Closed Loop or Open Loop

Dear All,
I'm currently using the DAQ card PIC-6221 to control a DC servomotor with an encoder displacement feedback. I'm using the labview program with a motor amplifier to start the motor and stop it at a specific rotational displacement, where it works by stopping the motor when the encoder feedback value reaches the input desired displacement value. The final displacement of the motor before it stops always exceeds the desired value marginally. May I know for my case,am I using an open or closed loop control? Is a PID controller a MUST for closed loop control? Is my DAQ card capable of desigining a PID controller? If so may I know how?
Also, my project requires me to toggle between the current and voltage mode control. May I know if that is possible to carry out digitally using the Labview Program as my current motor amplifier requires me to push a switch manually to toggle between Voltage and Current mode. Finally, I hope my last question is not out of this world but is it possible to use 2 amplifiers(drivers) for a single motor such that there's a chance to use both the current and voltage modes of control?
Really looking forward to the replys of you kind souls out there! Thanks!
Regards,
Oompa

Oompa,
to learn more about the general concepts of motion control, please visit this link.
In general I widely agree with JB's suggestions. I'll just add some more comments to your questions.
If you need closed loop control depends on what you try to accomplish. If you just want to generate a certain torque (current mode) that is proportional to the output voltage of your control device, you may not need to run a closed loop system. A similar statement is true for the voltage mode.
Closed loop control is especially important if you need to control the position, or if you need to run a special trajectory profile (position/velocity/torque over time)
Maybe it's possible to control the current/voltage mode of your drive with a digital signal. You will have to find out in the manual or check with the vendor.
Using two drives with a single motor doesn't seem to be recommendable. If you want to do something like that, you will need to switch the power signals with relays between the two drives, but as you are working with inductive loads this might become quite tricky and there is a risk to burn both, the motor and the drives...
Regards,
Jochen Klier
National Instruments

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Difference between open loop and closed loop mode when finding the index on a stepper motor axis.

I have a test system which uses a PXI-7352 motion controller with a MID-7602 stepper motor drive to position a 200 step/revolution stepper motor with an attached 1000 line encoder. The MID-7602 is configured for 64 microsteps per step, giving 12,800 microsteps and 4000 encoder counts per revolution. The system is controlled using LabView 7.1 and NI-Motion 6.13. To home the motor, I've defined a sequence in MAX which first finds the reverse limit switch and then moves forward to the encoder index pulse.
When finding its reference in open loop mode, the system its reference at the same microstep each time. When findings its reference in closed loop mode, the system finds its reference somewhere within about a 20 microstep wide range.
I hope somebody out there with more experience with this controller can describe the difference between finding the encoder index in open loop mode and finding the encoder index in closed loop mode.
Thanks,
Mark Moss

Hello Mark,
I suspect that the behavior you are experiencing has today
with the resolution difference between your encoder and your
microstepping. Let me illustrate with an
example:
Example Setup
Stepper
Motor
NI
Motion Controller
10
steps per encoder pulse (Every 10 steps the controller gets an encoder
pulse, therefore positions 0-9 all appear to be 0 to the controller)
The
system has a maximum of 3 pull in moves set
Open Loop Scenario
The
system is commanded to go to position 9.
The
controller sends out 9 pulses.
The
motor moves to position 9.
Closed Loop Scenario
The
system is commanded to go to position 9.
The
controller sends out 9 pulses.
The
motor moves to position 9.
The
controller checks its position and sees that it is still at position 0
because it has not received an encoder pulse.
Because
the controller thinks it is still at position 0, it sends another 9 pulses
as its first pull in move.
The
motor moves to position 18.
The
controller checks its position and sees that it is at position 10 because
it has only received one encoder pulse.
Because
the controller thinks it is still at position 10, it sends1 reverse pulse
as its second pull in move.
The
motor moves to position 17.
The
controller checks its position and sees that it is at position 10 because
it has only received one encoder pulse.
Because
the controller thinks it is still at position 10, it sends and additional
single reverse pulse as its third pull in move.
The
motor moves to its final position of 16.
I believe something similar is happening with your
application as it looks for the index pulse.
Because the controller does not compensate for what it perceives as
position error in open loop mode, the motor always goes to the same commanded
position. In closed loop mode, the motor
is bouncing around inside the single index encoder pulse trying to find a
certain position.
There are several recommendations I can make towards
correcting this behavior. These
recommendations are in order of effectiveness:
Setup
your system so that there are more encoder counts per revolution than
steps per revolution.
Turn
off pull-in moves by setting them to 0 in MAX.
Use an
open loop configuration.
Play
around with the pull in window in MAX.
Regards,
Luke H

