| Hydraulic force standard machine, which has two sets of ram-cylinder assembly, can realize large standard forces by hydraulic multiplication of deadweight forces. Electro-hydraulic servo control system is an important part of hydraulic force standard machine. Generally, it consists of (measuring ram) displacement transducer, servo amplifier (controller) and servo valve. The objective of the work described in this dissertation was to deal with electro-hydraulic servo control system of the machine. A new scheme of double loop (displacement and pressure) feedback computer control system, which is based on virtual instruments technology, is put forward to improve control performance. A PID algorithm commonly used in many control system has been adopted by controller.Based on this scheme, a digital electro-hydraulic servo control system has been made and experimented on NIM 5MN force standard machine. Industrial computer with data acquisition cards is used as the controller. Graphics language — LabVIEW, which has a powerful function, is used as a software development tools. The displacement loop and the pressure loop with different PID parameters are introduced under different circumstances. The result of experiment shows that the relative overshoot of the applied force after a step changes is smaller than 5% and the force stability is better than 0.02% (over 5 minutes). The application of the new digital electro-hydraulic servo control system in hydraulic force standard machine has proved to be successful. It has improved the performance of servo control system and advanced the level of automation of the machine. Details of the new system and the relative theories are described, the experimental results are demonstrated and discussed in the dissertation. |
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