U.S. patent application number 09/904428 was filed with the patent office on 2003-01-16 for constant cfm control algorithm for an air moving system utilizing a centrifugal blower driven by an induction motor.
Invention is credited to Eichorn, Ronald L..
Application Number | 20030011342 09/904428 |
Document ID | / |
Family ID | 25419145 |
Filed Date | 2003-01-16 |
United States Patent
Application |
20030011342 |
Kind Code |
A1 |
Eichorn, Ronald L. |
January 16, 2003 |
CONSTANT CFM CONTROL ALGORITHM FOR AN AIR MOVING SYSTEM UTILIZING A
CENTRIFUGAL BLOWER DRIVEN BY AN INDUCTION MOTOR
Abstract
A method and apparatus for achieving constant air flow rate
economically utilizes an induction blower motor with control
settings determining the motor excitation voltage. A memory stores,
for a variety of motor speeds, a graph of the speeds plotted
against a proportionality constant of flow rate to fan speed for
the selected motor system on one axis and motor control settings on
the other axis. The only monitoring necessary to achieve the
constant flow rate is thus the sensing of fan rotational speed. The
measured fan speed is compared against the proportionality constant
needed for the selected constant air flow rate and a motor
excitation voltage is derived to achieve that proportionality
constant. When the system load changes significantly, thereby
causing significant fan speed change, a cascaded control loop is
used whereby the speed changed induced by each control voltage
adjustment is monitored until the desired constant flow rate is
again attained at the new load level.
Inventors: |
Eichorn, Ronald L.; (Soquel,
CA) |
Correspondence
Address: |
Roland W. Norris
Pauley Petersen Kinne & Fejer
Suite 365
2800 West Higgins Road
Hoffman Estates
IL
60195
US
|
Family ID: |
25419145 |
Appl. No.: |
09/904428 |
Filed: |
July 12, 2001 |
Current U.S.
Class: |
318/727 |
Current CPC
Class: |
Y10S 388/929 20130101;
Y10S 388/93 20130101; G05D 7/0676 20130101; Y10S 236/09
20130101 |
Class at
Publication: |
318/727 |
International
Class: |
H02P 001/24; H02P
001/42; H02P 003/18; H02P 005/28; H02P 007/36 |
Claims
I claim:
1. A method of achieving a selected constant mass air flow rate for
a blower motor of the induction type exhibiting nonlinearity
between motor rotational speed, motor control voltage, and air flow
rate, comprising: a) controlling motor speed through a motor
controller by varying an excitation voltage to the motor; b)
determining proportionality constants of flow rate to motor speed
for various motor speeds over the operating range of the motor; c)
providing a graph of curves for the various motor speeds plotted
against the proportionality constants and the excitation voltages
in a memory accessible by the motor controller; d) setting the
controller at a first excitation voltage; e) measuring a rotational
speed of the motor; f) deriving the proportionality constant for
the measured motor speed to determine a fan speed corresponding to
the selected air flow rate.
2. The method of achieving a constant mass air flow rate for a
blower motor of the induction type according to claim 1 further
comprising: adjusting the excitation voltage according to the
determined fan speed to achieve the desired air flow.
3. The method of achieving a constant mass air flow rate for a
blower motor of the induction type according to claim 2 further
comprising: deriving a second proportionality constant according to
a speed of the motor at the adjusted excitation voltage.
4. A method of achieving a constant mass air flow rate for a blower
motor of the induction type exhibiting nonlinearity between motor
rotational speed, motor control voltage, and air flow rate,
comprising: a) controlling motor speed through a motor controller
by varying an excitation voltage to the motor at selected control
settings representative of varied excitation voltages; b)
determining proportionality constants of flow rate to motor speed
for various motor speed curves over an operating range of the
motor; c) providing a graph of motor speed curves plotted against
the proportionality constants of step b) and control settings of
step a) in a memory accessible by the motor controller; d) setting
the motor controller at a first setting for a selected constant
mass air flow rate; e) measuring a rotational speed of the motor;
f) deriving a proportionality constant for the measured motor speed
to determine a control setting corresponding to the selected air
flow; g) adjusting the RPM setting according to the proportionality
constant of step f); h) adjusting the control setting to increase
or decrease motor speed to achieve the desired RPM setting; i)
deriving a second proportionality constant when the measured speed
of the motor has achieved the desired RPM setting; and j)
iteratively adjusting the rotational speed of the motor by
repeating steps e)-g).
