In hydraulic motion control, with older flapper-nozzle and jet-pipe servo valves, or for low-performance servo or proportional valves, the valves may exhibit significant static friction so that after a period of holding position, the servo valve sticks, resulting in a delay in the next motion. To prevent this, valve drivers apply dither to the PWM signal going to the valve, which results in just enough small motion on the spool to keep it from sticking. This dither helps reduce hysteresis and can improve control.
With high-performance hydraulic motion, using high-performance valves, usually with a spool-in-sleeve design and on-board electronics, we at Delta Motion have rarely, if ever, encountered a need for dither. The spool-in-sleeve design reduces friction, the linear motors are much more powerful than the weak solenoids of lower-performing valves, and the algorithms in the valve electronics are very robust. In addition, with these precise systems, the small motion caused by dither can result in worse performance, such as greater errors when holding position. Consider a system that can hold position within 1 µm and smoothly move just a few microns over a set time - any disturbance, such as dither, causes great degradation of accuracy.
The requests we at Delta Motion receive for supporting dither usually appear to stem from very old, obsolete requirements, or apply to old valves, or apply to low-performing valves with solenoids and a basic valve driver, which are normally not used together with our high-performance motion controllers.
If your system uses a high-performance valve, you very likely do not need to use dither.
If you are using a lower-performing solenoid valve, you may need dither. It may be that the valve driver for such a valve may have the option of applying dither, which would be the best method in this situation.
If you feel your system really needs dither, and the valve driver doesn’t support it, it can be implemented in the RMC as shown in the attached RMCTools project, which applies a dither to the Output Bias. For best results, export the user program, then import it to your project, which will include the variables. You can choose the frequency and amplitude of the dither. This example is for the RMC75 and works for the RMC150 and RMC200, but keep in mind that the RMC200 Output Bias is in percent, whereas the Output Bias for the RMC75 and RMC150 is in Volts.
The values to use for the frequency and amplitude of the dither vary. Hydraforce suggests 70 - 250 Hz for their flow and directional control valves and 200-300 Hz for proportional pressure control valves, and recommends tuning the dither frequency as low as possible. Parker suggests 200 Hz for their traditional servo valves. The amplitude is set as part of a tuning process – adjusting it and watching for the control to improve.
Dither.rmcproj (29.5 KB)