Dual mode servo

A dual mode, digital servo positioning system controls servo motor current by a processor-generated current control set point in conjunction with velocity feedback. When the motor has advanced to the vicinity of a target position, the processor opens the velocity feedback loop and reduces servo amplifier gain, such that the set point controls motor current without velocity feedback. This open loop condition provides a springy servo response that eliminates oscillation due to small deviations from target position caused by electronic drift or gravity. Position information is provided to the processor by means of quadrature encoded tachometer pulses from the motor.

BACKGROUND 
The present invention is directed to servo control systems, and in 
particular, to velocity servo systems. The specific application of the 
present invention is a servo system control for positioning a 
vertical-travel paper picker for a multi-input-bin printing apparatus. 
Servo systems to control positioning an object such as the paper picker for 
a printer apparatus are of two general types--position servos and velocity 
servos. 
Position servos utilize position and velocity feedback to obtain high 
resolution (i.e. precise positional and translational control), including 
the ability to continuously hold a particular target position (zero 
velocity). However, such position servos exhibit a number of 
disadvantages. Analog implementations require extensive analog hardware to 
develop reliable analog position information, including cumbersome 
calibration circuitry. In digital implementations, executing the algorithm 
required to calculate velocity and acceleration for precise positional 
control requires extensive processing time. 
Digital velocity servos require significantly less processor time than 
digital position servos. The processor merely provides a servo 
current-control set point based upon relative distance from target 
position (typically by a look-up-table operation) to control a high gain 
amplifier in conjunction with velocity feedback. However, velocity servos 
are disadvantageous in that holding a target position (zero velocity) is 
problematic. When a particular target position is achieved, positional 
deviations unavoidably occur (such as from electronic drift) resulting in 
a positional error. While this positional error is relatively small, the 
resulting velocity derivative in the velocity feedback loop is relatively 
large (i.e. the velocity servo exhibits low resolution at the target 
position. Because of the relatively high gain of the amplifier (required 
for accurate translational movement), the set point correction for such 
positional deviations result in unavoidable target overshoot that causes 
oscillation about the target position. 
For example, to reposition a vertical-travel paper picker the 
current-control set point is gradually decreased until, at the target 
position, the set point goes to zero. However, drift in the control 
electronics and gravity cause deviation from target position. The 
resulting compensating velocity derivative of this positional deviation in 
the velocity feedback loop causes continual overshoot such that the servo 
motor is alternately driven positively and negatively, putting excessive 
strain on the power drive and needlessly dissipating power in the motor to 
maintain position. 
SUMMARY 
The present invention provides an improved servo positioning system for 
controlling a servo motor, for example, to translate a paper picker 
between target picking positions. The positioning system includes a servo 
power amplifier whose motor current output is controlled by a summing 
amplifier responsive (i) to a current-control set point and, during 
initial advance, (ii) to velocity feedback. When target position is 
approached, the velocity feedback loop is opened and the gain of the 
summing amplifier is reduced, such that the set point alone controls the 
output from the servo power amplifier in a low-gain, high resolution mode 
that minimizes over-compensation for positional drift about target 
position. Processor means is responsive to motor positional information to 
provide the current-control set point, and when target position is 
approached, to open the velocity feedback loop and switch the summing 
amplifier from a high gain to low gain state. 
For a preferred embodiment, the summing amplifier also receives current 
feedback to provide a higher-order velocity derivative feedback that 
smooths servo response. Velocity feedback is provided by a digital 
tachometer (quadrature encoded) coupled through a gate to a 
frequency/voltage converter. The gate is controlled by an open-loop signal 
from the processor to open the velocity feedback loop when target position 
is approached. Position information is provided by quadrature-decoding 
position logic that latches successive position-change and motor-direction 
indications for input to the processor means. This positional information 
is used by the processor to track motor (i.e. picker) position, and to 
provide the current-control set point and the open loop and gain control 
signals. 
