Electric drive system provides low speed, long term tracking of targets that move at a sidereal rate. It utilizes eddy-current energized actuators that are free from radio frequency interference generation and a solid state feedback amplifier with provisions for antibacklash biasing

Richter, H. L.

Stoller, F. W.

English

NASA Technical Reports Server

July 1967	 Brief 67-10220 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the ,
 public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Low Speed, Long Term Tracking Electric Drive System Has Zero Backlash 
Closed In Summing Amplifier 
Mode	
Velocity Modes  
Rela	 I'	 Summing And 
ompensati 
Closed InJ
ng
Eddy Current Coupling Coil 
MR2	
Amplifier	
Tachometer
 Summing and Compensating 
	
RCVR And	 Amplifier 
- Comp Modes 
i-i	
.	 MR0	
Input	
- e	 Power I 
Integrator 
ii	 1	 L____ 	
-	 Amplifier 
I
	
Servo 
I	 Pinion Gear Gear Box 
Mode	 Loop	 - 
	
— l	
Closed in Velocity	 L ---
Electric 
Motor 
Mode Relays
Computer Error A	 ____________ 
°	
MR4	 I 
A	 I	 Isolation	 i	 Receiver Error 
Rate/Sidereal Reference Voltage 
The problem: 
• To develop an electric drive system that will provide 
loiW speed, long term tracking of space probes, stars, 
and other targets that move at a sidereal rate. The 
system must be capable of moving a large structure, 
such as an 85-foot tracking antenna, a specified 
distance with essentially zero backlash, and without 
generating electrical noise and radio frequency inter-
ference. Prior art drive devices used silicon controlled 
rectifiers which caused radio frequency interference. 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
The solution: 
An electric drive system that utilizes eddy-current 
energized actuators that are free from radio frequency 
interference generation and a solid state feedback 
amplifier with provisions for antibacklash biasing. 
How it's done: 
The electric drive system cdntains 'four primary 
servo loops: a current feedback loop, amotor rate 
loop, a pinion rate loop, and velocity and position 
(continued overleaf) 
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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loops. The current feedback loop serves to decrease 
the effective inductive time constant of the large induc-
tance inherent in the eddy-current coupling within the 
actuator by utilizing a feedback of the current in the 
actuator coil. The current loop is closed with an open 
loop gain such that the effective time constant of the 
eddy-current field coil is reduced to 0.01 second. 
The motor rate loop comprises an eddy-current 
actuator, tachometer, summing and compensating 
amplifier, power amplifier, and eddy-current coupling 
coil which controls the amount of torque transferred 
from the constant speed electric motor to the eddy-
current coupler to the output shaft and gear box. 
A servo loop operating in opposition comprises 
an eddy-current actuator, tachometer, summing and 
compensating amplifier, power amplifier, and eddy-
current coupling coil which controls torque transfer 
from the electric motor to the gear box. The antiback-
lash feature is inherent in this design Since some power 
is applied to the dragging actuator when its opposite is 
driving the load. A bipolar dc signal, applied to the 
input of the differential network, controls the direc-
tion of the drive of the motor rate loop. 
A pinion-rate loop operates in conjunction with 
the motor rate loop. The pinion-rate loop provides for 
damping of the gearboxes and senses the motipn of 
the antenna. The upper pinion gear, which drives the 
antenna, is physically coupled to the upper tachom-
eter. Similarly, the lower pinion gear and lower tach-
ometer are coupled. The output (rate of speed) of 
these tachometers is summed in the summing ampli-
fier, the output of which closes the pinion rate loop 
through the summing and compensating amplifier, or 
through the-upper left mode relay and integrator when 
in one of the velocity modes of operation. When in 
the position modes of operation, the pinion rate 
signal closes a conventional rate loop around the
pinion but in the velocity modes this signal, when 
summed with a rate reference, provides the error 
necessary for precise rate control. 
In the receiver and computer modes (position loop), 
error signals are derived from receiver output or 
antenna axis resolvers respectively. The error signal is 
transferred through mode relays MR4 or MR5 di-
rectly into the integrator and then summed with the 
pinion rate signal in the summing and compensating 
amplifier. 
Notes: 
1. The drive system does not generate noise that could 
interfere with reception of very low level radiom-
etry or space probe signals. The system is capable 
of multimode operation in that signals from-several 
sources can be integrated to control the overall 
motion of the structure. 
2. The system is compatible with electrohydraulic 
systems for slewing. 
3. This drive system may be used to advantage wher-
ever several signals must be combined to move a 
large structure a specified distance and where 
backlash is not tolerable. Such applications may 
include large machine tools controllable by analog 
or digital-to-analog devices. 
4. Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B67-10220 
Patent status: 
No patent action is contemplated by NASA. 
Source: Hans L. Richter and Floyd W. Stoller 
Jet Propulsion Laboratory 
(NPO-10173) 
Brief 67-10220	 Category 01


Low speed, long term tracking electric drive system has zero backlash

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