PID control with an H bridge

Hi everyone,
I have been working on a project with labview for the last few months now and I have reached a point where I am in need of some assistance.
Basically, this next part of my project requires me to drive a linear actuator to apply a force to the end of a shaft, with a load cell in between the actuator and shaft, providing me with the actual force applied.
I need to use a PID controller to maintain a constant specified weight onto the end of the shaft, this in itself should be simple but I have an issue with the way the actuator is driven.
The actuator is being driven via an H bridge that has 3 seperate inputs to drive the actuator:
direction = logic high/low 5v high 0v low
break= logic high/low 5v high 0v low
Speed= Pulse width modulation
My plan was to use a basic PID controller to adjust the PWM going into the H-bridge by adjusting its duty cycle between 0.10 (fast) 0.90(slow) and then using basic comparison functions between the set point and process variable to control the logic to direction and break. So for example.....
I want it to apply 5kg onto the end of the shaft
my set point is 5 and the variable is 0 so labview applies a logic high to direction (to go forward) and logic low to break (to let it move) and the PWM is adjusted to zero in on the set point.
I cannot get this to work however and was wondering if there was a better way to do this?
I appreciate that this may seem unclear, so i hope I have explained it well enough!
Many thanks
HCook
Solved!
Go to Solution.

Hello,
Thank you for very much for your reply.
I really should have explained this better, but I only had a few minutes to post my code. I should also mention that Im teaching myself labview, so Im sure I could do this a lot better.
OK, so here is whats happening...
This is just a secttion of my code, as Im not allowed to upload the whole programme.
This section is meant to control the weight being applied onto the end of a shaft that is defined by the user.
By flicking between auto and manual the user can choose between the PID and comparison moving the actuator or themselves manually.
The two express vis that are outputting an analog voltage are to provide the logic to the h bridge.
To give me Proportional control, I have simply used a comparison function to look at the weight on bit i have requested (elsewhere in the programme and hence the local variable) and the actual weight signal from a loadcell being brought into labview from an anolog input. So for example, If i request 2kg then the comparison between the two signals then tells the h bridge to move the actuator to apply more or less wait depending on the signal.
I have got this bit working fine.
Now, the speed with which the actuator needs to move, must be continuosly adjusted to provide a precise adjustment to the weight and I wanted to do this by continuosly adjusting the duty cycle of the PWM through the PID. Thank you for pointing out the loop issue, preventing the PID from passing on the adjustment, I had not spotted this!
I will re build the code and condense it down into a smaller number of loops.
The WOB speed task, which is outputting the PWM to the h bridge keeps telling me that the specified resource is reserved and will not function after my inital run. Does this have anything to do with the PID not working ?
Also, when I run the programme in manual mode, where by I move the actuator manually and adjust the weight based on the load cell signal, I get an error when I adjust the duty cycle. It informs me that the PWM must complete a full cycle before the duty cycle can be adjusted. Now the rate at which im changing the duty cycle manually is no where near the same speed a PID may adjust it and therefore I can see myself having the same problem in auto mode.
Thank you for your help!
Hcook

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PID-RichLevelControl.vi ‏91 KB

Rich-MechatronicsStudent wrote:My questions are;
- How do I connect the 'output variable' of the PID subvi to my DAQ assistant digital output(both DAQ assistants are set at 1 sample on demand)?
I have a 'number to boolean array' function, but this gives an error saying that the number of tasks generated are not the same...
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Solved!
Go to Solution.

Hi blokk,
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Attachments:
Control.vi ‏277 KB
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Hello Zach!
First of all, I want to thank you for your kind interest on my problem!!
I must describe you the desired process with the Control.vi. There are two AI channels writing down the acceleration of a structure in two different points (for velocity reasons I use Binary Type storage for my data). One AO channel for structure sinusoid excitation (shaker) and two CO channels for controlling two solenoid valves with which I want to control my structure acceleration (minimize it as well and fast as possible)!!
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I have to mention that I work on a PCI-6289 and Labview 8.5.
With best regards!
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Attachments:
Control.vi ‏288 KB
equal within tolerance.vi ‏9 KB
ReadBinar.vi ‏163 KB

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Go to Solution.

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asdfs wrote:
Per my other posts, I am testing a friend's software to see if this is what we would like to use. I would prefer to use their software for our initial work to help offset the cost of the software. Problem is their software is 8.0 pro without the extra PID stuff. I am curious how one gets started creating a custom PID algorithm to be used in series between an input and an output. I have viewed several tutorials but all seem to simply point at the PID toolkit that is not availalbe to me right now. I realize it might be more work to create one but probably the only option at this point.
To simply the process, lets just compare a single input and output where the software will control the output to obtain to desired input. in tutorial, I was following right along until the PID loop was added and that seems to be just a drop and go type of deal in which would rarely apply to a real work application. I am simply trying to learn how PID systems are written into the block diagrams.
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PID_notch filter.png ‏21 KB
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How can I make and wire reffercence signals(ramp,sine,square) to excite and evaluate the behaviour of the system wih PID control?

Dear all,
I am Tri, I am a beginer with LV program. Actually I used LV for my reasearch, I had tried wiht very simple thing but the problem arise because I am not familiar with LA. Why I know that problem is simple because I compare with MatLab that I had somtime used before.
I make a loop of PID control, It worked properly but I don't know how to make a refference signal to wire to the loop for excitating to evaluate the behaviour of the system. Please give me your help.
Thanks

Hi Tri,
I've employed the "simple PID.vi" without a problem, but your example is more complex. I think you use a Function generator to modulate DAQ inputs?
Perhaps someone else will be able to infer your intention and identify a flaw in the logic.
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1) \File\Save with Options
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But first, It would be helpful to us if you clean-up the code - straighten wire-paths, minimize bends in wires, and try to avoid "tall" diagrams. Scrolling diagrams up and down is "bad form". If scrolling is necessary, left&right is preferrable!
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Attachments:
feedback design1.JPG ‏25 KB
feedback system1.JPG ‏42 KB

Dear Garry,
Do you have a motion controller to interface the MID-7654 to your
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Why do I get this error when running the PID Control-Single Channel.vi: "Requested value is not a supported value for this property."

Error -200077 occurred at Property Node DAQmx Timing (arg 3) in DAQmx Timing (Sample Clock).vi:1->PID Control-Single Channel.vi
Requested value is not a supported value for this property.
Property: SampQuant.SampMode
You Have Requested: Hardware Timed Single Point
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Attachments:
PID Control-Single Channel.vi ‏53 KB

Actually I have already tried a few things you suggested and got as far as the DAQmx Write vi. The error at the DAQmx was a buffer error indicating i needed to set it to 2 instead of the default of 1. Tried using the DAQmx Input and Output Buffer vi to change buffer, with no luck.
Attachments:
PID Control-Single Channel.vi ‏53 KB

PID control with an H bridge

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