5. The method of achieving a constant mass air flow rate for a
blower motor of the induction type of claim 4 further, comprising:
checking the proportionality constant for the measured motor speed
by interpolating between the curves to determine a control setting
corresponding to the selected air flow.
6. Apparatus for achieving a constant mass air flow rate for a
blower motor of the induction type exhibiting nonlinearity between
motor rotational speed, motor control voltage, and air flow rate,
comprising: a) a motor controller having a microprocessor for
receiving a selected constant CFM rate value; b) a memory having
stored therein motor speed curves plotted against proportionality
constants of fan speed to air flow on a first axis and motor
excitation voltages on a second axis, the memory operably connected
to the microprocessor; c) means for controlling an excitation
voltage to the motor, the means for controlling being operably
connected to the microprocessor; and d) a tachometer for sensing
the speed of the motor, the tachometer being operably connected to
the microprocessor.
7. Apparatus for achieving a constant mass air flow rate for a
blower motor of the induction type according to claim 6 further
comprising: means for inputting variety of constant CFM rates.
8. Apparatus for achieving a constant mass air flow rate for a
blower motor of the induction type according to claim 6 further
comprising: means for sensing changes in a line voltage and
adjusting excitation to the motor based on the changes in line
voltage.
9. Apparatus for achieving a constant mass air flow rate for a
blower motor of the induction type according to claim 6 further
comprising: means for sensing temperature changes in the motor and
adjusting excitation to the motor based on the changes in
temperature.
10. Apparatus for achieving a constant mass air flow rate for a
blower motor of the induction type according to claim 6 wherein the
memory is a look up table.
11. Apparatus for achieving a constant mass air flow rate for a
blower motor comprising: a) means for inputting a selected constant
CFM rate to a motor controller; b) a motor controller having a
microprocessor for receiving the input constant CFM rate; c) memory
means having stored therein motor speed curves plotted against
proportionality constants on a first axis and motor excitation
voltages on a second axis, the memory means operably connected to
the microprocessor; d) means for controlling the excitation voltage
to the motor, the means for controlling operably connected to the
microprocessor; and e) means for sensing a speed of the motor, the
means for sensing a speed operably connected to the
microprocessor.
12. The apparatus for achieving a constant mass air flow rate for a
blower motor according to claim 11 further comprising: a motor of
the induction type exhibiting nonlinearity between motor rotational
speed, motor control voltage, and air flow rate, the motor being
operably connected to the tachometer and the motor controller.
13. A method for achieving a constant mass air flow rate for an
induction blower motor according to claim 11 further comprising:
performing an algorithm having the steps in order of: a) select air
flow rate value; b) set motor speed value; c) change control motor
voltage until motor speed value is reached; d) derive system
constant for current control voltage and current motor speed value;
e) divide air flow rate value by derived system constant to derive
new motor speed value; and f) iterate steps c)-e) until motor speed
value and system constant balance at selected air flow rate
value.
14. A method for achieving a constant mass air flow rate for an
induction blower motor according to claim 11 further comprising:
performing an algorithm having the steps in order of: a) enter air
flow rate value; b) set first motor speed value; c) step motor
control voltage until first motor speed value is measured; d)
derive first system constant for current motor control voltage and
first motor speed value; e) divide air flow rate value by first
system constant to derive second motor speed value; f) subtract
second motor speed value-first motor speed value to derive third
motor speed value; g) divide third motor speed value by x to get
fourth motor speed value; h) step motor control voltage up until
fourth motor speed value is measured; i) derive second system
constant for current motor control voltage and fourth motor speed
value; j) divide air flow rate value by second system constant to
derive fifth motor speed value; and k) iteratively step up motor
control voltage, measure motor speed and step through motor speed
values until motor speed value and system constant balance at air
flow rate value.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to controls for
induction motors used in constant mass air flow, sometimes also
called constant CFM (cubic feet per minute), applications.