From the foregoing summary, and the following detailed description of a 
preferred embodiment, the present servo positioning system provides the 
advantages of velocity servo control, while avoiding over-compensation and 
oscillation in maintaining a target position (zero velocity). By 
eliminating velocity feedback and lowering amplifier gain, the servo 
system responds slower and with higher resolution to small deviations from 
target position. In addition, the processor can be programmed to ignore a 
predetermined narrow range of position deviation (dead band). The result 
is a springy response to deviations in maintaining target position, with 
resulting decreased compensating power demands.

PREFERRED EMBODIMENT 
The digital servo positioning system of the present invention (shown 
schematically in FIG. 1) will be described in relation to controlling the 
translation to a new target position of a paper picker mechanism for a 
multi-input-bin printer apparatus. The invention is not limited to such an 
application, but rather, has general applicability to servo systems in 
printers, robotics and other fields. 
The servo positioning system controls a motor M in response to 
commands/signals from a processor P which receives status inputs from the 
servo system. Neither the motor nor the processor form a part of the 
present invention. An exemplary motor comprises a standard, permanent 
magnet, DC motor incorporating a quadarature tachometer wheel to provide 
quadrature encoded tachometer pulses. The processing function can be 
performed by special purpose logic, but typically is provided by a general 
purpose printer-control processor executing servo control subroutines in 
response to interrupts from the servo positioning system. (Instead of 
responding to interrupts, the processor can poll the servo system for 
status inputs as desired.) 
The current to drive motor M is provided by a power amplifier stage 10 in 
response to the current control output from a gain-controlled summing 
amplifier stage 20. 
Summing amplifier 20 is responsive to a current-control set point from a 
digital/analog circuit 30, with feedback inputs from both velocity and 
current feedback loops. Velocity feedback is derived from a velocity 
feedback loop indicated at 40. Current feedback is derived from a current 
sense network 50. The current feedback input to the summing amplifier 
provides a higher order velocity derivative feedback that smooths servo 
response. 
The velocity feedback loop 40 includes a digital tachometer 42 that 
receives quadrature encoded tachometer signals from motor M and outputs 
quadrature encoded position-change pulses. The tachometer output is 
provided both to an AND gate 44 and a quadrature decoder 60. 
AND gate 44 couples the position-change pulses to a frequency/voltage 
converter 46, unless disabled by an open-loop signal from the processor. 
For each position change pulse, quadrature decoder 60 decodes quadrature 
to provide the frequency/voltage converter with a sign input corresponding 
to the direction of motor rotation. Frequency/voltage converter 46 
provides the velocity feedback input to summing amplifier 20 corresponding 
in magnitude to the frequency of the digital position-change pulses (i.e. 
the velocity of the motor), with feedback sign being determined by 
corresponding motor rotation signals from quadrature decoder 60. 
Summing amplifier 20 includes a gain control network 22, switchable from a 
high gain to a low gain state in response to a gain control signal from 
the processor. In a preferred implementation, gain control network 22 
comprises an additional resistor that is FET-switched into the amplifier 
gain circuitry of the summing amplifier in response to the gain control 
signal, thereby lowering the gain for the summing amplifier. 
Position information from decoder 60 is latched in a latch/interrupt logic 
network 65 for retrieval by the processor. The quadrature-encoded 
position-change pulses from tachometer 42 are applied to decoder 60, which 
outputs both position-change signals and corresponding quadrature-decoded 
motor-direction signals. Successive position-change and direction signals 
are independently latched in the latch/interrupt logic, which 
simultaneously provides an interrupt signal to the processor. 
The operation of the servo positioning system will be described with 
reference to the schematic representation in FIG. 2 showing 
current-control set point (vertical axis) in relation to position 
(horizontal axis). A target position is represented at TP on the position 
axis, while initial starting positions above and below the target position 
are represented at SPA and SPB. 
