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Because the motor loads vary over such an extremely wide range, the characteristic time constant of the motor accelerations exhibit a correspondingly wide range. The maximum acceleration of the motor is of the order of 3500 rad/sec^2 which corresponds to a time constant of approximately 17 milliseconds. The tachometer must track these rapid changes in motor rate without significant phase la to avoid instabilities in the rate loop. Analog tachometers provide good high frequency characteristics and a rate feedback signal with very little phase shift at these acceleration levels. However, at very  low rates, drift and accuracy can be a problem. A digital tachometer on the other hand, provides excellent low-noise drift-free responses at very low rates, but suffers from undesirable processing delays at high frequencies. A proper mix of the two types of tachometric feedback provides the desirable characteristics of each type while attenuating the undesirable characteristics. This is accomplished by using a quasi-complementary filter, with a high-pass component in series with the analog tachometer output and a low-pass component in series with the digital tachometer output. 

Because of the high amplification required in the analogue tachometer processing (in order to bring the extremely low level Inductosyn tachometer signal to required levels), an additional capacitively couples stage of amplification was required. The capacitive coupling prevents passage of any DC offset to the MDA input. 

An integrator is added to the forward path to provide high gain at low frequencies to break motor and drive-train stiction and to null small errors. The output of the integrator is limited to avoid undesirable large error offsets when, for example, the joint is being back driven. The limiter reduces overshoot in the large signal transient response, but most not prevent the integrated error signal from driving the motor under worst case stiction levels. 

The resulting servo compensation is shown in Figure 10. 

[[image]]

Figure 10 Servo Compensation Block Diagram

To maintain approximately equal maximum torques for the forward and back drive conditions, the current limits are set such that: 

IF = To max / Nf NKT
IB = IF NF NB

where, To max = maximum output torque of joint
N = Gear ratio
KT = Torque constant of motor
NF = Forward drive efficiency of drive train
NB = Backdrive efficiency of drive train
IF = Forward drive current limit
IB = Backdrive current limit

The logic used to switch between IF and IB is as follows:

If I[[?]]>0 and |[[?]]|>[[?]]T
Ilimit = IF
Otherwise Ilimit = IB

where
I is the motor current,
[[?]] is the motor speed, 
[[?]]T is a constant (0.1758 rad/sec)

The above logic ensures that the motor is current limited to IB when at rest. 

When the integral trim is operating within its limits, LT, the rate servo is a type 1 servo. However, when the integral trim saturates, it reverts to a type 0 servo which requires an error to maintain a one to one correspondence between the command and actual steady state rates. Consequently to maintain an accurate steady-state rate response over the entire operating requion, the scaling gain for the rate servo is shaped by a dual gain function implemented in the software.

In the position hold mode, a position loop is closed around the servos where the set point is normally the joint angles at the time position hold is initiated. The position hold computations are implemented in the GPC software. A round-up procedure is used to increase the gain of the error signals around null.

Potential SRMS Modifications

The present SRMS design resulted primarily from considerations of the payload deployment and retrieval system performance specifications and weight and size constraints.  Even though the SRMS is primarily intended for the deployment and retrieval of satellites, it can also be used to perfom the such additional tasks as follow in and around the cargo bay, sometimes with the aid of special purpose and effectors or tools:

- Inspection, photography, and possible 
  override of spacecraft system, 
  mechanisms and components.

- Cargo transfer.

- Deployment, retraction, and repositioning 
  of antennas, solar panels, booms and 
  radiators.

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