[0003] 2. Discussion of the Related Art
[0004] In modern heating systems, it is sometimes desirable to
regulate the amount of air flow through the heat exchanger to a
constant CFM volume. References addressing air flow control by a
motor driven blower system are known to exist. The reader is
referred to U.S. Pat. Nos. 5,736,823, to Norby et al.; and
5,202,951, to Doyle; for examples of the known art.
[0005] Generally the constant mass air flow blower controls of the
known art may require fan speed sensing, motor current sensing,
torque calculations, or some combination of the above which may
make the systems expensive in terms of the sensing apparatus,
mathematical processing power and the like. Such control systems
may also incur time delays during control calculation.
[0006] Known examples of control systems in the art may otherwise
rely on an assumption of a linear relationship between fan, or
motor, speed and mass air flow. While a degree of linearity may be
achieved in certain systems with expensive variable speed
controllers such as a pulse width modulated (PWM) controller, the
attendant cost may be prohibitive. Also, for readily available and
inexpensive induction motor driven systems the above assumption of
linearity does not hold true, and systems based on this assumption
may not yield adequate control stability or performance with or
without more expensive electronic motor controllers, resulting in
operation which is not optimally smooth or quiet.
[0007] Therefore to solve the shortcomings of the known art, there
is needed an inexpensive motor control system for induction motors
utilized in constant mass air flow systems.
SUMMARY OF THE INVENTION
[0008] The present invention provides an inexpensive and reliable
constant mass air flow controller for induction motor driven blower
systems. The present system requires monitoring only of the blower,
i.e. fan or motor, speed in conjunction with a motor controller
which does not assume linearity of speed, motor control voltage and
flow rate.
[0009] Instead, according to the present invention, the controller
is provided with a look up table covering the operating range of
the motor, which is accessible by the motor controller processor.
The look up table contains a family of fairly straight curves for
several motor speeds plotted against proportionality constants of
air flow rate to fan speed on one axis and the control voltage
settings on the other axis.
[0010] In order to select the proper motor control voltage setting,
the motor controller compares the measured speed of the motor, or
fan, of the blower (hereinafter referred to as just "motor" or
"fan" synonymously) against the control voltage setting to derive
the proportionality constant known to give the proper mass air
flow. The controller then derives the proper motor speed, or "RPM
setpoint", to achieve the desired mass air flow. The excitation
voltage is then increased or decreased to achieve the proper motor
speed. The control voltage setting will sometimes also be referred
to as a "control point" or "control setting" since voltages may not
be directly represented under the scheme of excitation used to
control the motor, as will be understood by the person of ordinary
skill in the art.
[0011] A cascaded control loop is used for the motor controller of
the invention to attain a constant mass air flow. The outer loop of
the cascade control has an input of the selected constant CFM rate
and an output of the RPM setpoint to the inner control loop. The
inner control loop has an input of the RPM setpoint and outputs to
the outer loop the control voltage setting when the RPM setpoint is
achieved.
[0012] The outer loop uses the measurement of the motor speed and
reported control voltage at that speed to derive a proportionality
constant of the system operation for that motor speed. The
proportionality constant contains the air flow information
necessary to select the next RPM setpoint for operation of the
motor to achieve the selected constant CFM mass air flow. If
necessary, a new RPM setpoint is selected, and the control voltage
adjusted, to increase or decrease fan speed to achieve the desired
air flow; with a rechecking of the proportionality constant for the
new fan speed attained under the given system load. Iterative
adjustment of the RPM setpoint is performed until the desired mass
air flow is reached.
