PID for Danfoss proportional valve

Hardware Autosteer
5

That is a great explanation even for me, who has dabbled with PID for 5+ years now.

6

What type of a Danfoss valve is the 7530 using? If it has the PVE head (the black box type), that should have a built-in closed loop control like this, so you shouldn’t have any issues with the stuff @ruan is mentioning, if I’m not totally off?

image

I’m currently putting in a PVG16 valve with the PVHC head (two individual coils), does the JD use this then:

image

Here the coils aren’t directly driving the main spool, but operate two pilot hydraulic valves that move the main spool. The hysteresis limits look quite large on the diagram…

image

7

The Danfoss valve on the 7530 has 4 wires. Error, Ground, U DC, and U S.

It’s a PVE valve, but we never were definite if it’s a A, H, or S model. John Deere applied their own number to the valve, which obfuscates things.

PVEH - PVES
PVES

As far as I can tell, both these valves should perform somewhat the same, using a different operation. I see that in the spec sheet the PVES is listed as a 0% hysteresis.

Later today, we will be working on this again. Hopefully we will have some more info to report. Maybe even success?!

Thanks @ruan and @nut for chiming in here, as I wasn’t thinking along the lines of hysteresis. I’ll keep an eye open today and maybe test the hysteresis and PWM speed.

8

Update: My friend just told me he gave Danfoss the number on the valve, and they responded and said the valve is a PVES model, which has 0% hysteresis. Sorry for any confusion.

9

Ok, so we got the proportional valve working like a top. The important parts are:

  1. find the hysteresis on the PWM for valve engagement (no, it doesn’t have 0% hysteresis)
    using analogWrite():
    *128 did not move
    *118, 138 put pressure on the wheels without moving left or right
    *108, 148 moved slowly left and right
    in real life we used minimum PWM of 30 (98, 158)

  2. use less Proportional (after all, a hydraulic valve is way faster than a steering motor)

  3. add Integral

  4. add Derivative

which leads us to changing lines around 530 in Autosteer_USB.ino to:

//Settings Header has been found, 8 bytes are the settings
if (Serial.available() > 7 && isSettingFound)
{
isSettingFound = false; //reset the flag
//change the factors as required for your own PID values
steerSettings.Kp = (float)Serial.read() * 0.2; // read Kp from AgOpenGPS
steerSettings.Ki = (float)Serial.read() * 0.1; // read Ki from AgOpenGPS
steerSettings.Kd = (float)Serial.read() * 0.1; // read Kd from AgOpenGPS
steerSettings.Ko = (float)Serial.read() * 0.1; // read Ko from AgOpenGPS
steerSettings.steeringPositionZero = 1533 + Serial.read(); //read steering zero offset
steerSettings.minPWMValue = Serial.read(); //read the minimum amount of PWM for instant on
steerSettings.maxIntegralValue = Serial.read() * 2; //
steerSettings.steerSensorCounts = Serial.read(); //sent as 10 times the setting displayed in AOG
EEPROM.put(10, steerSettings);
}

and replacing function calcSteeringPID in AutosteerPID.ino with:

void calcSteeringPID(void)
{

//Proportional
pidOutput = steerSettings.Kp * steerAngleError ;
//Integral
outputSum += (steerSettings.Ki * steerAngleError);
if (outputSum < -steerSettings.maxIntegralValue){
outputSum = -steerSettings.maxIntegralValue;
}
else if (outputSum > steerSettings.maxIntegralValue){
outputSum = steerSettings.maxIntegralValue;
}
//Derivative
dInput = steerAngleError - lastSteerAngleError;
pidOutput += outputSum - (steerSettings.Kd * dInput);

pwmDrive = (constrain(pidOutput, -250, 250));

//add min throttle factor so no delay from motor resistance.
if (pwmDrive < 0 ) pwmDrive -= steerSettings.minPWMValue;
else if (pwmDrive > 0 ) pwmDrive += steerSettings.minPWMValue;

if (pwmDrive > 255) pwmDrive = 255;
if (pwmDrive < -255) pwmDrive = -255;

if (aogSettings.MotorDriveDirection) pwmDrive *= -1;

lastSteerAngleError = steerAngleError;

}

Now do note that this only works with the 4.1.12 release, as Integral, Integral max, and Output gain have been removed since. It would be nice of @BrianTee_Admin to put Integral, Integral max and Derivative back in again. Hint, hint :smiley: The Output gain isn’t really useful, as that only multiplies the final result, which is better adjusted individually in Proportional, Integral, and Derivative.

The whole .ino sketch I used can be found here.

