Abstract
Cryocooler vibrational stability is an important issue in IR sensing. Unfortunately, several common approaches to this problem require that the cryocooler transfer function be accurately measured. We present a digital solution using a simple iterative algorithm for an axially aligned dual-piston cooler. We derived a formula for this algorithm to predict the vibration force attenuation Ak equals [1 - (mu) (cos(phi) + j sin(phi) )/1 + (epsilon) ]k where k is the algorithm iteration number, (mu) is an algorithm parameter, (phi) is the maximum absolute error in the measured transfer function phase, and (epsilon) is the relative error in measured transfer function gain. If (mu) is chosen so that 0 less than (mu) less than 2 (1 + (epsilon) ) cos(phi) , the algorithm will provide an exponential vibration attenuation. As long as (phi) less than (pi) /2, it is possible to find a value for (mu) so the algorithm will converge. To demonstrate this property of the algorithm, we constructed a cryocooler vibration model using two large axially-mounted audio speakers mounted on a rigid structure with numerous vibration modes. Speakers were driven using 2 D/A channels and vibration forces were measured using an A/D and an accelerometer mounted on the structure. After accurately measuring the vibration response transfer function of the model, we corrupted phase angles between -(pi) /2 and (pi) /2. In each corrupted transfer function case, the control algorithm quickly converged to greater than 25 db below uncompensated vibration power and within 5 db of the static model vibration floor. Tests were then conducted on a Hughes 65K SSC cryocooler. The algorithm was able to significantly reduce vibrations and remain stable under a variety of changing operation parameters and cooling loads.
