing devices of the optical
encoder,
[0029] FIG. 2 is a block diagram of the high resolution signal
processing part of the encoder of FIGS. 1a--1c,
[0030] FIGS. 3a-3d are timing diagrams of signals and states of
components of the high resolution signal processing part of the
encoder of FIGS. 1a-1c,
[0031] FIGS. 4a-4d are timing diagrams of signals and states of
components of the high resolution signal processing part of the
encoder of FIGS. 1a-1c illustrating the adaptation of an interval
between sampling pulses,
[0032] FIG. 5 is a more complete block diagram of an encoder,
[0033] FIG. 6 is a timing diagram illustrating the data transfer in
a system including two encoders using a common cable with separate
command and response channels,
[0034] FIG. 7 is a block diagram of a system including three motors
provided with encoders, three motor current controllers and a
common system central processor that communicates with the encoders
over a common bus cable,
[0035] FIG. 8 is a sectional view of the assembly of an encoder
using non-optical primary signals, and
[0036] FIG. 9 is a block diagram of the encoder of FIG. 8.
DETAILED DESCRIPTION
[0037] In FIG. 1a is shown, in a view perpendicular to an axial
direction, two reading heads of an encoder for detecting the
angular position of a rotatable part such as a shaft or a rotor of
a motor. The encoder and related parts will hereinafter be
described as mounted to detect the angular position of a shaft or a
rotor of an electric motor/generator, the term electric motor used
to also encompass generators. A encoder disc 101 is assembled to
the motor shaft 102 using a hub 103 that presses against the rear
motor bearing 104 in a way similar to that described for item 104
of FIG. 1 in the cited International patent application WO
99/54683. This gives a very stiff mechanical connection between the
shaft or rotor and the encoder disc 101.
[0038] The encoder disc 101 has one or more encoder or encoding
tracks, see also FIG. 5. Light from at least one emitter/lens
system 105 passes a track of the encoder disc 101 and is read by a
reading head 106a or 106b including one or more photo receivers.
The receiver in the head can advantageously have its photosensitive
area configured in such a way that it acts as a combined mask and
receiver. The signals output from the heads 106a, 106b are
substantially sinusoidal.
[0039] Two light emitter/reading head combinations are provided,
using the same incremental track of the encoder disc 101 for
detecting the angular position. As described with reference to
FIGS. 2, 3 and 4 in the cited U.S. Pat. No. 5,998,783, the summing
of the position information from at least two such reading heads
will cancel the effects of small radial movements of the encoder
disc 101. Such movements are likely to occur as a consequence of
the play in the bearings such as 104 and the various loads on the
motor shaft 102.
[0040] The encoder has a stationary part or stator 107 that is
radially centred relative to the motor rear end shield 108 by a
flange 109 in the motor rear shield that fits the outside of the
encoder stator 107 in a way similar to the assembly of
resolvers.
[0041] In the view of FIG. 1b taken in an axial direction, i.e.
parallel to the axial direction of the shaft 102, the mounting of
the encoder stator 107 is seen. The encoder stator is axially
centred relative to the motor rear end shield 108 by a flange 110
of the encoder stator that is by screws 11 pressed towards a flat
surface in the motor rear shield. Elongated holes 112 are provided
in the flange 110. This permits the encoder stator to be rotated in
a way similar to the assembly of resolvers in order to adjust the
phase of the signals from the encoder which are e.g. used for
commutation of an electric motor to the phase of the emf. of the
motor. In most applications, this alignment is achieved through the
storing of parameters during a calibration run as will be described
hereinafter.
