How to use to control something with both forward and reverse controls?

[hoeken]

I am working on a project to automate a reverse osmosis watermaker on my boat. Part of that is controlling a multi-turn, high pressure gate valve with a pressure sensor for feedback. Currently I'm using a continuous rotation servo, I'll probably switch to a simple DC motor next.

In the code below, you can see I'm using QuickPID to control the valve. It works okay, but there is some oscillation when it has to switch directions. It takes ~30s to hunt down to the proper set point. If I make that switch between forward and reverse smaller, then the oscillations get worse. Tuning the PID loop has been challenging because the control has a bigger effect when the pressure is high (near the setpoint) and a small effect in the beginning.

The QuickPID library supports forward / reverse modes, but you have to switch between them. I added some code to do that, I'm wondering if this is the right way to approach this?

Is it possible to setup QuickPID to give an output that represents both direction and magnitude? Like if it outputs +255, I rotate the valve CW, and if it outputs -255 I rotate the valve CCW?

void Brineomatic::manageHighPressureValve()
{
  float angle;

  if (currentStatus != Status::IDLE) {
    if (hasHighPressureValve()) {
      if (membranePressureTarget >= 0) {

        // change directions if we are too far off our target
        // this is only for RUNNING mode when we are close to our target
        if (currentStatus == Status::RUNNING) {
          if (currentMembranePressure < membranePressureTarget * 0.975 && membranePressurePID.GetDirection() == (uint8_t)QuickPID::Action::reverse) {
            membranePressurePID.SetControllerDirection(QuickPID::Action::direct);
          } else if (currentMembranePressure > membranePressureTarget * 1.025 && membranePressurePID.GetDirection() == (uint8_t)QuickPID::Action::direct) {
            membranePressurePID.SetControllerDirection(QuickPID::Action::reverse);
          }
        }

        // run our PID calculations
        if (membranePressurePID.Compute()) {
          // forwards / Clockwise / Close valve / Pressure UP
          if (membranePressurePID.GetDirection() == 0)
            angle = map(membranePressurePIDOutput, 0, 255, highPressureValveCloseMin, highPressureValveCloseMax);
          // reverse / Counterclockwise / Open Valve / Pressure DOWN
          else
            angle = map(membranePressurePIDOutput, 0, 255, highPressureValveOpenMin, highPressureValveOpenMax);

          // actually control the valve
          highPressureValve->write(angle);
        }
      }
    }
  }
}

[hoeken]

Just in case anyone every has a similar issue, you can simply set the output range to be both negative and positive which will allow you bi-directional control like this:

 pid.SetOutputLimits(-255, 255);

If its positive, control forwards, if negative, control backwards.