home and limit

[Gram]

I will do motion control with an open loop stepper system, could anybody give me some hints on how to set a reference point (home?) and consequently limit/switch setting, preferably software limit, to restrict the range of travel in my application. As far as I understand, a reference point is firstly to be defined/set, and then all subsequent motions are based on this reference point? Can anybody direct me to some links or examples I can refer to? Thanks in advance.

[Gram]

I do not quite understand what the function of find reference.flx really is. How can it set up a base point without specifying a particular point. My application is a step motor coupled with a lead screw, and how can I set the reference point as one end of the lead screw? Consequently, I can define the subsequent motions based on the distance away from this point. It would be perferable if somebody can direct me to a similar example code.

[Jochen]

This issue is not really related to programming so I can't provide example code. With Find Reference you make your axis search for a hardware reference point. This can either be a switch that is located somewhere within your travel range or the index pulse provided by a quadrature encoder. If you have limit switches at the start and the end of your travel range you also can configure Find Reference to position the axis in the center between these two limit switches.
In any case you need real hardware signals that indicate your reference positions. For detailed information please refer to the "Using this VI" section in the LabVIEW online help.

I hope this helps,

Jochen Klier
National Instruments Germany

[Buechsenschuetz]

As stated by Jochen, setting a reference point is rather a hardware issue, i.e. you use a sensor with TTL output (mechanical or opto-electronic switch) to detect whether your motion system has reached a certain position.

To limit the motion of your system at extreme positions hardware limit switches are not required. I think it is not possible to set software limits with MAX but it should be possible to implement this in your own code.

Even if you don't need limit switches, using a reference switch is almost a MUST. If a system runs in open loop mode and the motor/actuator is stopped mechanically or by driver faults the motor controller will not stop emitting stepper pulses, and you will not be able to detect where the system actually has moved without a reference switch.

[Jochen]

I just want to add that in fact there are software limits that can be set in MAX (Axis settings) but still setting these limits is completely worthless if you don't have a reference switch.

Regards,

Jochen

[Gram]

Thanks Jochen and Buechsenschuetz,

Now I got a picture what it's about. We do not have a limit switch as you mentioned. could you give me some hints on where to find it? Probably we can put them some places along the guild rail of the travelling system.

[Jochen]

I have made a quick search in Google, searching for the German word "Endlagenschalter" (= limit switch) and I found lots of offers. Here is an example:
http://www.mercateo.com/c/139-2175/Endlagenschalter.html

Searching for "limit switch" in Google should give you similar results for more international vendors. Depending on your mechanical setup the limit switches may not meet exactly your needs but the vendors who offer these types of switches should also provide switches that should be a better fit for a reference switch.

Regards,

Jochen

[Gram]

Further to my previous post.

I am working with a 7334 motion controller and an UMI7764. Can these be used for anolog i/o purpose should there is need from the limit switch senors?

Besides, I also have data acquisition system with sufficent number of Analog i/o and digital i/o, which all works under LabVIEW, although the cards are not from NI.

Looking forward to your advice.

[Jochen]

Please refer to your NI 7330 user manual. You will find that the 7334 provides four analog inputs, no analog outputs and inputs for limit switches.

Jochen

[Gram]

Thank you very much for your help.