[0013] The system relies on the fact that for a constant system
load, flow rate is proportional to fan speed. Because the system
load for a blower motor generally remains constant and changes by a
significant amount only occasionally, the system need only monitor
the motor rotational speed, which is a function of the system load,
and check the motor speed and voltage control point to derive the
proportionality constant. The selected CFM value is then divided by
the proportionality constant and used to select the next RPM
setpoint for the motor and the control voltage is changed
accordingly. During most periods of use little adjustment is
needed, so the motor controller may monitor speed changes at a long
time constant, or may operate with a lower allowable system
adjustment, or "gain", to make sure small transients in motor speed
do not affect system stability. When the system load changes
significantly, thereby causing significant fan speed change or
control voltage adjustments, a short time constant for the control
loop is used whereby the RPM setpoint and control voltage
adjustment occur more frequently until the desired constant mass
air flow rate is again attained at the new load level under a new
control voltage.
[0014] By using the cascade control loop algorithm of the present
invention minimal hardware is required since the cascade control is
merely a software implementation. Also, direct control of the motor
speed removes speed variations due to drifts in motor temperature,
line voltage, air temperature, etc. The lookup table storage for
motor/fan characteristics of the present invention promotes
efficiency of operation since the family of control curves tends to
be close to a set of straight lines. The addition of the adaptive
control in the outer loop of the control for the present invention
will provide very stable motor control that is responsive to system
load variations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A preferred embodiment of the invention is described below
along with the appended drawing figures in which:
[0016] FIG. 1 is a schematic representation of apparatus suitable
for practicing certain embodiments of the present invention.
[0017] FIG. 2 is a graph of motor and fan speed/torque
characteristics of an induction motor blower system which may be
selected for use in a system according to the present invention
[0018] FIG. 3 is a graph of CFM air flow characteristics for the
selected induction motor system plotted against motor speed and
control voltages for the motor.
[0019] FIG. 4 is a graph similar to the graph of FIG. 2 with
various system load lines also plotted thereon.
[0020] FIG. 5 is graph of CFM versus voltage control setting used
for explanatory purposes.
[0021] FIG. 6 is graph of motor speed versus voltage control
setting as seen by the inner control loop of the cascade control
system of the present invention.
[0022] FIG. 7 is a graph of motor speed curves for the induction
motor plotted against the control voltages and the air flow
rate-to-fan speed proportionality constants of the selected
system.
[0023] FIG. 8 is a graph of temperature effect on air flow at 400
RPM for the induction motor.
[0024] FIG. 9 is a graph of temperature effect on air flow at 1050
RPM for the induction motor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] In modern heating systems, it is sometimes desirable to
regulate the amount of air flow through the heat exchanger to a
constant CFM volume. FIG. 1 depicts one embodiment of the present
invention which is suitable to achieve this purpose economically by
utilizing inexpensive components, such as an induction motor with
triac speed control. As illustrated, the basic components of the
present constant CFM system 11 include a flow selector 13, a motor
controller 15, a motor 17, a speed sensor, or tachometer, 19 and a
fan 21.
[0026] The present control method requires only a sensing of the
motor speed in conjunction with the algorithm of its cascade
control scheme. Low cost apparatus known in the art are readily
available to accomplish this speed sensing purpose. It is important
to note that since air flow is directly proportional to fan speed,
the system accuracy will depend on how accurately motor speed is
measured.
[0027] The air flow control depends on knowing the speed-torque
characteristics of the selected motor for any particular control
voltage supplied to the motor. The fan characteristics of torque
versus speed for constant air flow can be can be determined from
the manufacturer's performance curves (FIG. 2). The points where
the very flat fan curves of constant air flow intersect the motor
speed/torque curves will yield a set of curves for motor control
voltage versus speed for constant air flow (FIG. 3). It will be
appreciated that airflow is determined by motor speed and motor
control voltage. Both of these quantities are accurately known,
from the tachometer and the fact that the motor controller 15
generates the control voltage. The system load will effect the
overall speed of the motor under a given control voltage.
[0028] The examples herein were derived using analytical
techniques. Motor characteristics were generated from an equivalent
circuit model using Mathcad.TM.. software, from MathSoft, Inc. of
Cambridge, Mass. The motor modeling closely simulated a standard
three-quarter horsepower 1075 RPM induction motor. The triac
control illustrated in FIG. 1 may yield slightly different results
compared to the linear voltage control model utilized in the
analysis set forth herein. Speed/torque characteristics were
generated by modeling a varying AC voltage applied to the main
winding of the motor. Fan characteristic were taken from graphical
data supplied by the manufacturer for a Lau Industries, Inc., of
Dayton Ohio, Model DD10-10A centrifugal fan. It was assumed that
the standard ideal fan laws apply within the intended operating
conditions of this device.
