7 Initial Inconsistent Performance   When  the BSM Engineering Model was available for testing, it was mounted to an optical bench and initial  frequency r esponse testing was performed. The controls engineers were finding inconsistency upon taking  successive  frequency responses and it was further found that changing the torque of  the mounting  hardware changed  those responses. Due to the fact that proper hardware for mounting the BSM to the  bench was not yet ready, the BSM housing was clamped to the bench using machine tooling.  The surface  of the honeycomb optical bench is made of a thin section of steel with thr eaded hol es on 1- inch (2.5-cm)  centers. There certainly was not concern over the stiffness of the bench that  also happened to sit on a very  thick granite table, which was  floating on a seismic block. However, local attachment  threaded holes  did  not provide adequate stiffness, thus af fecting response measurements. The issue was eliminated by adding  a half -inch-thick (1.3-cm) aluminum plate between the bench and BSM hardware.   

Undesirable Structural Mode  Measurement of the f requency response found there to be a very undesirable structural mode near  900 Hz.  While this frequency was quite far from the controller bandwidth, the mode had a very  high Q, which made  it troublesome. Very unusual was the wide frequency range of the base of this mode, such that it did not  have behavior consistent with that of a structural mode. One of the controls engineer s was able to  implement filtering in order to deal with this mode, sacrificing some phase margin, but still leaving adequate  phase margin.  

Careful study was made of the modal analysis to determine the source of this behavior, but it is typical to  find that modes revealed in the model do not match the measured modes.  Using FEM modal  analysis, we  were unable to determine the sourc e of this very troubling behavior. The modal analysis software tool does  not specify an input location, or  an out put location. Furthermore, guesses are made to choose dam ping  values for the materials  to input into the analysis  tool. In the physical world, t he frequency response input  is the current to the actuators, which becomes l ocalized force inputs, and the frequency response output is  the position sensor signal, which is the motion of the sensor targets. So, the modal analysis cannot be  expected to give an accurate representation of the measured frequency response. We would be pleased  to be made aware of analysis tools  that can better predict frequency response for a mechanism with  actuators and sensors intended for closed- loop control.  

Certainly, management was concerned that effort was being expended to correct this modal behavior issue. The question was posed, “Do we meet requirements?”  The answer was that we did on the bench, but the  possibility of structural mode behavior al changes after integration, or even on -orbit, requires having  capabil ity to deal with such changes. The controls engineer had designed in the capability for a maximum  number of poles for filtering, but with the unusual mode, there was risk that filter capability could be insufficient to deal with structural mode changes that might potentially come about.      We hit a dead end using FEM analysis, so we chose a  brute force method to find out what was causing the  troubling structural mode. The B SM does not have many parts. Trial and error c hanging of parts  with  modified parts  was the technique implemented. Since the BSM required a rather heavy tungsten  counterweight, it was  theorized that a lack of stiffness of the titanium counterweight shaft could be  responsible. The drawing was revised and parts were put into fabrication. After  planned testing of the BSM  Engineering Model  was completed, the new and stiffer ribbed titanium counterweight shaft was installed.  The new frequency response showed that the massive and wide structural mode was replaced by small,  typical looking structural modes. Problem solved.  The before and after open loop frequency responses (of  mechanism and controller  together ) are shown in Figure 7.