Optimum motor speed control system

The method for controlling a motor driving a load holds the speed of the output shaft of the motor substantially constant as the torque imposed upon the shaft by the load varies, then regulates the rate of change of the output torque delivered by the motor's output shaft as the motor's speed decreases.

FIELD OF THE INVENTION 
The invention relates to a method and a system for controlling the speed of 
a motor's output shaft as the torque or force imposed upon the shaft by 
the load being driven by the motor varies, and for controlling the output 
torque delivered by the shaft as the motor's speed drops to zero. This 
invention can be used to control an electric motor in a powered system of 
a motor vehicle. 
BACKGROUND OF THE INVENTION 
Many powered systems in motorized vehicles are controlled by permanent 
magnet electric motors. Examples include window lifts, sunroofs, sliding 
van doors, vehicle trunks, tailgates, and seat and seat back recliners and 
adjusters. The variability of certain parameters in many powered systems 
(e.g., the supply voltage, operating temperature, and the load driven by 
the motor) produces a variable speed vs. torque performance characteristic 
in the permanent magnetic electric motor used to power such a system, 
leading to undesirable variation in the powered systems' travel time, 
noise level, and sealing force. A control method and system is needed for 
optimizing the speed vs. torque performance characteristic of motors that 
drive powered systems in motor vehicles notwithstanding the variability of 
operating parameters in these systems. 
SUMMARY OF THE INVENTION 
The inventive method for controlling a motor driving a load is comprised of 
two phases. In the first phase, the speed of the motor's output shaft is 
held substantially constant as the torque imposed upon the output shaft by 
the load, and, therefore, the output torque of the motor, varies. In the 
second phase, the torque delivered by the motor's output shaft is 
controlled in a predetermined manner as the speed of the output shaft 
drops to zero. 
The inventive method may be applied in an automotive or motor vehicle 
setting to any motor that drives a traveling component in a powered 
system. In the first phase of the inventive method, a traveling component 
is driven by the motor at a constant speed to some intermediate position. 
During this first phase, the output speed of the motor is held constant 
regardless of variations in operating environment parameters. In a second 
phase as the traveling component moves from the intermediate position to a 
second position, the output speed of the motor is continuously decreased 
and the output torque delivered by the motor's output shaft is controlled.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings in which each numeral represents the same 
component among the several views, FIG. 1 depicts the speed vs. torque 
performance curve 20 of a typical permanent magnet electric motor with 
open loop control, i.e., with no control. The revolutions per minute or 
speed of the motor's output shaft is shown as a function of the input 
torque imposed on the motor's output shaft by the load driven by the 
motor. As the load torque increases the motor's speed decreases linearly. 
Variations in the input torque arise as a result of variations in certain 
operating parameters, for example, variability in the load imposed on the 
motor, in the operating temperature of the motor, and in the voltage 
supplied to the motor. As a result this variability, using open loop 
control for the permanent magnet motor in a powered system of an 
automobile disadvantageously leads to variation in the powered system's 
travel time, noise level, and sealing force. 
Because FIG. 1 represents open loop control of the motor, the same curve 
may also be used to show the revolutions per minute or speed of the 
motor's output shaft as a function of the torque delivered by the output 
shaft of a motor to drive the load. The speed vs. output torque 
performance characteristic for the motor of FIG. 1 operating under closed 
loop control according to the present method is shown in FIG. 2. 
The multiple curves 52 in region 50 depict various paths for controlling 
the large increase in the motor's output speed as the motor is started and 
begins to develop output torque. Controlling the motor's speed in region 
50 reduces the impact load on the components in the powered system' that 
are being actuated by the motor and on the mechanical components of the 
motor itself as the tolerances in the motor and in the powered system's 
mechanisms are taken up at startup. 
The segment of the operating curve 56 in region 54 reflects a control 
scheme in which the motor's output speed is held constant at a medium 
value within its open loop operating range as the load torque and, 
therefore, the output torque required to drive the load, increases. This 
uniformity in speed promotes constant travel times, smooth performance of 
the powered system's sealing components, and reduces undesirable system 
noise variations. 
The multiple curves 60 and 62 in region 58 depict various paths for 
controlling the decrease in the motor's output speed as the motor's output 
torque is increased at the end of the travel of the traveling component in 
the powered system. Control curve 60 corresponds to the uncontrolled open 
loop performance characteristic of curve 20 in FIG. 1. The closed-loop 
control method of the present invention represented by control curves 62 
effects a soft stop for the traveling component as the motor's output 
speed drops to zero. Such a control scheme minimizes impact loads on the 
powered system's closure mechanism as it reaches the end of its travel and 
on the mechanical components of the motor itself. 
At the end of the travel control curve 62 for the traveling component, the 
control method of the present invention applies a constant stall torque 
64. The stall torque 64 holds the amount of force applied to the powered 
system's mechanism through the motor to a known value regardless of 
variations in the operating environment parameters. 
