0
Answered

Ajdusting Dither Amplitude

Aurelio Romano 4 years ago in IQANdesign • updated by Gustav Widén (System support) 3 weeks ago 3
Is there a way to change the dither amplitude in the IQANdesign software? We have an application where we believe the dither amplitude is causing the valve to go unstable and would like to decrease the amplitude to do some testing.
Answered
Hi Aurelio,

On the IQAN-XT2, there are two properties for each COUT, one for dither frequency, and one for amplitude.
The XT2 is able to control the dither amplitude independent of the dither frequency by using a high-frequency PWM for the COUT control, and superimposing a sinosoidial low frequency (e.g. 100 Hz) dither on top of this.
For some applications, like the control of hydrostatic pumps that are very sensitive to the dither, the XT2 can still be the best choice.


On the other IQAN modules, such as the MC2, MC3 or XA2, the control of dither amplitude is indirect. 
The dither amplitude is given by a combination of the low frequency PWM used for the COUT control, and the inductance of the coil.
These modules are also used in applications where a smaller dither is needed (e.g. some hydrostatic pumps). To reduce the dither amplitude, increase the COUT PWM frequncy.

Is there a table showing the relationship between PWM frequency and the dither amplitude?


As an alternative solution, would it make sense to add an artificial dither by creating a sinusoidal wave (timer + math) to add on top of the actual control signal?

I don't have any table like that, but some scope measurement could help illustrate how the dither amplitude change with frequency.  It is important to understand that the load characteristics and the commanded current affect the result. 

Here are two scope measurements from a 24 PVC25 coil (predecessor of the PS25 coil, used on e.g. the Parker L90LS valve) 

Using an IQAN-XA2.   

 PVC25 24V coil, COUT command 200 mA. Comparison between 100Hz (recommended)and 333 Hz:

 PVC25 24V coil, COUT command 500 mA. Comparison between 100Hz (recommended) and 333 Hz:

For our mobile directional valves, the Parker L90, K220, P70 and so forth, I would not recommend any additional ripple, these valves perform best with an IQAN COUT for closed loop current control and the ripple you naturally with a 90 -125 Hz PWM frequency.