Why are my steppers misbehaving in closed loop mode, but work fine in open loop

I have a 7344 controlling a 4 axis prototype machine, which is configured with 50,000 steps/rev and linear encoders providing 50800 counts per inch. The physical properties of the machine are less than perfect (prototype!), so there is a considerable amount of backlash in the hardware (about 0.008" - 0.010").
The observed behaviour is as follows: when repeatedly blending in the X,Y plane using vector space 1 in closed loop mode, an axis will be driven (uncommanded) to the zero encoder reading (home position) occasionally (about 1 move in 600). I have kept a log of the commanded positions being passed to the controller (via the flex_load_vs_position function) and a move to zero is not being com
manded. The application is single-threaded, so there is no chance of race conditions. The ready state of the board is being confirmed by means of the flex_check_blend_complete function call before any blend is commanded. The programmed relationship between steps and counts/revolution have been independantly confirmed as being correct.
An A-B-A test with the only change being to initialise the X & Y steppers in open loop instead of closed loop then 'works' - although the physical positioning of the hardware is unsatisfactory due to the physical characteristics of the prototype machine already mentioned, hence the necessity to run closed loop.
I am inclined to interpret the evidence as suggesting that the 7344 is causing an 'occasional' uncommanded move to zero as a result of some interaction between pull-in moves and commanded position on a system where there is a considerable degree of 'elasticity' between the commanded position and the achieved position allied to a rapid-fire de
livery of blend moves.
This problem can be replicated under versions 5.0.1 and 5.2 of the FlexMotion software.
I can clearly run without bumping into the problem by running open loop, but unfortunately that is not acceptable in this situation. Any ideas, anyone?

Hi, and thanks for coming back so promptly. I think I need to give you some more information - sorry if it turns into a long post!
The system behaviour when it 'misbehaves' is that it starts the anomolous behaviour well clear of any home or limit switch (>6"). When it 'goes wrong', it then slews one or both axes to the zero position. Having arrived at the zero position, it then carries on with the next blend without killing the motor.
The initialisation functions include a seek to home switch, a search for the nearest index point, and a zeroing of the encoder count at the index pulse: hence the zero point is a little displaced from the home microswitch.
I believe that once the system has 'gone wrong', it is stopping at the zero point *not* the h
ome switch.
I have carried out some further tests that may be of help:
If I place a large (1 sec) inter-blend delay in the calling functions, the anomalous behaviour is not noticed. However, the start/stop nature of each move means that this test is knocking the bejezus out of a rather fragile prototype machine, so I can't do this too often!
The encoder positions are being read back on each move cycle and monitored, and they are not showing any anomolous behaviour, so I don't think they are the culprit. I have placed a 'scope on the quadrature encoder inputs and the edges are good and sharp with no real indications of noise.
I could test with a blend factor other than -1 if you think that might be illuminating?"

Is it possible to use software limits on 7354 board when switching control modes from open to closed or closed to open loop?

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Hello Richard,
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National Instruments
Applications Engineer
Machine Vision

Error -70006 when using stepper motor in Open Loop Mode

We have a LabView program that runs a test in which a table is rotated. At the end of the test, the table is supposed to rewind. Instead, the following message appears:
Error -7006 occurred at Read Trajectory status.flx
Possible reasons: For FlexMotion, an invalid axis number or other resource ID (Vector Space, Encoder, I/O Port, and so on) was used.
We have the PCI-7344 interfaced with a UMI-7764 and a Zeta4 that is controlling a ZETA83-93 stepper motor. There is no encoder in the system for this motor. The stepper loop mode was originally set to closed loop but the motor behaved erratically (moving multiple times when instructed by MAX to move once). The stepper loop mode was changed to open loop and the m
otor started behaving properly, but that is when the error message starting appearing. Are there any other configuration settings, such as Primary Feedback (which is now set to Encoder) or ? that I may need to correct to resolve this error?

