+1 for what Peter said. Position control over hydraulics, with any degree of finesse, is not a trivial task for a PLC.
Nick
Yes, in fact I was going to start a thread to that effect on a Chinese hydraulic forum. We see a lot of people try and give up after wasting much time and money. Many can't even figure out the basic like how to control the speed when the actual position is at 100mm and the command position is 200mm. The actuator jumps quickly to the new command position shaking the machine and over shoots the command position. The PID in most PLCs is not suitable for these simple motion applications.
PLCs shouldn’t be used to directly control closed loop hydraulic servos systems.
1. PLC scan times are too slow
2. PLC scan times are not consistent. Adding more code to the PLC makes the scan times slower and more inconsistent.
3. PLC analogue inputs and outputs are slow and may not have enough resolution.
4. PLCs do not have a trajectory planner. It must be written.
5. Diagnostics and debugging tools are lacking.
6. The PLC PID is written for process control applications. Time constants are in minutes instead of seconds or milliseconds.
7. PLC's do not have feed forward control to reduce following error.
8. Most important! Writing optimal hydraulic servo control code requires understanding hydraulics, control theory and programming.
PLC不应用来直接控制闭环液压伺服系统。
1. PLC的扫描时间太慢。
2. PLC的扫描时间不具备一致性。加入更多的代码使PLC扫描时间更慢,更加不一致。
3. PLC的模拟输入和输出缓慢,没有足够的分辨率。
4. PLC没有运动轨迹规划功能,必须人为编写。
5. PLC缺乏诊断和调试工具。
6. PLC的PID是为过程控制应用而编写的,其时间常数是分钟级,而非秒或毫秒级。
7. PLC的不具备前馈控制以减少跟踪误差。
8. 最重要的!编写出最佳的液压伺服运动控制程序,须要具备对液压系统和控制理论的理解,以及编程优化的综合能力。