[0042] In FIG. 1c is shown another view perpendicular to the axial
direction, this view being taken in a direction also perpendicular
to that of FIG. 1 a. In FIG. 1c also the electrical connections
between light sensors, light sources and signal processing devices
are seen. The heads 106a, 106b that include the light sensors are
assembled on a non-flexible PC (Printed Circuit) board 113
connected to a second non-flexible PC board 115 through a flexible
PC strip 114. The light emitters/lenses 105 are placed on the board
115. Temperature sensitive devices shown at 118 are assembled on a
third non-flexible PC board 117 and connected to the second PC
board 115 through another flexible strip 116. The third board 117
can be mounted to a heat sink 119 that can be used as a rear end
cover of the motor. A third combination of a light emitter 120 and
receiver 121 uses light sent through another track, not shown, of
the encoder disc 101. This third combination is shown as a separate
optical assembly but can alternatively be integrated in one or both
of the reading heads 106a or 106b for detecting position in a high
resolution.
[0043] FIG. 2 is a block diagram of the signal processing part of
the encoder used for detecting the angular position with a high
resolution. The encoder disc 101 can be assumed to have a track
comprising in the conventional manner for example 1024 periods per
full turn. The light passing through such a track of the disc
towards the head 106a is captured by two photo receivers 511 and
512 comprised in said head which have their light sensitive
surfaces arranged so that the signal from the receiver 511 is
approximately 90 degrees out of phase from the signal from the
other receiver 512, again in a conventional manner. Photo receivers
513 and 514 comprised in other head 106b are arranged to provide
signals in a similar manner.
[0044] The signals from the high resolution receivers 511, 512 and
513, 514 are commonly or at the same instances sampled by
sample-and-hold devices 1701-1704. The sampled values are then
converted to digital form using a multiplexer 1705 multiplexing the
signals in time and an analog-to-digital converter 1706. The
digital values obtained are converted to angular positions by a
microprocessor 1707 performing suitable calculations, possibly
using corrections to handle non-sinusoidal shapes of the signals
and varying amplitude for different angles of a full turn of the
encoder disc as disclosed in the cited U.S. Pat. No. 6,191,415. The
two signals indicating the position obtained from the head 106a and
the head 106b are then added to each other in order to cancel out
vibrations in the up-down directions of the encoder disc 101.
Vibrations in the left- right directions will have negligible
effect on the position signals.
[0045] The sampling commands or sampling pulses for sampling, at
the same common times, in all the sample-and-hold devices 1701-1704
are issued by a device 1709 that advantageously is a counter, in
particular a programmable divide by n counter, where n is a
positive integer, the counter counting the pulses of the clock
signal of the microprocessor 1707. Such a counter permits very
precise sampling times and thereby very accurately defined periods
or intervals between the sampling pulses. If position values of
high resolution are obtained at highly precise times, a
mathematical division of the detected movement by the time elapsed
between the sampling pulses can give a very accurate estimation of
the average speed during the interval.
[0046] Data on the position, or on the change in position during
the interval, is sent to a central processor, see FIG. 7 to be
described hereinafter, through a communication link, not shown in
FIG. 2, connected to the communication unit 1708.
[0047] In the following "microprocessor" or "local processor"
refers to a processor in an encoder, "central processor" to a
processor controlling a system including one or more motors using a
"communication link" and "current controller" to a processor
controlling the switches that control the phase currents of an
electric motor. The communication link is the bus type and can be
used to transfer a wide set of commands. In the following,
"synchronous position send command" or "read signal command" refers
to a command on the communication link used to call for information
on the position of the motor rotor to be used by the current
controller to calculate the suitable current for a coming phase
current switch period.
[0048] To permit the communication unit 1708 to work on a bus
shared by other similar encoders and other devices, for example in
a system for controlling several motors of a machine as will be
described hereinafter, the periodic times when a synchronous
position send command from the central processor will arrive at the
communication unit 1708 will not always be quite regularly
repeated, i.e. the time intervals between these commands are not
always equal to each other, even if this the normal state, i.e.
these commands are normally or mostly issued at regularly spaced
times. It is however essential that the position signals from the
photo receivers like 511 are sampled at known times having known
intervals therebetween and that the sampling is done reasonably
close to the times when the current controller starts its
calculation of the electrical currents required for a coming time
frame. This is arranged by a device or function that can identify a
synchronous position send command.
