Closed loop configuration question

[Wayne H.]

I have a motor(with encoder feedback) attached to a linear actuator(with end limit switches).
The motor has a commercially bought servo drive for control. 
The servo drive will accept either a step/direction (2 seperate TTL digital pulse train inputs) or an analog -10 to 10vdc input for control. 
The purpose is to drive a linear actuator(continiously in and out) in closed loop operation utilizing a ( (SV) Setpoint variable)value from a file converted to a frequency to compare with an actual ( (PV) Position variable) measured frequency.
I have created and experimented with individual vi's allows analog control and digital pulse train control (thankfully with the help of examples). 
Before I pose my question, I would like to make the following observations:  It is my understanding that Closed loop control means that I dont need to know an exact position at which to drive, but constant comparision of PV and SV through PID applictation.  Without getting into any proprietery information I can say that the constant positioning of the linear actuator will produce a latency of 2 to 3 seconds based on the time the actuator moves to a new position and when the PV will change.  While experimenting with the analog input, i noticed imediate response to motor velocity, but after the motor is stopped, position is not held in place.  However, while experimenting with the Digital pulse train input, I noticed that the servo drive can only accept one command at one time; if, halfway through a move, position error produces a response to move the linear actuator in the opposite or different direction, the origional move must finish first. 
Can anyone recommend the proper configuration for the closed loop control i have described?
If I can make the system work with the servo drive/motor I plan to use the simple (pci 6014) daq card with the Analog out, or utilize the digital out.
If I cant get this to work, we do have a pxi with 7344 motion card(I would like to exhaust all efforts to use the PCI 6014 card).
Depending on where I go from here, I planned to use the PID vi's for the loop control.
Thanks,
Wayne Hilburn

[Jochen]

Wayne,

in general due to the non-deterministic behavior of Windows you will never be able to create a reliable motion control system with a PCI-6014. I have answered similar questions multiple times in the forums (e. g.  here and here).

Best regards,

Jochen Klier
National Instruments Germany

[Wayne H.]

Thanks for the reply Jochen.  I realize there is a built-in latency with windows but I think the I/O control would be ok.  A change in actuator position will not result in an immediate change in process variable;  Is there a way to measure the latency or is it calculated?  A satisfactory reaction time could be from 1 to 1.5 sec.
Use of the PCI-6014 is to supply the control output to the servo drive/amp, and not to drive the motor itself.  As stated earlier, while using the 6014 board, I have the choice of digital or analog output.
Currently I am at a point where I must choose which configuration, analog control or digital control(in the form of digital pulse train), (i am inserting from first message)
While experimenting with the analog input, i noticed imediate response to motor velocity, but after the motor is stopped, position is not held in place.  However, while experimenting with the Digital pulse train input, I noticed that the servo drive can only accept one command at one time; if, halfway through a move, position error produces a response to move the linear actuator in the opposite or different direction, the origional move must finish first.  .
I dont claim to understand all the limitations with the specific boards, however, i am using an approach that is showing me the characteristics(a couple are listed in the above paragraph)  of the hardware and software configurations.
So I am really back to my origional question;  Which configuration would be better for closed loop control, analog or digital pulse train?

Thanks,
Wayne Hilburn

[Brian_Beal]

What I don't understand from your post is how you are closing the loop.  Is it truly a servo system, or are you using a stepper motor?  What is the encoder wired into?  What is the brand/model of the drive?  This will help us to answer your questions.

[Wayne H.]

The motor is a brushless DC servo motor bought in a package with the drive.  The drive is a world servo SST drive.  This drive can be configured for analog or digital control.  The Motor has a quadrature encoder which, at this point, I dont intend to use it in the loop. 
The world servo website is http://www.worldservo.com/html/fbm.htm

The labview system  will have two inputs:  one from a data file with values converted to a frequency.  The second input will be a raw frequency input to compare to the previous input.  Once the error is detected between the two values, a process output will drive the motor/linear actuater to a specified position.

Realizing that I have to know how far and how fast  to move the actuator, I can work that out depending on which configuration to use for the loop control output; possibly a table or just established limits, or if necessary the encoder input.

The whole cause of misunderstanding on my part is the fact that I may not see an imediate change in the process variable with a change in output.

Thanks
Wayne

[Brian_Beal]

I would use the analog input, but the tuning may be somewhat difficult.  Keep in mind that the analog input will only tell the motor how fast to move and in what direction.  It doesn't care about position.  If you were getting movement with zero volts before, you may want to make sure that the drive was seeing zero volts, and if there is some kind of hysterisis setting that you can change to help with settling.

[Wayne H.]

Thanks Brian,

I admit i am not looking forward to the tuning process .  The "how much how fast" to operate the actuator bothers me most about the whole operation, all based on a slow response in feedback to a seemingly fast change in actuator position.  From this point i will attempt the analog approach.

Thanks,

Wayne Hilburn