First of all I would check a couple of things. The reason why your motor was not behaving properly in closed loop stepper configuration is most probably related to the fact that you don't have the encoder counts per revolution adn the stepper steps per revolution properly configured for your motor. Please follow the instructions found in the following knowledgebase:
http://digital.ni.com/public.nsf/websearch/29edb56146f274a386256a1c00624310?OpenDocument
When you configure an axis in closed loop stepper mode, we use the information of the encoder to generate the adequate velocity profile. If the encoder counts and steps per revolution are not properly matched, then the motor will not execute the moves properly. What you need to do is the following:
1.-Configure your axis as closed loop stepper in the Axis Conifguration Section in MAX.
2.- In thbe 1-D Interactive Basic panel, kill the motor and click on reset position.
3.- Manually rotate the motor one revolution.
4.- In the advanced panel on 1-D interactive, read the Encoder Value. This is the value you should enter on the Axis Encoder Counts per Revolution under Axis Configuration. Press Apply.
5.- Go back to the 1-D interactive Basic Panel. Change the bullet selector unbder Stepper Loop Mode from Closed to Open. Run a 10000 step move.
6.- When the move is finished, read the encoder counts from the advanced panel. This will tell you how many counts you have in 10000 steps. You can now calculate the stepper steps per revolution by the following formula:
(# of steps per count)*(# counts per revolution) = # steps per revolution.
Your motor should work fine in closed loop stepper configuration now and the error should not appear. Please make sure you are using the latest version of our driver (Ni-Motion 6.0.1)to avoid older firmware errors and fixed bugs from affecting your system. You can get it from:
http://digital.ni.com/softlib.nsf/webcategories/85256410006C055586256BB9002C16DF?opendocument&node=132070_US

Open Loop Problem of PCI-7344

Dear Partners
I have a big trouble....
Any suggestion would be appreciate!
Here is the hardware and software that I am using
S/W : MAX 3.1.1
           LabVIEW 7.1
           NI Motion 7.0
H/W : PCI-7344
           UMI-7764
           YASKAWA Servo Driver
I've tested my motion B/D step by step
1) Start Max3.1.1 / MySystem / Devices and Interfaces /NI Motion Devices /
       PCI7344(1)/Interactive/1-D Interactive
2) Slect "OPEN LOOP"
3) Input steps : Velocity, Acceleration, Deceleration
4) Input steps : Target Position = 100000
5) Click "Apply"
6) Click "Start"
But sometimes it stops generating steps.
it occurs very very irregularly.
I can't understand because I use "OPEN LOOP"
and "Current Trajectory Position" must increase until "TargetPosion" whether the asis move or not.
Also there is no limit signals..
Please see the image file attached.
It's very simple I control motion b/d with OPEN LOOP MODE.
and, I think the servo Driver works well because command steps and encoder pulses are equal.
Please Help me!!!
Crane
Attachments:
7334error.GIF ‏117 KB

Thank you Jasper S.
I think you understand me well.
I tell you more informations.
1) The computer is working in Windows XP Professional (Just Theme).
2) It happens also at low velocity and acceleration.
3) I think when the motor stops moving, it fails to translate into hardware steps.
Because when the motor stops, "Current Trajectory Position" and "Encoder Counts" are equal.
4) I checked status registers on the card when it works abnormally, but can't find any error message and flgs.
5) To make matters worse it happens occasionally, so it's difficult to find out the reason.
6) Strange as it may sound, when I touch the motion connector correctly,  the abnormal behavior frequency decrease.
So I tried to exchange a new motion b/d and a motion cable but it was just same error situation.
I can't understand because I just use "OPEN LOOP",  it is no matter whether servo driver works well or not.
7) Do I have to change PC? I already tried to exchange PCI Slot position.
I need more information about this.
I'm in trouble because this system is operating at foreign country.
It is fatal error even if it happens once per day....
Please help me once more. Thank you.

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Thanks
Adam

adamkse wrote:
I want to control my plant with lead/lag control but I do not know the plant model which makes this very difficult. I cannot theoretically calculate it because the information I need to obtain a numerical transfer function or state space model is unavailable.
I have it controlled with PID at the moment but this is not ideal.
I am trying to use the system identification toolkit to obtain a transfer function but I do not know what to do because the examples do not work for open loop unstable plants.
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Adding hardware (sensors) is the most likely solution.
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Message was edited by: Jump_Over

Thank you Jasper S.
I think you understand me well.
I tell you more informations.
1) The computer is working in Windows XP Professional (Just Theme).
2) It happens also at low velocity and acceleration.
3) I think when the motor stops moving, it fails to translate into hardware steps.
Because when the motor stops, "Current Trajectory Position" and "Encoder Counts" are equal.
4) I checked status registers on the card when it works abnormally, but can't find any error message and flgs.
5) To make matters worse it happens occasionally, so it's difficult to find out the reason.
6) Strange as it may sound, when I touch the motion connector correctly,  the abnormal behavior frequency decrease.
So I tried to exchange a new motion b/d and a motion cable but it was just same error situation.
I can't understand because I just use "OPEN LOOP",  it is no matter whether servo driver works well or not.
7) Do I have to change PC? I already tried to exchange PCI Slot position.
I need more information about this.
I'm in trouble because this system is operating at foreign country.
It is fatal error even if it happens once per day....
Please help me once more. Thank you.

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Hi Jon,
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