Motion Control and Motor Drives

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Closed loop feedback problems

I am using a Vexta 2 phase motor with a HEDS-5600 encoder and a Micos VT-80 motor/stage/encoder. The stepper motors work in open loop mode, do not in closed loop with encoder feedback. The steps in document 27QARKOA that discuss this issue do not help to resolve my problem.
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By accident I have been working with a very similar setup by myself and I didn't encounter any problems. So far I haven't seen a problem with closed loop stepper motors that wasn't covered by the hints given in the document you mention. Especially this part of the document generally solves about 90% of all closed loop stepper problems:

"Verify that the correct values for Stepper and Encoder resolution are entered under Axis Configuration in Measurement and Automation Explorer. If you are not sure what the "steps per revolution" value should be, you can configure the stepper motor to run in open loop mode. You can then move the stepper motor until it has turned one revolution. You can enter the total number of steps for the Stepper "Steps Per Revolution" value
under Axis Configuration. To figure out what the "encoder counts per revolution" value should be, you can configure the axis for closed loop stepper mode. Press "Kill," which should de-energize the axis. Then turn the motor shaft one revolution to get the encoder counts per revolution."

Have followed these steps accurately? Could you please be more specific about the symptoms of the failure? If you can't find the solution please export your motion settings (xml file) in MAX and attach them to your reply.

Best regards,

Jochen Klier
Applications Engineering Group Leader
National Instruments Germany GmbH
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Jochen -
Thanks for the information ... I think that I am working through the issue with the Vexta stepper and Agilent encoder. The on-line knowledge base and discussion forum has been very helpful.

I am still somewhat in a quandary regarding the Micos stage/motor and the fact that "limit switches are wired as normally closed". How do I test them using MAX?

Refer to attached document.

Any thoughts on this would be greatly appreciated.

Jerry
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Jerry,

the best way to test your limit switch settings in MAX is to use the 1D-Interactive tab. The axis status LEDs indicate if the limit switches are active or not.

If your stage is somewhere in the middle of the travelling range but the axis status indicates that one or both limit switches are active you should go back to the Motion I/O tab of the Axis Settings and change the limit switch polarity.
If you haven't done any wiring mistakes your limit switches should work like expected then. For testing your configuration move the axis in 1D-interactive slowly to the end positions. Motion should stop automatically before the stage has reached the mechanical end of the travelling range

Regards,

Jochen Klier
Applications Engineering Group
Leader
National Instruments Germany GmbH
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Jochen -
Thanks ... this really helped. I was able to get the limit switches working properly.
Regards,
Jerry
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BTW if you have the opportunity to design your own limit switch setup I would recommend to wire them as normally-closed, and use pull-ups on the input buffer or on the inputs of the 7344. In case there is a wiring failure, in most cases the wires will be open and the failure will be obvious. If the limit switches are wired as normally-open, there is risk of the system not detecting any limit switch(es) when there is a wiring failure. In case you are using slotted opto-switches the same rules apply, since all those I encountered have a logical high output when the input led is not powered. So in case of failure the system will not run over the defective limit switch.
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Just one more comment: It's not necessary to use any external pull-up resistors for the limit switches of the 7344 as there are already 3.3k pull-ups on the board itself.
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I see... however, when connecting the limit/home switches to any other TTL compatible circuitry you will need pull-up resistors. Leaving a TTL input open (with a n/o switch) in most cases will yield a high signal on the input but this is not what the TTL circuit designers had in mind. With a high level input current of approx. 40 uA and a threshold voltage of 2.4V for high level the maximum value for TTL input pull-up resistors is more than 50kOhms, but a lower value is recommended for better S/N ratio and will do no harm. Depending on the sink current and/or current transfer ratio of an optoswitch the resistor value required might be somewhat higher but I would not recommend to use more than 22kOhm.
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