10 conductive structural adhesive. This increased the effective electrical resistance of the bobbin in the  appropriate direction without comp romising the thermal conductive properties of the bobbin. When  implemented on the prototype mechanism, there was no measurable performance difference between the  slotted Be-Cu bobbin and a polymeric one.    The voice coil drive method is quite different to the electrically commutated motors that power the rest of  actuators in the Drill. Due to its late development in  the MSL project life cycle, the voice coil actuator  needed to be compatible with the existing motor dr ive electronic hardware. Additionally, the tight  packaging constraints and the severe percussive (sho ck) environment precluded the implementation of a  sensor for closed loop feedback control of the DPM. The challenge remained to develop a drive method that did not require sensory feedback.    The DPM is driven by an open loop voltage waveform. Near the impact, the voltage waveform is set to zero so the hammer will coast just prior to and after the impact. This zero voltage zone, called the coast  period, provides a tolerance to perturbations in the hammer motion cycle. This prevents the voice coil  from fighting the motion of the hammer. Otherwise, t he voice coil could be quite inefficient using energy  and generating heat to slow down the hammer. The voltage values are defined such that the voice coil generates a prescribed force to retract (upstroke forc e) and extend the hammer (downstroke force). The  forces are defined during a tuning procedure to create a desired percussion output: kinetic energy level just prior to the impact. A position sensor was us ed to determine the kinetic energy; it was removed  before the installation of the DPM into the Drill top-leve l assembly. During tuning, the two forces are also  modulated to adjust the timing of impacts such that  they occur in the middle of the coast period.    For a desired impact hammer velocity and assumed rebound velocity, the motion profile of the hammer  (i.e. position and velocity time history) is estimated a priori  (Figure 8). Using a discretized motion profile,  the desired actuator force,  Fdesired, the voice coil force constant, KF, (back-emf, KV, constant equivalently)  as a function of hammer position,  x, and the total roundtrip resistance of winding circuit, Rtotal, the voltage  waveform is defined as follows:  ௘௠௙  ௞ሻ  where k is the index of the voltage table   The voice coil winding temperature, and thus resistan ce, will increase due to self heating during  operation. The temperature of the wi ndings is modeled onboard by the flight system software to prevent  overheating and to estimate the resistance parameter for the voltage waveform. The DPM tuning process yields two tables: desired current draw and t he feed-forward back-emf voltage compensation.     A project schedule constraint mandated the use of a voltage driver for the voice coil to minimize the  impact to the rover avionics development. Fortunately,  the voltage drive method features inherent speed  regulation and a reduced sensitivity to variations of t he voice coil force constant. If the velocity of the  hammer is lower than expected, the voice coil will draw more current and thus output more force. This is  equivalent to having a built in proportional feedback c ontroller on hammer velocity. One source of velocity  error is the variation of impact coefficient of restitution due to rock strength. In a current driver, a drop in the force constant (K F) would directly result in a voice coil outpu t force. In an open loop current driver  system, the known sources of K F variation would require compensation complicating the software  implementation. With the voltage drive, a decrease in K F (and thus force generation of the actuator) is  counteracted by a lowering of the back-emf voltag e generated by the windings. Since the unreduced  back-emf is compensated for in the voltage waveform , this “found” voltage is converted into additional  current to generate more force. Some sources of K F variation are the presence of the steel ball screw  within the bore of the voice coil field assembly (whi ch varies with Drill feed position) and voice coil magnet  temperature.     NASA/CP-2010-216272