Imin, Imax, Deadband and Ramp in Proportional Valve Amplifiers

Why Settings Are Needed if the Valve Is "Proportional"

The word "proportional" can create the impression that everything should work immediately and linearly: give 5 V out of 10 V and get half the flow. In practice, this is not always how it behaves.

The valve has deadband, friction, spool overlap, tolerances and maximum-current requirements. The hydraulic system has its own limitations too: flow must not open abruptly, pressure shocks must be avoided, and sometimes the maximum value has to be limited intentionally.

That is why the amplifier should not merely accept an input signal. It has to convert that signal correctly into coil current.

What Imax Is

Imax is the maximum coil current the amplifier will output at the maximum command.

This is one of the most important settings. If Imax is set above the rated value, the coil can overheat or the valve can work outside the intended mode. If it is set too low, the valve may not reach the required flow or pressure.

Imax is usually taken from the coil or valve datasheet. Sometimes the documentation specifies rated current; sometimes it gives power and voltage. If only resistance and voltage are available, the current can be estimated, but it is still better to check the manufacturer's documentation.

What Imin Is

Imin is the minimum working current from which the valve starts responding usefully to the command.

It is needed because many valves do not start moving exactly from zero. There is a spring, spool overlap, friction and hydraulic forces. So the first percent of command may change nothing.

Imin removes the empty part of the range and makes control more convenient.

Example: the valve starts to open noticeably only around 0.45 A. Then Imin can be set to 0.45 A. When a working command appears, the amplifier immediately outputs current near the start-of-opening point, and then increases current smoothly up to Imax.

Input and Output Deadband

The deadband can be not only in the valve, but also in the input signal.

For example, a controller outputs 0-10 V, but only the 2-8 V range should be used. Below 2 V, the command is treated as zero; above 8 V, the output is limited to maximum. This is useful when part of the input range is reserved, noisy or unnecessary.

Input boundary settings make this possible:

X < Xmin        -> current 0
Xmin..Xmax      -> current linearly from Imin to Imax
X > Xmax        -> current Imax

Example: 0-10 V -> 0.5-2.0 A

Assume there is a 0-10 V input, but the working range is 2-8 V. The coil should operate from 0.5 to 2.0 A.

Settings:

Result:

This solves four tasks at once: input threshold, output threshold, scaling and maximum current limiting.

What Ramp Is

Ramp is a limit on the rate of current change. If the input command jumps sharply from 0 to 100%, the amplifier does not have to output full current immediately. It can bring the current to the required value smoothly over a specified time.

This is useful in hydraulics because abrupt changes in flow and pressure create:

• jerks;
• pressure shocks;
• noise;
• oscillation;
• higher mechanical loads;
• unpleasant behavior for the operator.

Why Actual Ramp Time Can Differ from Expectations

The actual rate of current change depends not only on the setting, but also on coil physics, supply voltage and the demagnetization method.

During current rise, available supply voltage matters. The higher the coil resistance and inductance, the more slowly current will rise.

During current decay, the important factor is how the driver dissipates coil energy. A freewheel diode gives a softer decay, but not the fastest one. Harder circuits can speed up decay, but they produce higher voltages and place more requirements on the power stage.

How to Tune in Practice

A typical working order is:

1. Set Imax according to the coil datasheet.
2. Apply a small command and find the current at which the valve starts to respond.
3. Set Imin slightly above the range where the valve still does not respond.
4. Set input boundaries if the signal is not the full 0-10 V or 4-20 mA range.
5. Set a moderate ramp, for example 200-500 ms for current rise.
6. Check movement, pressure or flow.
7. If the hydraulic system jerks, increase ramp.
8. If the system is too sluggish, reduce ramp, while watching for jerks and overheating.
9. After that, tune dither.

Typical Mistakes