Hydraulic system provides smooth control of large tracking and antenna drive systems at very low tracking rates. This configuration modifies a series connection of the drive motors with compensating orifices to offset the effects of drain line loss. Linearization of response by eliminating cogging or cyclic operation is thus obtained

Parker, G. L.

English

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Motor 
P1
November 1967	 Brief 67-10418 
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. 
Hydraulic System Provides Smooth Control of Large Tracking and

Antenna Drive Systems at Very Low Tracking Rates 
Pinion	 ç )Antenna  
Gear Box	 Upstream Downstream 
Orifice	 Orifice
Pinion 
Gear Box 
Dtor 
Return I Power 
Supply çUnit 
Drain 
The problem: 
The primary disadvantage of the conventional par-
allel motor control systems commonly used on the 
heavy structures of the DSIF and other tracking an-
tenna drive systems is that at very low speed tracking 
an oscillatory condition often exists because the 
structure is driven cyclically by first one and then 
another motor. This effect is caused partly by the 
spring constant of the gear train and minute differ-
ences between the motors. The small servovalve open-
ing at very low speed operation causes the pressure to 
the drive motors to build up relatively slowly when 
the motors are stalled. As the pressure increases, one 
motor will break away or begin to run before the 
other due to small differences between the units. When 
this happens, the pressure immediately drops due to
the flow through the motor. The running motor then 
turns until all the backlash is removed, the gear box 
wound up, and the antenna moves. This motor then 
stalls, and the pressure increases until the other motor, 
now unloaded by accumulated backlash, starts and the 
cycle continues. Smooth control of the heavy structure 
at very low tracking rates, with no degradation in per-
formance at high rates is required. 
The solution: 
A concept Of a simple hydraulic system configura-
tion modification to a series connection of the drive 
motors with compensating orifices to offset the effects 
of drain line loss. Linearization of response by 
eliminating cogging or cyclic operation is thus ob-
tained. 
Adak	 (continued overleaf) 
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
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.
https://ntrs.nasa.gov/search.jsp?R=19670000418 2020-03-11T21:09:32+00:00Z
How it's done: 
Two very small orifice elements are provided in par-
allel with each of the series connected motors to com-
pensate for the drain line loss. If the servovalve is 
displaced to the right, the fluid driving Mi is not all 
available to drive M2 due to drain line loss, but both 
motors are constrained to move together by the gear 
train. This causes the pressure, P2 , between the drive 
motors to drop when the backlash has been removed 
and the differential pressure, P1—P2 increases while 
P2—P3 decreases. Since the now rate through an ori-
fice is proportional to the differential pressure, more 
fluid flows through the upstream than the downstream 
orifice. This results in a net flow into the junction 
between the motors which compensates for the drain 
line loss. To obtain the same rate and torque char-
acteristics, it is also necessary to double the dis-
placement of the motors by replacing the cam plates. 
The relative pressures, Pt , P2 , and P3 , can be con-
trolled in a linear fashion to insure gear backlash 
compensation by suitably sizing the two symmetrical 
orifices to have an appropriate now constant. 
When the system is first started, backlash will be 
present but since the orifices are the same size the 
flow in each will be equal until drain line loss causes 
the removal of the backlash. At this point the pres-
sure in the center line drops until the differential 
flow between the orifices equals the drain line loss. 
In this condition, the differential pressure is auto-
matically adjusted to a value sufficient to maintain 
backlash-free operation by the sizing of the orifices.
Notes: 
1. Use of this concept appears possible in servodrive 
systems for reversible control of heavy loads for 
various types of construction equipment (cranes, 
gantrys, etc.), where precise wide range control is 
required. 
2. Possible advantages of this concept are: 
(a) Elimination of very low speed cogging or un-
stable operation of the drive motors. 
(b) Significant simplification over other series drive 
configurations. 
3. The concept of this system configuration was 
implemented utilizing the low-speed skid of an 
85-foot tracking antenna to prove the feasibility. 
4. Inquiries concerning this invention may be di-
rected to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B67-10418 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: Gary L. Parker 
Jet Propulsion Laboratory 
(NPO-10316) 
Brief 67-10418	 Category 05


Hydraulic system provides smooth control of large tracking and antenna drive systems at very low tracking rates

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