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Developing the rig has required a substantial amount of work taking over 5 years of effort.  The first potential problem arose from the degree of friction in the guidance system of the test mass reducing the acceleration of the test mass which could unacceptably slow the test mass speed ┬ábeneath that specified in the European Standard EN 1891:1998.  We also had a problem relating to the degree to which the test mass could rotate around the 3 axes along with moving across in the horizontal plane.  (Obviously we wanted the mass to freely move in the vertical direction.)  We eventually developed a rattle reduction system which reduced the degree of play to an acceptable level whilst not impacting on friction. 
Another point of detail was pinning down the time of release.  Obtaining sub milli second accuracy has taken us down some tortuous paths.  And because we are interested in sub milli second events, we have had to demonstrate our electrical equipment has suitable time constants. 
One theoretical point not realised in the early days was that the time at which peak force was reached was not the same as the time at which the test mass reversed direction of travel.  The initial intent of the set up was to derive sample length from the measured force time data incorrectly assuming a boundary condition that the time at peak force was also the time of reversal of direction of travel.  When this error was pointed out we tried several different approaches to measure sample length resulting in adopting the HE sensors system to measure the sample length at sub milli second times. 

The clarity of the signals from the HE sensors approach to measuring sample length then caused us to realise we had a problem with timing errors.  Tackling these has required close attention to sources of electrical interference.  We also found some problems with our quick release mechanism.