[0029] FIG. 2 shows both the motor and the fan speed/torque
characteristics on the same plot. The control input variable for
variable speed motor control, used by way of example in the
illustrated embodiment of FIG. 1, is the ratio of the main winding
29 excitation voltage, as controlled by a triac 31, to the
auxiliary winding 33 voltage excitation. The control voltage input
variable varies in value from 0 to 1. The auxiliary winding voltage
is a constant line voltage 115 V. Thus, a control input of 0.5
means that the auxiliary is excited with 115 V and the main winding
is excited with 57.5V. Motor excitation curves between input
control points of 1.0 and 0.25 are labeled with even reference
numbers 40-52 with specific values as seen on the right of the
graph of FIG. 2. Blower CFM curves are labeled with odd reference
numbers 41-55 for specific values between 500 CFM and 2500 CFM as
seen on the right of the graph of FIG. 2.
[0030] The fan curves seen in FIG. 3, which are a family of curves
representing constant airflow in CFM, were generated by first
graphically extracting the data from the manufacturers performance
curves. Using Excel.TM. software from Microsoft, a curve fit was
obtained for each constant air flow data set using a second order
equation. This provided for smoothing of the data errors produced
by taking the data from a graph, and additionally provided a means
by which the fan curves could be entered in to Mathcad.TM.. With
these curves in Mathcad.TM., the intersections of motor and fan
characteristics were determined. This results in the set of curves
seen in FIG. 3, which show the control voltage, or control point,
(X axis) versus speed characteristics, or RPM (Y axis) for constant
air flow in the system between values of 500 CFM to 2500 CFM,
labeled with odd reference numbers 57-69 for the specific values
seen on the right of the graph of FIG. 3.
[0031] A visualization of one control strategy thus becomes
conceptually simple. One could provide a means by which the curves
of FIG. 3 can be stored and accessed by the motor controller 15.
Upon receiving an input variable, such as an operator's input
through flow selector 13, representing desired air flow, the motor
controller 15 must vary the control voltage, as represented by
control line 27 until the RPM, i.e., motor speed, measurement and
control voltage converge to the desired air flow. However, there
are several issues involved in this control strategy. Among them
are calculation time and memory requirements, the control
stability, and errors due to power line variation and
temperature.
[0032] Referring again to FIG. 2, it is observed that in the lower
RPM ranges, i.e. less than about 850 RPM, the curves of constant
air flow are nearly parallel to the motor torque curves. One might
think that control stability could be compromised in this operating
region since the curves do not intersect at a well defined point,
such as they do beyond the motor torque peak. However, this
operating region is not so problematic when one considers that as
the control voltage to the motor is varied the speed will vary
according to the system load line, whereas the torque varies with
the square of speed.
[0033] FIG. 4 shows system load lines added to the plot of FIG. 2
(minus the curve 47 for 1500 CFM). The first load line 35 shows a
system with a heavy load, i.e. very little restriction in the
system resulting in a high flow rate. The second load line 37 is a
light load, with a restricted flow. The third load line 39 was
taken from the fan performance curves at zero static pressure. The
system cannot operate above and to the left of this line. It will
be observed that for a heavy load, the first load line 35 crosses
both the dotted constant air flow lines and the solid motor torque
curves at a reasonable angle. This is consistent with the known
experience that the speed of an Induction motor is most easily
controlled when it is heavily loaded. In FIG. 4, it may be further
observed that the second load line 37 of a lightly loaded system
closely follows the constant air flow curve of the 500CFM curve,
reference number 41, particularly above about 900 RPM. It is in
this area that control stability might be difficult to achieve
since the motor RPM can vary over a wide range without inducing a
corresponding change in the CFM output. Put another way, the
requirement of nearly constant CFM can be satisfied over a wide
motor speed range. But, although the CFM control might be adequate
over such a range, the possibility of rapidly varying motor speed
due to lack of fixed control points could be annoying.
