Detail of our test on 6601,anything wrong ?

[jimmyjimmy2000]

Hi,
Thank you for your privious answer.
First, I'll describe our try in detail.
In hardware, we had a 7344 motion controller. And we used 6601 as an auxiliary postion mearement device. We had a servo motor conneted to an increamental encoder. The encoder signal being conneted to 6601 and 7344 has a resolution of 900000 line per revolution.
In software, we imitated the example provded by CVI(TIOquadEncoderPosMeasure.RRJ). To measure a high frequency signal, we configed the filter to 100ns. So far as we know, the 7344 position measurement works under x4 mode. So we set 6601 to x4 mode, Without changing any other parameters. We used a test program which do the following thing,
1.initial 7344 and 6601

2.set the 7344 and 6601 counter to zero.
3.start measurement
4.print the value coming from 7344 and 6601 to screen.
Now, we started our test. First, we started servo motor at low speed. When it stoped, the positon values coming from 7344 and 6601 were exactly same. We increased the speed of rotation. We found ,when the motor rotated faster than 30 rpm , the value coming from 6601 seemed less than that coming from 7344.

To meed our demond, the values coming from 6601 and 7344 should be exactly same , when the encoder rotates slower than 133.333 rpm. In another word, 6601 should measure a quadrature encoder signal. The maximum frequency of the signal is 133.333*900000/60=2MHz, right?
I am not sure the 6601 can measure such a high frequency encoder signal. And I wonder if the maximum frequency coming from encoder relates to the timebase of the counter.

regards to you,
Jimmy Gan

[<Mike Ross> mross@smpcorp.com]

>>jimmyjimmy2000>
I am not sure the 6601 can measure such a high frequency encoder signal. And I wonder if the maximum frequency coming from encoder relates to the timebase of the counter.
>mross>
Jimmy,

Your question is, simply stated, can the 6601 do what is required? Or, is it failing due to limitations of its design?

If your encoder signal is TTL and free of noise, then the specifications for the 6601 indicate:

It will increment if the source frequency is less than 20MHz.
It will increment if the pulse duration is greater than 3.5ns

A 20MHz source has a period of 50ns. Encoders have a 50% duty cycle. Therefore, at 133 rpm (as you say this speed translates to 20MHz with 900 000 lines/rev) you will have a pulse duration of 25ns. The 6601 can handle this.

You indicate that the counter is registering less counts than the motion controller. Only 2 things can cause this:

The motion controller is seeing noise and counting false signals.

The counter is not receiving pulses of the proper Voltage and duration and is failing to count.
This too can be caused by noise. If a slow noise signal is riding on the encoder output, perhaps the noise causes a long duration high signal, thus overiding the lows of the pulse train. Think of this: if you have power line noise at 60Hz you might see spikes of high voltage the take 1000th of a 60Hz period. 60Hz => 16ms period/1000= 16us. 16us/50ns = 320. In other words this hypothetical noise could obscure 320 encoder pulses.

Do not assume that this is the fault of the counter or motion controller hardware. Check the signals with an oscilloscope. Maybe you can determine the speed threshold at which the errors begin, then you may see the problem on the scope. It may have characteristics which suggest a particular noise source. One of the simplest ways to filter out noise is to turn off the offending machinery that generates the noise signals.

Another possible problem: The encoder may need to drive some sort of load. A typical encoder load is 10mA, but look at your specification sheet. Or the load may be too great and thermal protection may be shutting the encoder down intermittantly. The scope will show you if the encoder is not functioning.

In summary, you should trust the 6602 card to increment at 20MHz, if the signal it senses is correct in nature.

Mike Ross