For a preferred embodiment, the processor's servo positioning software 
includes two subroutines--position and set point. Processor P stores a 
starting motor position corresponding to the starting position of the 
paper picker (i.e. SPA or SPB in FIG. 2). In response to an initial 
position-seek, the position subroutine is called to provide an indication 
of distance to target position (i.e. SPA to TP). Next, the set point 
subroutine is called to provide an appropriate set point level by a 
look-up table function. With each position-change and motor direction 
update received from the latch/interrupt logic, the position and set point 
subroutines are called to update the processors set point output. When 
target position is approached to within a predetermined distance, the set 
point subroutine causes the processor to output, in addition to a new set 
point, an open-loop command causing the velocity feedback loop to open and 
a gain-control command lowering the gain of the summing amplifier 20. When 
target position has been reached, and the processor provides a null set 
point; small deviations from target position are permitted in a dead band 
(IP1-IP1' in FIG. 2) without the set point subroutine causing the 
processor to change its null set point output. 
Thus, in response to the position-seek instruction to move the paper picker 
to a new target position (TP), the processor provides an initial, maximum 
servo current control set point to set-point D/A converter 30. At the same 
time, the processor enables AND gate 44 to close the velocity feedback 
loop, and switches gain control network 22 to a high gain state. 
Summing amplifier 20 provides a current control output in response to (a) 
the set point from D/A converter 30, and (b) velocity and current feedback 
inputs. The current control output from the summing amplifier drives the 
power amplifier stage, providing current to motor M which accelerates to 
the set point velocity. Quadrature encoded tachometer pulses from the 
motor are converted by digital tachometer 42, quadrature decoder 60 and 
frequency/voltage converter 46 to the velocity feedback input to the 
summing amplifier 20. 
While the motor is translating the paper picker toward the target position 
(TP), the processor receives continual motor picker position information 
via decoder 60 and latch/interrupt logic 65. In response, the processor 
provides a predetermined set point to D/A converter 30. By way of 
illustration, FIG. 2 shows five discrete set point levels (0 to 4) between 
starting (SPA or SPB) and target positions (TP). Thus, as the motor 
translates the paper picker between the starting position and intermediate 
position IP4 motor current (velocity) is set at a maximum by a level 4 set 
point. Between intermediate positions IP4 and IP3 the processor provides a 
lower level 3 set point, while between intermediate positions IP3 and IP2 
the processor outputs a level 2 set point. 
When motor position (i.e. paper picker position) is a predetermined 
distance from target position, the processor provides an open-loop signal 
to disable AND gate 44, inhibiting the input of position-change pulses to 
frequency/voltage converter 46 thereby opening the velocity feedback loop. 
In addition, the processor switches gain control network 22 to the low 
gain state for summing amplifier 20. Referring to FIG. 2, when the motor 
has translated the paper picker to intermediate position IP2, the 
processor reduces the current control set point to level 1 and at the same 
time opens the velocity feedback loop and switches the summing amplifier 
to the low gain state. When the paper picker passes intermediate position 
IP1, the processor nulls the set point (level O), shutting off current to 
the motor. 
With velocity feedback eliminated, the current control signal output from 
summing amplifier 20 is determined by the current-control set point (level 
1 or 0), together with the higher order derivative current feedback. Thus, 
over-compensation for small deviations from target position which would 
result from the combination of velocity feedback and high summing 
amplifier gain is eliminated. The resulting high resolution current 
control output from summing amplifier 20 causes the power amplifier stage 
to respond slowly to positional deviations. This springy servo operation 
in the vicinity of target position avoids unnecessary over-compensation 
and reduces power dissipation in holding the motor, and the paper picker, 
at the target position. 
For a preferred embodiment, a predetermined dead band (IP1 to IP1' in FIG. 
2) is provided around the target position (TP) permitting the paper picker 
to deviate from the target position without causing the processor to 
switch from the null level 0 to the level 1 set point. If positional drift 
exceeds this dead band deviation such that the paper picker passes 
intermediate position IP1 or IP1' the processor outputs the level 1 set 
point to cause position correction without closing the velocity feedback 
loop or raising amplifier gain. 
While the invention has been described with respect to a preferred 
embodiment, those of ordinary skill in the art will understand that the 
invention is not thereby limited, but rather that the limits of the 
invention are to be interpreted only in conjunction with the appended 
claims.