[0034] The stability problem can be solved by considering the fan
laws, which state that, for a constant system load, flow rate is
directly proportional to fan speed. The fan laws thus tell us that
when the system load changes the fan speed will, or must, change to
maintain a constant CFM. A control implementation for an induction
motor with nonlinear behavior can thus make use of this law by
storing the proportionality constants of flow rate to fan speed
over the operating range of the selected fan/motor components, and
controlling the motor RPM according to the proportionality constant
for that speed, to achieve the selected quantity of mass air
flow.
[0035] Referencing FIGS. 5 and 6, the graphs therein help to
demonstrate one advantage of using the cascade control loop of the
present invention instead of a direct, one loop, CFM control. The
FIG. 5 plot graphs the CFM value on the Y axis versus control
voltage setting on the X axis for various system constants (K).
These lines could be used as a nonpreferred direct control loop
that would directly try to control the motor to an air flow
setpoint. For a heavily loaded system (K=3) this would not
represent a problem, as there is a linear response of air flow to
control setting. On the other hand, in a lightly loaded system,
represented by K=0.5, e.g. all the dampers closed, the line becomes
very flat. One would get just a little over 500 CFM for any control
setting between 0.5 and 1. The motor speed could conceivably drift
greatly with small system variations, e.g. motor temperature, air
temperature, etc. Such a control loop could satisfy the requirement
for CFM control, but motor speed variations might be very
annoying.
[0036] The FIG. 6 plot shows a graph of motor speed in RPM, plotted
on the Y axis, versus control voltage settings, plotted on the X
axis, for four different system constants (K). This is the
characteristic information seen by the "inner" cascade loop of the
present invention as further explained below. While FIG. 6 also
shows some "flattening" for the unloaded system (K=0.5), it is not
nearly so severe. It should also be noted that RPM on the Y axis is
a direct, accurate measurement of the actual system, and not
dependant on another system variable such as motor or air
temperature. Therefore, a speed control inner loop of the cascade
control will still have the ability to control speed reasonably
well, avoiding annoying speed variations.
[0037] FIG. 7 shows the curves for several given motor speeds
between 400 RPM and 1100 RPM, with specific values listed to the
right of the graph, plotted against the proportionality constants
(Y axis) and motor control inputs (X axis) determined for the
selected blower system. The maximum system constant for this fan is
3.3, as shown by the dotted line. This would represent a fan
sitting in open air with no restrictions, resulting in maximum
possible air flow, i.e. zero static pressure. Practically, this
motor/fan combination would not be used in a system where the
constant exceeded 3 because at full power the motor would likely be
overloaded and overheat.
[0038] Fortuitously, this family of system constant versus control
setting curves is a set of nearly straight lines Thus, use of
simple lookup tables, stored in the motor controller 15, e.g., in
ROM 23, can be used in combination with linear equations, if
necessary for interpolation, to derive an appropriate
proportionality constant for a selected constant mass air flow, and
will be easier and quicker to implement with a microprocessor 25
than the control voltage/speed curves of FIG. 3.
[0039] The system load in a heating system is typically constant,
with only occasional abrupt changes due to opening and closing of
dampers. Thus a cascade control loop can be used, with the outer
loop having an input of the selected CFM value; and the inner loop
having a speed control input from the outer loop. The cascade
control loop of the described embodiment has the inner loop being a
speed control loop, and the outer loop being the CFM control loop.
The outer loop supplies an RPM setpoint by deriving a system
constant from the currently known control voltage value and the
currently measured fan speed value. The selected CFM value is then
divided by the derived system constant to derive the new RPM set
point. The new RPM set point is then provided to the inner
loop.
[0040] The new fan speed value, or RPM set point, is then provided
to the inner control loop. The inner control loop steps the control
voltage value up or down and compares measured fan speed to the new
RPM set point. When the measured fan speed equals the new RPM set
point, the current control voltage value is reported back to the
CFM control loop, a new RPM set point is calculated, and the fan
speed/control voltage is again adjusted, and so on iteratively
until the system constant and the fan speed are in the proper
control range.
