Figure 27:  Brake Test (Good Sequencing)  Future Work  To potentially reduce the incremental step size of PLADD down into the 2-5 nm range, we plan on taking  the following steps:  Mechanical Modifications MicroE Mercury II Encoder— Micro E recently introduced its next generation Mercury II encoder capable  of obtaining 1.2-nm LSB. Installing this improved read head would allow the controller to attempt moves as small as 4.8 nm. Mechanical Disturbance Isolation— While the current test setup utilizes an extremely quiet building, an  isolation pad, and an air table there are potentially further layers of isolation that could be added to the test setup. Potential gain can be had by installing an isolation layer of foam padding between the test fixture and the table. The row of actuators is also currently mounted along one edge of the air table. Control Modifications PLADD’s control logic is derived from code developed by JPL’s Ted Kopf for the Mars Exploration  Rovers. Many elements of the code can potentially be improved to optimize PLADD’s performance. The following is a list of potential adjustments. Filter Encoder Count— The value read from the encoder has high frequency noise of +/- several bits.  Currently, this value goes directly into the PID controller. Placing a filter in front of the controller would clean up the signal so that the PID command does not react to small noise variations. This could improve performance for very small step sizes.Adjust Rate Filter— There is already a filter inside of the PID controller on the internally generated rate  signal. However, the time constant of this filter was set for an entirely different application (MER motor control) that used very different encoders. Adjusting this filter could help make the derivative gain useful for this application. Adjust Controller Scaling— The PID controller scaling was set for a very different application (MER motor  control) and is not ideal for the SID ballscrew. Adjusting this scaling could result in performance benefits due to finer tuning of the PID gains. Add Bits to PID Controller Position Value— The PID controller currently uses 22 bits for the position value.  For this application, the limited number of bits forces a choice between high resolution and large range. Increasing the bits allocated for the position value would allow high resolution control over the entire ballscrew range. This modification is tied in with the scaling modification listed above, as the scaling is  effected by the number of bits. Control Down to Encoder LSB— The PID controller does not allow delta position commands less than 4x  the encoder resolution. It might be worth investigating the reason for this in detail to figure out if it is feasible to remove that limitation. The idea is to find out just how far down we can push the incremental command resolution. Reduce Encoder Noise— The encoder noise is probably due to a combination of mechanical and  electrical sources. Investigation into the nature of this noise might lead to ways to reduce it. One idea is to improve the shielding of the cable between the encoder read head and the encoder interpolation module. This cable transmits sensitive analog signals that are susceptible to noise.  384