Things to do:

  1. Use a better way of resetting the Integral and Derivative when shutting down autosteer and turning it back on. Namely, when turning it back on, the kickback of Derivative or lack of Integral makes autosteer turn sharply (even when close to the AB line) until the values are built up again. Maybe the solution would be to keep on calculating when autosteer is turned off, but not set the valve. At this time, all Integral and Derivative is cleared when the autosteer is turned off.
1 Like
10

Did you see this post from grabik and also the next posts from him?

PID.ino

PID calculation:

*//Proportional
pValue = steerSettings.Kp * steerAngleError *steerSettings.Ko;

/* //Derivative
dError = steerAngleError - lastLastError;
dValue = Kd * (dError) * Ko;

//save history of errors
lastLastError = lastError;
lastError = steerAngleError;
*/

drive = pValue;// + dValue;
pwmDrive = (constrain(drive, -255, 255));

//add throttle factor so no delay from motor resistance.
if (pwmDrive < 0 ) pwmDrive -= steerSettings.minPWMValue;
else if (pwmDrive > 0 ) pwmDrive += steerSettings.minPWMValue;

if (pwmDrive > 255) pwmDrive = 255;
if (pwmDrive < -255) pwmDrive = -255;
*

8 byte PWM resolution on arduino nano(255) , for driver direction DIR pin, and PWM ,DIR pin LOW left side -255 max write to PWM 255 , right side DIR pin HIGH 255 max write to PWM 255 for max duty cycle.

For Danfoss valve not need DIR pin for change direction.

11

Sorry for not mentioning… Yes, I also included the

pwmDrive = pwmDrive / 4;
pwmDrive = pwmDrive + 128; // add Center Pos.

as explained in that topic. That got the IBT_2 working with the valve, but does not necessarily get AgOpenGPS working with the valve.

From reading through that topic, it also appears to me that he is not using Integral.

12

According to the following text from the pid for dummies, it is not important to have derivative.

Derivative Action – predicting the future

OK, so the combination of P and I action seems to cover all the bases and do a pretty good job of controlling our system. That is the reason that PI controllers are the most prevalent. They do the job well enough and keep things simple. Great.

But engineers being engineers are always looking to tweak performance.

They do this in a PID loop by adding the final ingredient: Derivative Action.

So adding derivative action can allow you to have bigger P and I gains and still keep the loop stable, giving you a faster response and better loop performance.

If you think about it, Derivative action improves the controller action because it predicts what is yet to happen by projecting the current rate of change into the future. This means that it is not using the current measured value, but a future measured value.

The units used for derivative action describe how far into the future you want to look. i.e. If derivative action is 20 seconds, the derivative term will project the current rate of change 20 seconds into the future.

The big problem with D control is that if you have noise on your signal (which looks like a bunch of spikes with steep sides) this confuses the hell out of the algorithm. It looks at the slope of the noise-spike and thinks:

“Holy crap! This process is changing quickly, lets pile on the D Action!!!”

And your control output jumps all over the place, messing up your control.

Of course you can try and filter the noise out, but my advice is that, unless PI control is really slow, don’t worry about switching D on.

13

Maybe a delay on power (so signal can be built up first)
If sharp turn is always to same side, it could be a Danfoss thing when powered on. Could be checked by applying center voltage to signal when turning power on. Edit: center voltage meaning 6v if you have 12v as supply)

14

Isn’t the Danfoss supposed to have an analog signal in since it has its own PWM generator with LVDT feedback?

15

That’s my understanding as well. You need DC supply voltage on one pin and then a control voltage on the other. Hysteresis shouldn’t exist as there’s a displacement feedback in the valve controller.

Here’s some discussion & working setups in another thread .ino for PCB+MD13S+Danfoss valve You should see something like 25-75% of U_DC in U_S, check out the pictures towards the back of the thread from @Kaupoi and @Larsvest

16

We tried putting PWM to the U S pin, and everything worked. A lot of analog input controllers out there accept true analog, and they also accept PWM.
Danfoss PWM connection

17

I think I have the PID ‘turn on’ figured out. Now to test it yet. :face_with_raised_eyebrow:

The Integral can not be calculating while not steering, since it would build up to the Integral max over time, even if off of the AB line by 1%, for instance.

The ‘last’ value of the Derivative needs to be saved constantly, as this will be used to calculate the Derivative once the steering is on again.

18

The only value used for PID is the P, the rest have been used for other purposes like low and high max pwm etc.