[0041] By way of an example, suppose an air flow of 1200 CFM is
required from the blower system. The system is turned on from a
full stop. When the system is turned on, the CFM control loop
provides an arbitrary RPM setpoint to the speed control loop, of
e.g. 400 RPM. The speed control loop raises the control voltage
until a fan speed of 400 RPM is measured. At this time, the voltage
control setting to maintain 400 RPM is known, and reported to the
CFM control loop. A system constant is then calculated from the
measured speed and control voltage value, as from the graph of FIG.
6. By way of example, the voltage control setting is found to be
0.193, and the system constant (K) is then calculated for the 400
RPM speed to be 2.00.
[0042] The CFM control loop can now calculate the desired speed as:
RPM CFM/K. Where CFM is 1200 and K is 2.00, the desired RPM is 600.
The CFM control loop thus raises the RPM set point and delivers it
to the speed control loop. The CFM control loop could raise the RPM
setpoint to the full 600 RPM, but this might not be prudent since
there could be a small error due to accuracy of the curve fitting
or other minor variables in the system as the system constant
changes with fan speed. Therefore, the CFM control loop may, in one
embodiment of the invention, place the RPM setpoint at one half the
difference between the present speed and the desired speed, which
in this case would be 500 RPM.
[0043] When 500 RPM is reached as measured by the tachometer, the
voltage control setting is again reported to the CFM control loop.
Suppose that the voltage control setting value at 500 RPM is now
0.265. A new system constant is calculated for the 500 RPM motor
operation, and found to be 1.975. The new desired RPM setpoint is
then calculated to be 608 RPM. Using the same algorithm as before,
the CFM control loop sets the RPM setpoint halfway to the desired
final speed, which would be 554 RPM. This process now continues,
until the speed converges to a value which produces only an
acceptable error between the desired RPM and the actual RPM. For
our example, the final voltage control setting is 0.352, resulting
in a speed of 605 RPM and a calculated system constant (K) of
1.984.
[0044] Essentially the described algorithm will follow the order
of:
1 START 1. enter CFM value (e.g., 1200 CFM) 2. set arbitrary RPM1
value (e.g., 400 RPM) 3. step control voltage (CV) up until RPM1 is
measured at tachometer 4. derive system constant K1 for current CV
and RPM1 5. divide CFM value by K1 to derive RPM2 value 6. subtract
RPM2-RPM1 to derive RPM 3 7. divide RPM3 by 2 and add to RPM1 to
get RPM4 (i.e. half step) 8. step CV up until RPM4 is measured at
tachometer 9. derive K2 for current CV and RPM4 10. divide CFM
value by K2 to derive RPM5 value 11. iteratively step up CV,
measure RPM and half step through RPM values until RPM value and K
balance at CFM value.
[0045] Preferably there is some intelligence provided in the CFM
control loop. As long as there is not an unacceptable error in the
provided constant CFM air flow, the CFM control loop should not
change the RPM setting. This will reduce annoying speed changes
which might come with minor drift and noise in the system, while
still controlling constant CFM air flow within a desired tolerance.
This is especially true in the operational areas of the motor where
CFM flow does not change much over a wide speed range. If a CFM
error outside the allowed error band is observed, the CFM control
loop could then command a new speed setpoint. This is sometimes
referred to as "adaptive" control.
[0046] In continuing our control example, suppose a damper is now
closed, restricting the volume of air which may be moved through
the system for a given time. In order to maintain constant CFM the
air flow must increase and the fan, being unloaded to some degree,
will speed up. It is again noted that the fan system here is the
centrifugal fan typically used in most home furnace applications.
The speed control loop will however, decrease the control voltage
to try and maintain the setpoint of 605 RPM. The faster fan speed
and lowered control voltage value are reported to alert the CFM
control loop. The CFM control loop may see either of these
occurrences as a sudden decrease in the system constant and command
the speed control loop to increase the speed. A new, lower, system
constant is derived for the fan speed and decreased control
voltage, thus leading to a new higher RPM value.
