Troulbe in measurement quadrature encoder with 6601

[jimmyjimmy2000]

Hi,
Thanks for Ross's help.But I cannot differ 6601's x4 mode from other counter/timer counting mode. In measurement position, what difference between them. And I just want to know whether my application viable.

We are trying to measure position with 6601. The frequency of quadrature encoder is from 100Hz to 8MHz. Now, we connet the A,B and Z channel directly to counter . Using the example in CVI, we change the filter to 100ns and use x4 mode. It works perfect at low frequency.
The problem is when the frequency of encoder is higher than 2MHz, the accuracy decreases greatly. I'd like to know about following:

1.Can 6601 do the previous task enough? Or I must change to use 6602.


2.If
6601 is available, is there any way to increase measurement accuracy ?(in software or wire connection )

[aasenjo]

Jimmy,

Based on your current questions:

1)I cannot differ 6601's x4 mode from other counter/timer counting mode

The board�s manual briefly explains the difference between X1, X2 and X4 measurement modes.

Basically the accuracy should be independent of the encoder frequency as long as you don�t exceed the limits set by your filtering parameters and the signal connections are done properly.

2) Can 6601 do the previous task enough? Or I must change to use 6602.

Yes, it can. I believe you have to be more specific about any incorrect behavior you might be seeing so we can suggest things to try.

Please refer to your original posting for more information:

Developer Exchange: Trouble in measurement quadrature encoder with 6601

3.If 6601 is available, is there any way to increase measurement accuracy ?(in software or wire connection )"

Basically X4 measurements are the way to go, you should immediately notice that you get more counts per revolution when using this mode, try it first at lower frequencies and then you can test it at higher rates.

Regards,

Alejandro Asenjo
Applications Engineer
National Instruments

[<Mike Ross> mross@smpcorp.com]

>jimmyjimmy2000
"Hi,
Thanks for Ross's help.But I cannot differ 6601's x4 mode from other counter/timer counting mode. In measurement position, what difference between them. And I just want to know whether my application viable.

>>mross>The application is viable. However, the use may be difficult.

>jimmyjimmy2000
We are trying to measure position with 6601. The frequency of quadrature encoder is from 100Hz to 8MHz. Now, we connet the A,B and Z channel directly to counter . Using the example in CVI, we change the filter to 100ns and use x4 mode. It works perfect at low frequency.
The problem is when the frequency of encoder is higher than 2MHz, the accuracy decreases greatly. I'd like to know about following:
1.Can 6601 do the previous task enough? Or I must change to use 6602.

>>mross>The 6601 is just as good as the 6602 at 2MHz. Therefore, a 6602 will have the same problem of inaccuracy.

>jimmyjimmy2000
2.If 6601 is available, is there any way to increase measurement accuracy ?(in software or wire connection )"

>>mross>Yes, in the wire connection you can make improvements.

6601 can count as fast as 20MHz (60MHz with prescaling). The signal is noisy (containing addtional, incorrect information).

You must use an oscillosope to look at the signal. Look at it when 6601 is counting correctly, and look at it when 6601 is "malfunctioning".

The counter will increment whenever the signal is high for more than 5 nanoseconds. The conditions for "High" and "Low" signals must be strictly met. If the signal is greater than 0.8V, the counter may increment. The signal must then drop below 2 Volts for a "Low" to be seen.

This sounds wrong, but is true. The counter is expecting TTL signal conditions (Transistor -Transistor Logic). This specification is very broad.

A "High" will absolutely be recognised if the signal is greater than 2.0V.
A "Low" will absolutely be recognised if the signal is less than 0.8 V.

However, the range from 0.8V to 2.0V is "undefined." This means the counter could increment at 0.8V. It has been my experience that National Instruments TTL will always see 0.8V as a "High." If your noise exceeds 0.8V you will never be able to count accurately.

The problem will be solved when your encoder signal is read cleanly by the 6601. This is why I gave you the URL for US Digital in the previous answer. The circuits they sell can help to clean up the counter signals. You will have to see what works best, since I don't exactly understnad you application. If the noise is very bad, you may wish to use the Encoder to Binary Number circuit. There will be no noise in the reading of a binary number. Since your encoder may run as fast as 8MHz, you must be sure the circuit you choose is also this fast.

The other solution is to eliminate the noise which is causing the counter to trigger falsely. THis can be a very interesting problem to solve. Please look a this NI link:

Field Wiring and Noise Considerations for Analog Signalsv
http://zone.ni.com/devzone/conceptd.nsf/webmain/01F147E156A1BE15862568650057DF15?opendocument

Follow these instructions very carefully.

Also, this book is helpful to explain noise and how one may reduce it.

From Amazon.com you can get Grounding and Shielding Techniques, by Ralph Morrison.
http://www.amazon.com/exec/obidos/ASIN/ 0471245186/104-3492923-2811911

Be very careful how you wire the circuits. Try to use only one ground point.

The worst source of noise can be the motor and drive that spins the encoder. Be particularly careful in wiring this apparatus. A simple solution can be to use an air powered motor insted of an electric motor.

Good luck,

Mike Ross

[aasenjo]

Jimmy, Mike,

Great answer by Mike, basically those suggestions will help your signal be optimal shape so it can be used by the counter board.

There is one more thing I'd like to add. The SH6868D1 cable has been manufactured so that it has twisted pairs inside. If you use these twisted pairs for your signals and connect the signal reference to the appropriate DGND pin you should get the benefits of twisted pair wiring inside the cable.

I attach a spreadsheet with the twisted pairs for a SH6868D1 cable when used with Counter 0 of the 660X boards.

I hope this helps,

Alejandro Asenjo
Applications Engineer
National Instruments