PWM, Dither and Hysteresis in Proportional Valves

Why This Topic Matters

Descriptions of proportional valve amplifiers almost always contain the words PWM and dither. They are sometimes mixed together, although they are different things.

In simple terms:

• power-stage PWM is the driver's internal way to create the required current;
• dither is a deliberately added small current modulation for the valve mechanics.

If these two concepts are confused, the system can be tuned incorrectly. For example, it is easy to assume that a high PWM frequency by itself will solve spool sticking. Sometimes it helps partly, but a proper dither setting is still more convenient and predictable.

What Power-Stage PWM Is

PWM is fast switching of the power transistor. Supply voltage is applied to the coil in pulses, and the coil current cannot jump up and down instantly because of inductance. The regulator changes duty cycle to get the required average current.

For example, if the system needs to hold 1.2 A, the driver measures current and adjusts PWM so that the actual current stays around 1.2 A.

PWM frequency affects:

• audible coil noise;
• power-switch heating;
• current ripple;
• electromagnetic interference;
• the quality of dither generation.

In VL-PVD1-24, the power-stage frequency is 16 kHz.

What Dither Is

Dither is a small low-frequency addition to the current setpoint. It is usually tens or hundreds of hertz, not kilohertz. Dither amplitude is much smaller than the working current.

Its job is to keep the moving parts of the valve in slight micromotion. This helps overcome static friction and reduce hysteresis.

The simplified formula is:

Icmd = Iavg + iDither(t)

where Iavg is the base command current and iDither(t) is the variable addition.

Important: correctly tuned dither should not change the average current. It should add a small motion around the working point.

What Hysteresis Is

Hysteresis means the result depends not only on the current command value, but also on the direction from which that value was reached.

For example, while current is increasing, the valve may start opening at 0.45 A, but while current is decreasing, it may close back at 0.35 A. This creates a loop. For control, that is inconvenient: the same command can produce slightly different flow.

Causes of hysteresis include:

• static friction;
• oil viscosity;
• magnetic hysteresis;
• forces acting on the spool;
• contamination and valve condition;
• mechanical clearances and overlaps.

How to Choose Dither Frequency

Too low a frequency can become noticeable as vibration, noise, pressure pulsation or flow pulsation. Too high a frequency may have almost no mechanical effect, because the coil and valve do not have time to physically follow the modulation.

A practical starting range for many applications is about 100-150 Hz. Further adjustment depends on the valve, oil, pressure and smoothness requirements.

If vibration or pulsation is noticeable, the frequency can be increased or the amplitude reduced. If the valve still moves in steps at small commands, the amplitude can be raised slightly.

How to Choose Dither Amplitude

The amplitude should be the minimum that is sufficient. This is a good rule.

You can start with a small value, for example 0.05-0.10 A peak-to-peak, and then observe the system near the start of opening. If there is sticking, a step or poor repeatability, increase the amplitude carefully. If noise, vibration or pulsation appears, reduce it.

In VL-PVD1-24, amplitude is set as peak-to-peak. So if 0.2 A peak-to-peak is set, the addition around the average current will be approximately ±0.1 A.

Why Dither Cannot Be Increased Forever

Too much dither can make operation worse:

• vibration appears;
• pressure pulsations appear;
• audible noise increases;
• the coil can heat more;
• sensitive hydraulics can start to "breathe".

So dither is not "the more, the better". It is a setting selected for the minimum sufficient effect.

Dither Anti-Clipping

There is another detail: what happens if the average current is close to zero or close to the upper limit? If triangular dither is simply added, part of the signal can be clipped at the 0 A boundary or at Imax. Then the average value becomes distorted.

VL-PVD1-24 automatically limits dither amplitude near the lower and upper current boundaries. This prevents the modulation from exceeding the allowed limits and keeps the average current predictable.

Practical Tuning Procedure

1. First set Imax according to the coil datasheet.
2. Set Imin so that the valve starts working at the required point.
3. Set a moderate ramp so the system does not jerk.
4. Enable dither with a small amplitude.
5. Check small commands; this is where dither is most often needed.
6. Increase the amplitude until the result is stable.
7. If pulsation or noise appears, reduce the amplitude or change the frequency.
8. Check several modes: small, medium and maximum commands.