FIG. 3 outlines the control method of the present invention depicted in 
FIG. 2. In step 100, the output speed of the motor is controlled along one 
of the control paths 52 in region 50 as the motor is started and begins to 
develop torque. In step 102, the motor's output speed is held constant 
along curve 56 throughout the region 54 regardless of any increase in the 
motor's torque under the load up to point 60 as the motor drives the 
traveling component in the powered system at a constant speed to an 
intermediate position near the end of its predetermined travel path. 
In step 104, the motor's speed is decreased along curve 62 in region 58 as 
the traveling component moves from its intermediate position to its final 
position on the travel path. The speed is decreased along path 62 as the 
rate of change of the output torque is regulated and minimized throughout 
region 58 as the traveling component nears the end of its travel path. At 
point 64 a constant-valued stall torque is applied at a position very near 
the end of the powered system's travel to effect a soft positive stop for 
the traveling and/or sealing components of the powered system. 
The control method outlined in FIG. 3 advantageously eliminates variations 
in closure time between multiple powered systems of the same vehicle. For 
example, if all windows in a vehicle are in the fully open position and 
are commanded to close at the same time, the method of FIG. 3 ensures that 
they will close simultaneously, thus improving sound quality in the 
vehicle by reducing variation between the moving systems. Reducing motor 
speed 62 in region 58 as the powered system approaches the end of its 
travel and seating the sealing components with a constant stall torque 64 
limits impact forces on the powered system and on the motor itself. In 
addition, powered systems may be optimized for a narrower range of forces, 
which reduces design and system costs. 
FIGS. 4, 5 and 6 illustrate the motor control method of the present 
invention applied to a powered window lift system 200 in an automotive 
vehicle. Referring now to FIG. 4, the powered lift system 200 includes a 
window 202 attached to a sash 204 that travels upward to seat the window 
202 against a seal 206. A permanent magnet electric motor 208 controlled 
by electronics 210 operates a gear sector 212 that causes translation of 
regulator cross arms 214 attached to the sash 204. The extended and 
compressed positions of the regulator cross arms 214 define the open and 
closed positions of the window 202. 
FIGS. 5 and 6 functionally depict two of the many possible motor 
controllers 210, which may be implemented physically using discrete 
hardware components or a combination of hardware and software with a 
microprocessor. In FIG. 5, a velocity sensor 302 provides digital data 
regarding the revolutions per minute or speed of the motor's 208 output 
shaft. A current sensor 304 positioned between the bus voltage 306 and 
ground provides analog data regarding the current flowing through the 
motor, from which the motor's output torque value is derived. 
The digital output from the velocity sensor 302 is converted to the 
frequency domain 308 and conditioned 310 to improve its signal quality 
then compared 312 within the controller 314 to an external adjustable and 
programmable reference control speed value 316. Developing a reference 
value may be accomplished by any of the implementations well known in the 
art. For example, a multi-turn potentiometer or a resistive network 
attached to the motor shaft may be used to vary the output voltage as a 
function of the rotor or "window" position. A programmed EPROM or an 
independent processor could also be used, as well as distributing sensors 
along the path of the window guides, or providing analog multi-segment 
piece-wise linear curves or wave synthesizers. 
The digital result 318 of the comparison 312 is supplied to a processor and 
pulsewidth modulation (PWM) control module 320 to determine the duty cycle 
of the motor control signal that will yield the performance curve of FIG. 
2. This motor control signal is transmitted by the controller 314 via the 
PWM speed control bus 322 to a motor pre-driver that regulates the speed 
of the motor. The controller 314 may also be implemented logically using 
discrete component circuits. 
In a similar fashion, output from the current sensor 304 is conditioned 324 
to improve its signal quality then compared 326 to an external adjustable 
and programmable reference or control torque value 328. The digital result 
330 is supplied to the PWM control module 320 to determine the duty cycle 
of the motor control signal that will yield the performance curve of FIG. 
2. This motor control signal is transmitted by the controller 314 via the 
PWM torque control bus 332 to the motor pre-driver 334 that also regulates 
the output torque of the motor. 
FIG. 6 depicts a one of the many alternate implementations of the motor 
controller 210. The analog output from the current sensor 304 is 
conditioned the converted from the analog to the digital domain 336 then, 
together with the conditioned digital output from the velocity sensor 302, 
compared within controller 338 against a programmed speed table or map 340 
that charts the desired speed to implement the performance curve of FIG. 
2. As in FIG. 5, a compare algorithm within processor and PMW control 
module 320 determines the duty cycle for the motor control signals 
required to control the motor in such a way as to yield the performance 
curve of FIG. 2. For example, the duty cycle may be increased if the motor 
speed falls below the desired speed, or decreased if the motor speed tends 
to exceed the desired speed at any point on the performance curve of FIG. 
2. 
A designer may choose the implementation of FIG. 5 or 6 depending on 
several factors such as complexity and calculation power of available 
processor and PWM modules 320, software memory requirements, bandwidth 
resolution of the PWM channels and the clarity of the motor control 
signals transmitted via control buses 322 and 332. 
The present inventive method for controlling the speed of a motor has been 
illustrated in the context of only one of many possible applications, 
i.e., controlling a permanent magnet electric motor that drives a powered 
system in a motor vehicle. Alternative and minor variations of the 
invention that are apparent to those skilled in the art may still properly 
fall within the scope of the claims, which follow.