[0049] A hardware device can be part of the communication device
1708, which sends a triggering signal to a register 1710 that on
receiving the triggering signal directly copies the content of the
sampling pulse issuing device or counter 1709 when a synchronous
position send command is received by the communication device. An
other alternative among many others is to let an interrupt software
routine, triggered by data received by the communication device
1708 from the central processor, contain a short program that when
executed checks whether the received data is a synchronous position
send command and in that case reads and stores the current value
hold in the device or counter 1709 used to generate the
sample-and-hold commands.
[0050] This provides the encoder system with information of the
extent to which the timing inside the encoder system fits the
requirements of the current controller. Ideally, the timing of the
operations performed by the encoder should be such that the
position calculations made by the microprocessor 1707 are completed
just before the command to send the position is received from the
central processor. Whether this is true can be easily found by
recording in the microprocessor 1707 the time intervals between the
sample-and-hold commands or pulses sent to the sampling devices
1701-1704 or possibly the commands sent to the device 1709
commanding it to issue said pulses and the times of arrival of the
triggering signals indicating the reception of a synchronous
position send command from the central processor or possibly the
times of receiving the synchronous position send commands. In the
case where this interval differs too much from the ideal value, the
next time interval for issuing the sampling pulses can be adjusted
by a suitable amount, i.e. be made longer or shorter, the next
pulse thus being issued earlier or later, in the later case
delayed. The sampling times following the next time will however
again have the normal time interval from the preceding sampling
time. Only some sampling times should for a proper basic setting of
the normal time interval between the sampling times be changed,
such non-standard time intervals thus occurring rather seldom.
[0051] The information so obtained can be used in many different
ways.
[0052] 1. The encoder can send data on position and speed to the
central processor. The microprocessor of the encoder always uses
the relevant, currently valid interval between sample-and-hold
pulses when calculating the speed. The calculated speed value will
then be correct. This solution has the advantage that the central
processor does not have to do any extra processing to handle the
synchronisation.
[0053] 2. The encoder can also send the deviation from ideal to
actual time between sampling time and the time of command arrival.
The central processor can then issue a command with a new interval
between sampling pulses in order to keep the synchronisation. This
solution has the advantage that the motor computer might know that
the synchronous send command was delayed, for example due to
retransmission of commands. The central processor may have a device
similar to 1708-1709-1710 and can then compare the time delay from
its sending of a correct synchronous send command to the period of
its motor current control loop.
[0054] 3. The encoder will adjust the sampling interval to remain
in synchronisation. It will send position data and the sampling
interval used since the previous synchronous position send
command.
[0055] The time diagrams of FIGS. 3a-3d show the values hold by a
programmable cyclic count down counter such as 1709, see FIG. 3a,
the sample-and-hold pulses or commands as derived from said values,
see FIG. 3b, the times during which the microprocessor 1707
calculates the position from the four analog values stored in the
sample-and-hold circuits 1701-1704, see FIG. 3c, and the times when
a synchronous position send command is received by the
communication device 1708, see FIG. 3d. As appears from FIGS.
3a-3d, all but one of the synchronous position send commands arrive
shortly after that the position calculations have been finished.
The third synchronous position send command has been delayed.
[0056] The time diagrams of FIGS. 4a-4d show the values hold by a
programmable cyclic count down counter, see FIG. 4a, the
sample-and-hold pulses or commands derived from these values, see
FIG. 4b, the times during which the microprocessor 1707 is engaged
in calculating the position from the four analog values, see FIG.
4c, and the time when a synchronous position send command is
received by the communication unit 1708, see FIG. 4d. As is
observed in FIGS. 4a- 4c, all synchronous position send commands,
compare FIG. 4d, arrive at times having equidistant intervals
therebetween. This interval length is however not identical to the
internal, normally used sampling interval having a predetermined
length, e.g. as set by the central processor, and shown in the
three leftmost periods of the counter value as seen in FIG. 4a. The
time period from the time when the position calculations have been
finished to the synchronous position send command increases for
every period. The fourth period shown is therefore prolonged to
maintain the synchronisation. The period adjustment is exaggerated
for the sake of clarity.
