Running two stepper motors at the same time from one port

Question

Hello,
I have to run two motors at the same time - one used to move a linear ball screw and one to turn a grip. I am doing tension and torsion testing on small wires, but tension and torsion have to be done at the same time. I also have to build in the ability for each motor to take multiple steps, ie. have a motor take x number steps at y velocity and then move x number of steps at z velocity. Both motors need to be able to do this. However, I am having problems getting the motors to run at the same time and to take all the steps. The motors are connected to two RMS Technologies R356 controllers. Both of the controllers are then connected to one RS485 to USB converter card, and that card is connected to one of the USB ports in the computer. Currently, I pass an array of commands to two for loops each containing VISA read and writes. I've attached the vi to look at. All of the timing stuff in the code is because a new command cannot be sent to the controller before the motor is done executing the previous command. If commands are sent right after one another, the controller will only execute the first command and then return with an error. 
Does anyone have a suggestion as to what I am doing wrong?
Thanks in advance for any help. 
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Attachments:
Motor Controller (Sub VI 3).vi ‏41 KB

Answer 1

I'm not too clear on exactly what you are trying to accomplish or some of the details.
Are there equal number of torsion and tension steps?
Is there a pairing between a torsion and tension?  For example, you want to tension and torsion simultaneously, but those two together make a complete step?
Do you want to wait until both the torsion and tension have completed before moving on to the next torsion/tension pair?
Is there any time delay you want to hold at before moving on to the next step.
When working with state machines, it is important not only to define the states, but also the logical stepping from one state to the next, or alternatives based on conditions.  It could be flowchard, but sometimes people call them state charts.
Right now your latest posting has goes to "Write to Motor" event though and will wait there forever because there is nothing in the queue to write out through the VISA port.
Let's assume your first state would happen to be Enqueue Tension (in order to get something in the queue), you will actually queue up all your tension commands at once with the For loop.  Then i will be 1 less than the size of the array it always will be because for an array size n, i will go from 0 to n-1 on each iteration.  So your false case will run and you will go to "Write to Motor".  Write to Motor will run and dequeue the first command then go to Waiting for Response.  Waiting for response will iterate through the while loop until 66 or 67 comes back.  I assume only 66 will come back for tension since you have yet to write a torsion command.  Once that comes back, it will go to Enqueue Tension again and insert a whole other series of commands.  Then go back to write Motor and will take out the 2nd queue element from the first set of commands your wrote.  It will stay in a Tension mode and the Queue will grow endlessly putting in a whole new series for each element that you dequeue.
A state machine is the way to go, but I can't really sketch one out for you because I'm not sure exactly how your applications is supposed to work.  If a tension and torsion command are paired together,  perhaps you should just queue them together and write them out at once.   But the key thing is to sketch out on paper your application and flowchart exactly how the program should progress from step to step.
PS:  One other thing, make sure you wire the queue reference wire through all states.