I understand they “work” with PWM, but have your tried with an analog signal? For example if you sent a 50% duty cycle, half the time the danfoss would think wide open , the other half closed - that would work however with the LVDT feedback I’m not so sure that is the best scenario for its feedback control.

All i’m saying is worth a try.

19

I’d like to disagree on the theory that the Danfoss thinks it should be wide open half the time and closed half the time with 50% duty cycle. We can get it to steer left by putting 12Vdc to the U DC and U S pins, but putting 12V to the U DC pin and 0 Vdc to the U S pin does nothing. We can get it to steer right by connecting the Nano’s 3V or 5V pin to the U S pin. The 3V steers faster to the right than the 5V. But like I said before, 0V on the signal pin gets ignored.

20

I recall the signal is 0.25 - 0.75 times U_DC, so with 12 volts 3 volts full the other way and 9 volts the other.

1 Like
21

Yes exactly. Outside of those limits, it should do nothing. It would seem the Danfoss was designed for use with like a potentiometer type of installation (analog signal) for flow control. Being 1/2 DC allows you flow in two directions without needing a corresponding negative supply.

Anyway, i hope someone at least tries the type of input signal the valve was designed for.

22

I am using an IBT2 to control the steering on an 8330 with Danfoss. I have 12v on the Udc pin, and the pwm signals from the arduino sets the Us. 6V steers straight ahead, 9V to the left and 3V to the right. I have some pics JD 8230 hydraulic steering . Its not a clean setup but it has been working nice on my one field test cultivating. @theGtknerd i noticed while i was originally setting up that with 1 hz on gps it was all over the place, but now at 10hz its smooth. Any chance that could be an issue for you? I have been using v3.9 with the coffeetrac button ino. I think that with the code written like i have it below that it seems to be working fine on my bench test setup but no actual field experience yet. I am a total arduino beginner so if anything in this coding seems wrong please let me know.

Autosteer PID

void motorDrive(void)
{
// Used with Cytron MD30C Driver
// Steering Motor
// Dir + PWM Signal
if (aogSettings.CytronDriver)
{
pwmDisplay = pwmDrive;

  //fast set the direction accordingly (this is pin DIR1_RL_ENABLE, port D, 4)
  if (pwmDrive >= 0) bitSet(PORTD, 4);  //set the correct direction
  else   
  {
    bitClear(PORTD, 4); 
    pwmDrive = -1 * pwmDrive;  
  }

  //write out the 0 to 255 value 
  analogWrite(PWM1_LPWM, pwmDrive);
}

else
{ // Danfoss: PWM 25% On = Left Position max (below Valve=Center)
// Danfoss: PWM 50% On = Center Position
// Danfoss: PWM 75% On = Right Position max (above Valve=Center)
// Used with IBT 2 Driver for Danfoss
// Dir1 connected to BOTH enables
// PWM Left + PWM Right Signal

pwmDrive = pwmDrive / 4;
pwmDrive = pwmDrive + 128; // add Center Pos.

pwmDisplay = pwmDrive;
analogWrite(PWM1_LPWM, pwmDrive),  // apply varying voltage to the Usignal valve pin
digitalWrite(PWM2_RPWM, HIGH);     // apply 100% DC current to Udc

}
}

23

danfoss should indeed do nothing when outside the 0.25-0.75 Udc on the Us pin (safety for ground or supply shorts) .
Udc should also be stabilized (not directly from battery/alternator).
When Udc is been fed with stable 12V, one can use a industry standard 0-10V analog signal to control Us.

24

So, after a hectic summer, and a still busy winter, I will post an update.

We got the Danfoss valve to work using the code I posted on Github. My friend (who owns the tractor) also spoke directly with a Danfoss representative, who tested a PVE valve on the test bench. The Danfoss tech guy recommended using PWM 4KHZ or higher. This said, our Danfoss valve worked fine on the default PWM speed. Well, that is after we had the PID set up properly. The end result was that we had to turn up the minimum PWM quite a bit, and keep fairly low values in the PID. Sorry, I don’t have all those values with me. However, this only worked with old versions of AOG, which brings me to:

We wanted to use the newest AOG (several reasons, I forget what all). When I discovered the ESP32 project that doesn’t use steer characteristics info coming from AOG, but can be set in the built in webUI, we switched to the ESP32. So far we are happy with the performance, and I am now working toward a PCB to plug the ESP32 into as our setup looks crummy, and we experience occasional disconnects. It’s not surprising, given the state of wires in our setup.

There’s some small changes I made to the ESP32 code, and when I have proved the code/PCB a bit, I will see about making a GitHub repository to share the PCB, code and BOM. Until next time, gotta run…

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