[0057] FIG. 5 is a more complete block diagram of an encoder also
showing the handling of signals from low resolution tracks. As has
already been said, the encoding tracks of the encoder disc can also
comprise low resolution tracks. For reading the low resolution
track a third reading head 501, also indicated at 121 in FIG. 1c,
is provided detecting light issued by a suitable light emitter such
as 120 in FIG. 1c.
[0058] The low resolution tracks can be absolute tracks or comprise
two sinusoidal tracks 90 degrees apart with one or a few periods
per turn. Alternatively, two sinusoidal tracks 90 degrees apart
having the same number of periods as the pole pairs of the motor
rotor can be used. In FIG. 5 a third alternative is shown
comprising one track 516 with sinusoidal transparency having 5
periods per turn and with opto receivers 517 and 518 displaced 90
degrees out of phase, one period being equal to 360/5=72 degrees
and 90 signal degrees being equal to 90/360*72=18 mechanical
degrees. An electric motor having five pole pairs will have five
complete commutation periods per turn of the rotor. A given reading
of the position according to the five period track 516 will
therefore directly give the information necessary for the
commutation of the motor. If the encoder is assembled on a motor in
a motor production line, the phase difference between the signals
from the low resolution track and the emf from the motor can be
detected during the test of the motor/encoder combination. The
observed difference can be stored in the memory of the encoder
microprocessor. This eliminates the need to angularly adjust the
encoder to the motor stator by moving the encoder stator relative
to the motor and would make the elongated holes shown at 112 in
FIG. 1b unnecessary.
[0059] One or several reference pulses 521 can be added. They can
be read by a photo receiver 519 and an amplifier 520. In the case
of a low resolution track giving two sinusoidal signals 90 degrees
apart with one or a few periods per turn, several reference pulses
can be used. They must be so far apart that the low resolution
sinusoidal signals can be used to identify one reference pulse from
all other reference pulses even during unfavourable conditions in
age, supply voltage, temperature, etc. The reference pulses 521
must be so short that each reference pulse can uniquely identify
one and only one of the high resolution channel periods of channel
515. The six reference pulses like 521 shown in FIG. 5 can be
easily identified from each other using signals from the track 516
since the positions of the six reference pulses will be 60 optical
degrees apart when measured by the five period track 516.
[0060] The third reading head 501 comprises three photo receivers
517, 518, 519 having amplifiers 502, 503 and 520 connected to
provide their output signals to the multiplexer 1705.
[0061] The encoder illustrated in FIG. 5 is suitable for the use of
a low power standby mode as described in the cited U.S. Pat. No.
6,084,234. A low power standby system requires a low resolution
track. During high power mode, the high resolution track 515 is
used for detecting the angular position. The low resolution track
516 is used for coarse positioning during low power mode and for
commutation during the initial stages of high power mode. During
low power mode, only the microprocessor 1707, the low resolution
track optical receivers 517, 518, their amplifiers 502, 503, the
multiplexer 1705 and the analog-to-digital converter 1706 must be
active, and the microprocessor can possibly switch to a lower clock
frequency. As described in the cited U.S. Pat. No. 6,084,234 the
light source for reading the low resolution encoder channel can be
switched on only periodically, which permits a continues recording
of possible movements of the motor shaft during low power
operation.
[0062] Battery backup can be provided using two power supply lines,
one of which has battery backup, see FIG. 9 as will be described
hereinafter. To reduce cabling, one power line can used for both
normal and battery supported power. In FIG. 5 means 508 to detect
when a low power mode should be entered are shown. The data
transmission lines 509 of a communication bus are through a
receiver/transmitter buffer 507 connected to the communication
device 1708 and are also connected to the multiplexer 1705 through
two respective RC-networks 508. Assuming that the data transmission
over the bus lines 509 uses signals according to the protocol
RS485, at least one of the multiplexer terminals connected to the
two RC-networks 508 will sense a voltage far above ground level.