[0047] For example, the voltage control setting drops to 0.269 to
maintain 605 RPM. The system constant is then 1.50, and the airflow
has dropped to 907 CFM at the maintained 605 RPM. This is outside
the allowed error band. Air velocity must increase to move the same
volume of air, i.e. 1200 CFM, though the now restricted space. The
CFM control loop calculates the new desired speed setting to be 800
RPM. The CFM control loop uses the same algorithm as previously
described, moving the RPM setpoint half the difference to 702 RPM.
The controller then iterates until finally achieving zero error at
700 RPM, with a system constant of 1.50. The voltage control
setting is now 0.397.
[0048] The controller 15 increases or decreases the control
voltage, or signal, to the motor 17, which in the illustrated
embodiment of FIG. 1 controls the switching of the triac 31, in
order to achieve the desired RPM speed of the motor/fan, as
provided by the outer loop. Iterations of the control loop cycle
will take place until the fan speed is within the desired
tolerances to achieve the desired constant mass air flow.
[0049] Adaptive filtering may be applied to the inner loop to
insure that the proportionality constant of the control system is
allowed to change only very slowly during normal operation, and
more rapidly when an abrupt change in RPM indicates a major load
transient such as a damper change. Thus the microprocessor 25 would
not normally have to access the lookup tables containing the
proportionality constants very rapidly.
[0050] Adequate processing power, including calculation time and
memory requirements, is commercially available for this control
solution, with the design choice left to the person of skill in the
art to select the components and balance these requirements against
the lowest cost. The cascade control implementation discussed above
is believed to be achievable in a low cost processor.
[0051] Thus a summary of the advantages of the cascade control loop
according to the present invention may include: no extra hardware
requirements because the cascade control is merely a software
implementation; no annoying speed variations of the motor due to
direct control of the speed; lookup table storage for the motor/fan
characteristics is easier since the family of curves tends to be
closer to a set of straight lines; the overall processing power
required is reasonable; and the addition of the adaptive control in
the outer loop should provide very stable control that is still
responsive to system load variations.
[0052] Additional considerations for certain embodiments of the
present invention may include power line supply variations and
temperature variations to be accounted for to ensure that a
constant CFM flow is maintained. The motor torque may be highly
sensitive to line power. Since constant CFM control is ultimately
based on assumptions concerning the motor torque, it may be
desirable that correction for line voltage be supplied, as
indicated in FIG. 1 at reference number 35. Line voltage is not
difficult to measure, but may require the addition of a low cost
analog to digital converter (ADC), which may possibly be integrated
into the selected microprocessor.
[0053] Motor characteristics also vary with temperature, although
the effect is not nearly as significant as with line voltage. The
temperature characteristics of FIGS. 8 and 9 were calculated in
Mathcad.TM. by varying the copper resistivity and the rotor
resistance based upon the temperature coefficient of aluminum. CFM
versus control input was then plotted at 50.degree. C. and
75.degree. C. for different RPM's. FIG. 8 is a plot for 400 RPM and
FIG. 9 is a plot for 1050 RPM. Note that at 400 RPM the temperature
effect is hardly discernible. At 1050 RPM, the effect of the
25.degree. C. change is on the order of 8% to 10%. If this amount
of error in air flow can be tolerated, then no correction for
temperature should be necessary. It would be feasible to include a
temperature correction by the addition of a temperature sensor to
the motor as indicated in FIG. 1 at reference number 37. Since most
commercial microprocessors with ADC's usually contain multiple
ADC's, the additional cost may be minimal. Further testing to
verify how well this temperature model agrees with real motor
characteristics may need to be performed empirically.
[0054] A control method for constant CFM using an induction motor
has been presented teaching inexpensive and robust control means to
achieve the method. Novel apparatus and methods to achieve the
present invention have been described. Persons skilled in the art
shall appreciate that the details of the preferred embodiment
described above can be changed or modified without departure from
the spirit and scope of the invention which is to be limited only
by the appended claims.
* * * * *