Fluke PM6681R, PM6685R manual Supervising a Process, Obvious Method, Optimal Method

How to Measure Fast

Supervising a

Process

One typical use of a counter in the indus- try is to measure a parameter and alert the adjusting machinery when the parameter gets close to the correct value. The ma- chinery now slows down for an accurate final adjustment of the parameter, and stops the adjustment procedure when the value is correct.

An example of such a procedure is when a laser adjusts the value of a resistor that is connected to an oscillator. You mea- sure the frequency of the oscillator and the laser cuts the resistor until the oscilla- tor oscillates at the correct frequency.

Obvious Method

The most obvious way to do this may be as follows:

–Let the counter measure the frequency.

–Send the result to the controller.

–Let the controller, that controls both the la- ser cutter and the counter, decide when to slow down the cutting procedure, and eventually switch the laser off when the correct frequency is obtained.

This method works fine for slow pro- cesses but the bus transfer rate of the counter limits the measuring speed to around 125 measurements/s for PM6680B and PM6685, and 250 mea- surements/s for PM6681. If all speed in- creasing actions are taken, and only around 10 measurements/s if no speed in- creasing actions taken.

Optimal Method

An experienced CNT-8X programmer knows that he can increase the process speed to over 300 measurements/second, by letting the counter do more and the controller less of the job:

–Set up the counter to measure continuously with low resolution, without displaying or reading out any results.

–Set up limit monitoring so that the counter issues a service request when the frequency reaches the limit where the laser should slow down.

–Proceed with one of the following:

–Alternative 1: The program slows down the laser and recalls new narrower limits from the internal counter memory and selects higher resolution.

–Alternative 2: The program slows down the laser and reprograms the counter to make high resolution measurements and reports each measurement result to the controller.

–The controller stops the process when the desired result is obtained.

See also the limit monitoring program- ming example in Chapter 4.

7-10 Supervising a Process