During standby mode, the central processor that normally drives the
electric motor to which the encoder is attached is disabled and the
lines can be kept at ground level using a pull down resistor, not
shown. The two multiplexer inputs connected to the two RC-networks
508 will then sense a voltage essentially at ground level,
indicating to the microprocessor 1707 that stand-by mode should be
selected.
[0063] To track the high resolution position data, comparators 504
and 505 compare the level of each signal to a reference level
symmetric between the positive and negative peak values of the
sinusoidal signals and thereby convert the analog sinusoidal
signals to conventional digital signals which in the conventional
manner are fed to an up-down counter 506. This counter can
advantageously have a capture register that stores the counter
value at the same time as the sample-and-hold circuits are
triggered; alternatively, the counter can be reset at the same
time, thus only recording changes compared to the previous sampling
command.
[0064] To get the correct position, data in the counter 506
representing quarters of a period of the primary signal data are to
be combined with high resolution data obtained by processing the
analog values captured by the sample-and-hold circuits 1701-1704,
which give data on the fraction within one period of the primary
signal data.
[0065] In FIG. 6 the data transfer in a system comprising two
encoders A, B using a common cable with separate command and
response channels with low data rates is illustrated by some
waveform diagrams. The waveforms shown in FIG. 6 illustrate a
system with a relatively low data rate. This can be advantageous to
permit long cables with low shielding requirements in a high noise
environment such as a welding robot. To reduce the amount of data,
the data are preferably sent as differential data, i.e. the
position data are sent as the difference since the previous
sampling and the sampling interval is sent as the deviation from
the sampling period initially agreed upon. For a resolution of 23
bits per turn, a maximum speed of 6000 rpm and a sampling interval
of 250 microseconds, the value of the position difference between
two successive sampling intervals will require 19 bits. If another
two bits are added to permit three sampling intervals of for
example 249, 250 or 251 microseconds and another three bits are
allocated to identify the responding encoder, a total of 24 bits is
required for a regulator loop synchronous position reply, i.e. the
response signal including a position difference sent from the
microprocessor of an encoder.
[0066] In FIG. 6, the signal 601a illustrates the value in the down
counter 1709 in encoder A. Analog position data is sampled on the
rising flank of the signal 601a and the data obtained is processed
during the time interval 602a directly following the sampling time.
At 603a the transmission of the synchronous position request
command from the central processor is indicated. The end of the
stop bit from this single byte command generates an interrupt at
time 606a. This interrupt will identify the received command byte
in the communication device 1708 as a synchronous position send
command for encoder A. The encoder A then enables the encoder A
respond channel three state driver such as 507 during a period 607a
It will also read the value of its down counter value 601a to check
whether the down counter is in synchronisation with the central
processor. The position difference will be sent as a three byte
word 604a, which is driven out on the reply channel as indicated at
605a. In the following time slot used by the central processor, the
same command is sent to encoder B.
[0067] If the absolute position range is set to 64 bits, this
permits a 23 bit resolution of one turn within a maximum of plus or
minus 6000 turns per minute during 348 years. At a modest 1 Mbaud
data rate for the communication link with six time slots during a
250 microsecond cycle, five encoders can transfer their 24 bit
differential position data during 5 time slots, still permitting
the sixth slot to transfer absolute position, motor temperature and
encoder temperature for all five encoders at more than 100 times
per second.
[0068] The third time slot in FIG. 6 is used for auxiliary
information, for example to read a part of the absolute position or
temperature data from a selected encoder. In the third slot shown
in FIG. 6, the central processor sends a parameter address. It
consists of a command byte, all of which has one bit, pointed at by
arrow 608, low, while the two address bytes such as 609 has the
corresponding bit high. This permits an address of 14 bits to be
sent to one of the encoders as a data pointer that can be used by a
later parameter read or write command. For example, a certain value
of this address may indicate the temperature sensor. To read the
temperature of the motor connected to encoder A, the third slot
would first contain a command to motor a to set its data pointer to
point at the temperature reading, and the following third slot
would contain a command to encoder A to send the data indicated by
the previously sent data pointer.
