Dither Generation for Proportional Valves

[cyber_dev]

Hi all,

I want to design a proportional valve controller using closed loop (current feedback) method. I have found some documents regarding it. Please see the links below. As you know a dither signal is used to reduce the effect of stiction and hysteresis.

**broken link removed**
https://www.axiomatic.com/pulse-width-modulation.pdf

In my design I will use a powerful MCU (Cortex M4 based) to generate high frequency PWM. All the parameters (freq, dither amplitude...) will be software controlled. For the output circuit I can use a FET or H bridge IC with current sense. For the dither circuit what should I do? How can I superimpose it on output current?

The dither signal should have following properties.

Frequency: 50 to 400 Hz
Amplitude: %10 of output current

Thanks in advance.

 

[FvM]

A powerful MCU can be expected to generate the dither in software, I think. In so far there won't be a particular circuit.

 

[WimRFP]

Hello,

I understand you will use "high frequency" PWM in combination with current measurement/feedback. When using > 5 kHz PWM, you can just add the required dither signal (frequency and amplitude) to the current setting (in SW). See it as having noise on the current setpoint value. Your current control loop will very likely be fast enough (first order process) to track the dither signal that you add. This is the easiest method (if the loop is fast enough)

If your current control loop isn't fast enough, you may add the dither by modulating the actual pulse width. For example when the actual PWM is 15% to get a certain current and you want 10% ripple, you should modulate the pulse width with 0.1*15% = 1.5%. However this requires more tweaking as the actual current ripple you introduce depends on the inductance of the selonoid (so it can be less then 10%) and you need to be sure the main current feedback loop does not respond to the inserted ripple.

 

[cyber_dev]

@FvM
How can I implement it?

@WinRFP
Indeed low frequency PWM might be sufficient.

I have investigated some of commercial valve drivers. The following parameters are sw configurable: Imax, Imin, ramp times in ms (up-down), dither freq. and amplitude. I will try to do the same thing. I am not experienced with valve drivers. I didn't understand you very well. Could you explain more? Assume that dither amplitude is configured to 250 mA at 100 Hz. Also PWM frequency is given 10 kHz. How can I produce it?

 

[cyber_dev]

Hi again,

I would like to learn that how I can superimpose PWM and dither. I had a look at a valve controller. It doesn't have additional circuit for dither.

 

[cyber_dev]

Hello,

I understand you will use "high frequency" PWM in combination with current measurement/feedback. When using > 5 kHz PWM, you can just add the required dither signal (frequency and amplitude) to the current setting (in SW). See it as having noise on the current setpoint value. Your current control loop will very likely be fast enough (first order process) to track the dither signal that you add. This is the easiest method (if the loop is fast enough)

Is 1 ms periodic timer sufficient for this purpose? I think the calculation of next value of PID should be less than a few hundred us with my MCU. Could you show me a piece of pseudo code to achieve it?