Controlling gantry aligment(PID)

Hello everyone,
I want to make the alignment of a mini grantry crane. For a known reason, I must re-align it from time to time. The specific features in my project are:

1. The user can JOG in one way or another(Jog up or jog down).
2. The speed is the maximum speed of the motors.
3. The sensors read the distance between the gantry deck and a straight reference (in this case it is the left rail)

I put a little diagram, top view shown :



I read a lot of about controller, I'm not sure if this is a position,speed control because it initially receive a control speed (Point 1) and my sensors read distance in mm. Do I need two PID (one for each motor)? What I expected is:
MaxSpeed-PIDout as the case of misalignment.
Can you help me?? Thank you
Here is a first draft that what im thinking:



Overall the function already exist named ''Anti Skew'': http://static.schneider-electric.us/docs/Machine%20Control/OEM%20Solutions/8000HO1026.pdf



Thank you
Raphael

Im sure theres several ways to do this. Is there a hoist or something that travels horizontal? what tolerance do you need?
The synchronization offset would be the difference between the values of your two sensors

yes there is a trolley with a hoist but i think i dont need to implement them in the gantry controller.
Remeber that the picture of my gantry is a top view.
You think the master folower is a good solution? two PID like your picture? one for the master and the other for the follower, isnt it?
What i the output of virtual axis? Speed?!
i want to follow the distance of 8mm for both sensor(SetPoint)

Thank you its appreciated
Scuse me for my english

I would go for motor1= motor2+your PID (if sensor 1>sensor2 {signal is positive for example The max is about 10% of full power, so your K is 0.1.

shooter said:

I would go for motor1= motor2+your PID (if sensor 1>sensor2 {signal is positive for example The max is about 10% of full power, so your K is 0.1.

Click to expand...

With two PID? One by sensor? with SP of 8mm on the both?

Summary:

1. Two PID, one for each sensor with my SP 8mm
2. Verification of the polarity of the measures of my two sensors
3. PID1 + PID2 or other manipulation'''' depending on the polarity checked at point #2.
4. M1 = M2 + ' result of manipulation of the point #3''

Point #3 is where I really do not know what to do with the handling of the two PID & polarity result.

Thank you
Raphael

the control of motor2 is independent of number 1 just lt #2 on how its done now. you may limit maxspeed to 90% to be able to give motor#1 enough speedcontrol to correct misalignment.

Think about it,

1) you are manipulating speed to control position (natural integrator)
2) You are trying to control alignment which is the result of the accumulated relative speed difference between the two motors


guessing motor top speed is 1600 rpm
what is going to happen when these situations occur?

• a 5 rpm speed difference at 10 rpm is a 50% relative speed difference
• a 5 rpm speed difference at 1500 rpm is a 0.3% relative speed difference

Work out the feedback loop for the forward direction (up) then when going reverse you may find that only the sign of the correction must change


As you are controlling "relative speed" only one PID is needed

Think about only "slowing" down the leading motor - it allows top speed to occur at all times

I would of course do this with 2 position loops following the same master profiler. But if you want to do it the way you are heading, then you only need a trim PID for motor#2. Give motor #2 a speed reference that is the same as the speed of motor #1, but use your sensors to "trim" that reference speed using PID. That way, your PID is doing less work, because it really doesn't need to.