[0069] FIG. 7 is a diagram of a system, e.g. a robot control
system, having three motors such as 701, each one provided with an
encoder 702, three motor current controllers 703, one for each
motor, a common power supply 713 and a common central processor 704
that communicates with the encoders over a common bus cable 705,
706, 707, 708. Commercial installations will depend on the
connector structure supplied by the supplier of the encoders in the
system and will therefore use the same connection structure on all
motors. The bus cable installation is illustrated to be provided
with connectors to the motors according to two alternative
embodiments. For the motor 701, the cabling consists of separate
parts comprising two connectors like 709 for one encoder. These two
connectors are interconnected inside the encoder, and the different
cable parts like 705 and 706 are therefore mechanically separated
but galvanically interconnected. This has advantages in
installations where the motor connectors must have a high
protection or shielding class. The connection for the motor 711 has
a single connector provided with two cable ends connected to each
other in the connector 710. This saves connectors but has the
disadvantage that the whole cable must be assembled as one piece
having several connectors.
[0070] Each current controller like 703 is connected to one motor
like 701 over a separate current cable like 714.
[0071] The central processor 704 issues the synchronous position
send commands over the communication link cable 705, 706, 707, 708
and receives the position replies from the encoders on the same
cable. This information is processed and used to issue current
commands to the current controllers, for example to permit a robot
hand or a machine tool to follow a required path. The means
employed to issue the current commands from the central processor
704 to the current controllers like 703 are illustrated as a bus
712. There are several standards for such control structures, such
as SERCOS.
[0072] The communication link cable 705, 706, . . . may also be
connected to each current controller like 703 so that each current
controller gets direct information on the position of its
associated motor. In FIG. 7 this is shown as cables 715, 716, 717.
This is suitable when the controllers are controlling movements
with lower requirements for synchronism. In such cases the central
processor 704 may issue more general commands to each current
controller 703. Many present current controllers like 703 have a
processing capacity required to execute commands for complete
movements. If the communication link cable 705, 706, . . . is
connected to both the central processor 704 and the motor current
controllers like 703, the motor controllers can use the replies to
the synchronous position commands to control the movement of its
associated motor whereas the system central processor can use the
same information to monitor the movements already commanded and to
decide when it is appropriate to issue commands for other actions.
The commands between the central processor and the motor current
controllers can in such cases also be sent over the communication
link cable 705, 706, . . . This would require a higher bandwidth
than that used for case illustrated in the diagram of FIG. 6.
[0073] The current controllers like 703 can use the synchronous
position send commands to synchronise their current loops in the
same way as that described above for the encoders. In this way, the
synchronous position send commands from the system central
processor 704 can be used to synchronise both the encoder 702 and
the current controller 703 so that recent position data from the
encoder 702 will be available when setting the current flowing from
current controller 703 for the next time slot.
[0074] Auxiliary sensors may be connected to the bus cable 705, for
example using the uncommitted end 708.
[0075] Although the timing diagrams of FIGS. 3a-4d illustrate a
case where the sample-and-hold circuits 1701-1704 are triggered
only once for every synchronous position send command, it is often
advantageous to trigger the sample-and-hold devices several times
during one motor current switch period. This permits calculation of
speed information on more recent data. Another use of a more
frequent sampling is the possibility to calculate the speed using
the same flank of the same primary signal. The sample-and-hold
timing generator can then issue signals as described with reference
to FIGS. 3a-4d synchronous to the synchronous position send
commands, but also another set of signals synchronous to the
frequency of one of the primary signals. At high speeds, this
permits the speed to be calculated using approximately the same
part of the same steep flank of the same one primary signal to
calculate the speed. In this way, the effects of signal distortion
and phase lag at high frequencies of the primary signal can be
cancelled out.
