Incremental Encoder

[timmer656]

I am designing a new product that uses 2 dcpm motors to control horizontal and vertical positioning. Due to cost we would like to use an incremental encoder. Has anyone ever used an incremental encoder and continuously saved the position so in case of a power failure, the position will be retained? This will be like pseudo absolute position. Upon power failure the unit we are controlling will not coast. The position would ned to be copied to non-volatile memory, I assume.

 

[ScottyUK]

Hi Timmer656,

Depending on your accuracy requirement, I suppose it is possible.

Are you sure there is NO coasting or over-run on power loss? It seems unlikely considering that the motor has an amount or rotational kinetic energy which has to go somewhere as it stops. Normally it would be expended in moving the load.

I guess the vertical axis must be highly geared to avoid
movement under gravity?

Bearing in mind the above, your stored information may well be worthless following power failure, unless the unit re-zeros itself from a datum point and then returns to the stored location. Providing the datum point can be repeatably located with an accuracy comparable with the encoder you are using, this may work.

 

[jbartos]

Suggestion: Visit

http://www.pepperl-fuchs.com/fa/rotaryencoders/incremental/main_e.html

http://www.photocraftencoders.com/Incremental Encoder Options & Features.htm

etc. for more info

 

[Yahoo1]

Hi, timmer656,

I am encountering the same problem that is to store the encoder counts in somewhere in case the power failure. I am using SmartMove from Baldor. I guess it should have this funtion to store data in memory but I couldn't find and don't know how to use it. How did you slove this issue?

Thank you very much for your reply in advance.

 

[avigodner]

In connection with phase initialization at power-up of sinusoidal brushless motor with incremental encoder without position Hall sensors see "Smart Initialization" article overview at

http://www.drbrushless.com/cgi-bin/articles.phtml

 

[timmer656]

I don't know about the Baldor product. We are in the process of designing a custom board to control a brush DC motor. This board will have a micro and will continually store the position in non-volatile memory. In case of power failure the position will be saved. Upon power up the position in the memory will become the actual position.

 

[snoogie]

Its fine to use incremental encoders as you intend. The accuracy is not an issue as it will be as good as absolute with respect to the encoder funtions. However what you should do is put one limit / prox switch on each axis which can be used used as a permanent known position such that if any axis needs to be re-refernced due to mechanical disassembly of the system (which will happen so do not try to ignor the fact) then it can be easily reset.
The proceedure is to acurately measure the physical position of each axis limit switch (drive the axis onto the switch as slow as possible (use the switch as end of travel one end then use a soft limit on the other based on this position count)how well you get the switch position point is your accuracy along with how well you measure its physical position) and then physically measure the actual position relative to a datum position and enter this value in your register. Do this with each axis then if the system is mechanically dissasembled of if any axis is moved when the power is off your resetting proceedure should be to slowly jog each axis onto its switch then when each axis is stopped on its switch (all switches made) reset the actual position for each axis to the value you stored in the registers when you did the inital setup. I have done this and it works pretty well but you have to watch out limit / prox switch accuracy and always approach from the same direction etc..
I hope this is what you were looking for Snoogie

 

[Yahoo1]

Thanks for all your kindly reply. They are helpful to me.
My situation is that now I have 2 proximity sensor at both side of the cylinder for limit and home purpose. But they are not as accurate as I need and I am worring about they are not 100% reliable.
Do you know anything about index sensor or position hall sensor? Are they more accurate and reliable than a proximity sensor?

timmer656: is it pretty easy to use a non-volatile memory? Do you need another memory card or something else?

Thanks again!

Wendy

 

[jbartos]

Suggestion: Visit

http://www.engineeringtalk.com/indexes/categorybrowsesp.html

etc. for more info