Verification of the Back-EMF Method for Piston Velocity Measurements | NIST
Verification of the Back-EMF Method for Piston Velocity Measurements | NIST
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发表时间：
2012-07
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通讯作者：
R. Radebaugh;M. Lewis;P. Bradley
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作者：
R. Radebaugh;M. Lewis;P. Bradley
直线压缩机用于驱动脉冲管和斯特林制冷机，当惯性管不能在流量和压力之间提供足够的相移时，可以用线性压缩机代替惯性管作为膨胀器。商用直线压缩机很少安装位置传感器，因此活塞处的光伏功率、流量和流量-压力相位通常是未知的。利用直线电机中的反电动势来测量活塞速度已被提出。将得到的速度与活塞上的加速度计测得的速度进行了比较，结果显示出很好的定性一致性。然而，目前还没有关于它的准确性或与位置传感器进行比较的测量报告。本文报道了用反电势法测量的活塞速度与商用直线压缩机活塞上的线性变位移传感器(LVDT)的速度测量结果的比较。文中还讨论了使用第二压缩机与激光位置传感器的比较。反电势是正常运行和活塞被锁定以防止相同电流移动的情况下的复杂电压差。速度就是反电动势除以马达的力常数。在两台压缩机中分别以16赫兹、30赫兹和60赫兹进行了测量。结果表明，用反电动势方法测得的速度幅值与用LVDT测得的速度幅值在7%以内。激光位置传感器在行程少于全行程的20%时显示出异常行为(20%至30%的偏差)。在较高的笔划下，一致性在约5%以内。在相位方面，在16 Hz和30 Hz时，激光传感器的速度与反电动势的速度在±2°内一致，而在60 Hz时，激光传感器的速度相位比反电动势的速度相位滞后13°。在16赫兹时，LVDT与反电动势的相位在2°范围内一致，但随着频率的增加，LVDT的相位逐渐变窄。在60赫兹时，差值为30°。在耦合储油罐中的压力测量也表明，LVDT引入了相位延迟。结果表明，只要压缩机效率大于30%左右，反电势法的测量不确定度就很低，可以作为一种精密速度传感器。
Linear compressors are used to drive pulse tube and Stirling cryocoolers, and they can be used as expanders in place of inertance tubes when inertance tubes cannot provide sufficient phase shifts between flow and pressure. Commercial linear compressors rarely incorporate position sensors, so PV power, flow rates, and flow-pressure phase at the piston are usually unknown. Use of the back EMF in linear motors to measure piston velocity was previously proposed. A comparison of this derived velocity with that determined from an accelerometer attached to the piston showed good qualitative agreement. However, no measurements have been reported on its accuracy or on comparisons with position sensors. We report here on a comparison of piston velocity measurements determined from the back-EMF method with those from a linear variable displacement transducer (LVDT) attached to a piston of a commercial linear compressor. Comparisons using a second compressor with a laser position sensor are also discussed. The back EMF is the complex voltage difference between normal operation and that where the pistons are locked to prevent movement for the same current. The velocity is simply the back EMF divided by the force constant of the motor. Measurements were made at 16 Hz, 30 Hz, and 60 Hz in both compressors. Our results show that the velocity amplitudes determined from the back-EMF method and the LVDT agree within 7 %. The laser position sensor showed abnormal behavior (20 % to 30 % deviations) for strokes less than about 20 % of full stroke. Agreement was within about 5 % at higher strokes. In terms of phase, the velocities from the laser sensor and the back EMF agreed within ±2° at 16 Hz and 30 Hz, but the velocity phase from the laser sensor lagged that of the back EMF by 13° at 60 Hz. The LVDT phase agreed within 2° with that of the back EMF at 16 Hz, but led as frequency increased. At 60 Hz the difference was 30°. Pressure measurements in a coupled reservoir also indicated the LVDT introduced a phase delay. We conclude that the back-EMF method has low uncertainty and can be used as a precision velocity sensor, as long as the compressor efficiency is greater than about 30 %.