[0076] The embodiments shown above are based on optical primary
signals using variations in opacity between different sectors of an
optical encoder disc. The basic principles of synchronisation can
also be applied to primary signal sources based on variations in
electrical conductivity, permeability or magnetic coercitivity.
[0077] FIG. 8 is a sectional view showing an assembly of a
non-optical encoder system. A modulator disc 801 is assembled to a
the shaft 802 of an electric motor using a hub 803 that presses
against the rear bearing 804 of the motor in a way similar to that
described for item 104 of FIG. 1 in the cited International patent
application WO 99/54683. This gives a very stiff connection between
the motor rotating shaft and the modulator disc 801.
[0078] The magnetic field from a set of primary coils in the coil
board 805 is affected by metallized sectors on the modulator disc
801 so that the sign and amplitude of voltages induced in two
reading coils in the coil board 805 vary with the position of the
modulator disc in a conventional prior art manner. For the purpose
of commutation of the electric motor, the number of periods per
turn of this signal should be the same as the number of motor poles
or an integer fraction thereof, such as 1, 2 or 4 for an 8-pole
motor. In order to obtain higher resolutions, a second coil set
with for example 16 times more periods per turn than that of the
commutation coil set can be added.
[0079] The coil board 805 is radially centred relative to the motor
rear end shield 808 by a flange 810 in the motor rear shield 808
that fits the outside of the coil board 805 in a way similar to the
assembly of resolvers. The coil board 805 is pressed towards a
plane in the motor rear shield by screws 811.
[0080] Thermally sensitive devices shown as 818 are assembled on a
PC board 817 and connected to the coil board 805 through a
connector 816. The board 817 can then be assembled on a heat sink
819 that can be used as a rear end cover of the motor. The assembly
816-818 is shown before mounting. Connection to the outside world
can be done through cables from the coil board 805 through the
channel 820 in the motor stator.
[0081] FIG. 9 is a block diagram of the signal processing part of
the transducer. The modulator disc 801 has in the conventional
manner a low resolution track 1817 with for example one or as shown
five periods per full turn. The magnetic field from a set of
primary coils 1814 in the coil board 805 will be affected by
metallized sectors 1817 on the modulator disc 801 so that the sign
and amplitude of voltages induced in two reading coils 1815 and
1816 in the coil board 805 vary with the position of the modulator
disc in the conventional way. The ground symbols in FIG. 5 denotes
signal base voltage, which in a 5 Volt single supply system
normally would be 2.5 or 2.048 Volts. For clarity, the coils are
drawn as if they covered only a small section of arc. They should
preferably cover the whole circumference, i.e. a full turn.
[0082] The modulator disc 801 may also have another higher
resolution track 1818 with for example 50 periods per turn as
shown. The magnetic field from another set of primary coils 1813 in
the coil board 805 will be affected by metallized sectors 1818 on
the modulator disc 801 so that the sign and amplitude of voltages
induced in two reading coils 1811 and 1812 in the coil board 805
vary with the position of the modulator disc in the conventional
way.
[0083] The signals from the high resolution receivers are
simultaneously sampled by sample- and-hold devices 1801-1802. The
values are then converted to digital form using a time multiplexer
1805 and an analog-to-digital converter 1806. The values obtained
are converted to angular position values by the microprocessor
1807, possibly using corrections to handle non-sinusoidal shapes of
the signals output from the coils and varying amplitude for
different angles of a full turn of the modulator disc as disclosed
in the cited U.S. Pat. No. 5,998,783.
[0084] The signal processing is made in the same way as described
above for optical encoders.
[0085] The tasks of the encoder microprocessor 1707 can be
summarised against the background of the partial explanations given
in the texts accompanying the figures above.
[0086] At the initial assembly of the encoder on a motor, a test
run can be made. In such a test run, the emf. from the motor
windings, the real angular position as given by a precision high
resolution encoder in the calibration station and the signals
obtained from the various tracks can be recorded. This information
can be used to get parameters for a linearisation of the high
resolution tracks as described in U.S. Pat. No. 6,191,415. The same
calibration setup can be used to get a relation between the high
resolution track periods and the low resolution channel and between
these two and reference pulses.
[0087] When the motor is assembled in a machine and the power is
switched on, the microprocessor may receive commands to define the
conditions to be used in the system. This may include the basic
communication or commanding principle, for example a selection
between the three basic principles given as alternatives 1. to 3.
in the description with reference to FIG. 3 above. If alternative
3. is given, the standard sampling period, for example, 250 .mu.s,
and adjustment periods, for example 249 and 251 .mu.s, can be
defined.
[0088] At the first power up, the microprocessor can read the high
and slow resolution tracks 515 and 516 to get the approximate
position. It must also initiate the counter 506 to initially be in
synchronism with the first synchronous position send command
received.
[0089] The position read from track 516 will be sufficient to
obtain the correct commutation of the motor. The position read from
track 515 will permit an excellent speed control of the motor as
the speed will be estimated from the movement of the high
resolution track between two sampling commands from the counter
506. The resolution of track 516 may not be high enough to uniquely
identify the period from track 515. By turning the motor slowly a
reference pulse such as 521 will be found. This will permit a
unique identification of the period of track 515. By slowly moving
to a mechanically fixed end position, the relation between data
from the encoder and positions of the machine can be
established.
[0090] When the machine is switched off, the encoders will enter a
low power mode. The microprocessor can then periodically enable the
light source for the low resolution track 516 and can thereby
detect movements that eventually were forced on the machine during
power off. When the machine is switched on again, the
microprocessor can use the data from the low resolution track and
its recorded history to get the coarse position, for example
212.42+-0.08 whole low track periods from the mechanical end stop
established during the first power on. In this stage the encoder
can give a high resolution position that however can be wrong with
one a few periods of the high resolution channel.
[0091] In the case shown in FIG. 6, speed data are independent of
the reference pulse. The central processor can therefore command
the motors to move slowly for a fraction of a turn and then stop.
As soon as the encoder microprocessor detects a reference pulse,
the encoder microprocessor knows the correct position. The exact
position can then be read by the central processor using the slower
communication of the third slot in FIG. 6.
[0092] During a normal period of counter 1709, the microprocessor
will run background tasks such as temperature control until an
interrupt from the counter 1709 occurs. The microprocessor 1707
will then read data from counter 506 to get the approximate angular
position. It will also read the receivers 511-514 using the
multiplexer 1705 and the analog-to-digital converter 1706. The
angular position and analog receiver data will be entered into an
algorithm using calibration constants obtained during the factory
calibration of the motor/encoder combination to get the angular
position of the disc 101 relative to heads 106a and 106b. By taking
the average of these and adding that to the counter 506, the
angular position is obtained. By subtracting the previous position
the movement during the preceding time slot is obtained. The data
for the next reply to a synchronous position reply command as given
in FIG. 6 can thereby be made ready by including information on the
counter period recently used by counter 1709.
[0093] The microprocessor will get an interrupt from the counter
506 communication receiver 1708 when data arrives from the central
processor. It then checks if the input data refers to its own
encoder identification. If the data received was a synchronous
position send command, it will send the reply already prepared. It
also reads the content or capture register 1710 to check whether it
still is in synchronism with the central processor and sets the
data to be used for the following period of counter 1709
accordingly.
[0094] While specific embodiments of the invention have been
illustrated and described herein, it is realised that numerous
additional advantages, modifications and changes will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details,
representative devices and illustrated examples shown and described
herein. Accordingly, various modifications may be made without
departing from the spirit or scope of the general inventive concept
as defined by the appended claims and their equivalents. It is
there- fore to be understood that the appended claims are intended
to cover all such modifications and changes as fall within a true
spirit and scope of the invention.
* * * * *