To see the effects of different debounce time settings simply view the analog

To see the effects of different debounce time

This preview shows page 71 - 74 out of 122 pages.

To see the effects of different debounce time settings, simply view the analog waveform along with the counter output. Use trigger before stable mode when the input signal has groups of glitches and each group is to be counted as one. The trigger before stable mode will recognize and count the first glitch within a group but reject the subsequent glitches within the group if the debounce time is set accordingly. The debounce time should be set to encompass one entire group of glitches as shown in the following diagram. Trigger after stable mode behaves more like a traditional debounce function: rejecting glitches and only passing state transitions after a required period of stability. Trigger after stable mode is used with electro- mechanical devices like encoders and mechanical switches to reject switch bounce and disturbances due to a vibrating encoder that is not otherwise moving. The debounce time should be set short enough to accept ZonicBook/618E 917695 Tach Channels 7-7
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the desired input pulse but longer than the period of the undesired disturbance as shown in the diagram below. To make an optimal noise rejection setting in software . . . 1. Setup the input for Voltage Mode (Select AC or DC Coupling). 2. Take a measurement. 3. View the Tach Signal on a TimeWaveform plot. 4. Set the Trigger Slope [Trig Slope] to Positive (POS) or Negative (NEG). 5. Set the Trigger Level [Trig Level] to a value between 50 and 75 % of the maximum excursion. 6. Set Edge Detect to “Immediate” or “Delayed.” An illustrated example making use of a 500 ns delay follows shortly. You may need to experiment with Edge Detect settings to determine whether “Immediate” or “Delayed” works best for your application. Optimum settings will vary, depending upon the noise present on the Tach input. 7. Set the Delay Time to a pre-set value between 500 ns and 25.5 ms. To have no delay time select “none.” The debounce time should initially be set as low as possible, i.e., to 500 ns or to “none.” If the velocity readings are jumping around, that is, they are either too high [as if there was an extra tach pulse] or too low [as if there was a missing pulse] increase the debounce value to higher than 500 ns. Some experimentation is to be expected. Increasing the debounce value too high will reject the tachometer pulse altogether, particularly at high RPM. Do not set the debounce value to higher than one-half of the minimum tach pulse period. The minimum tach pulse period is that which exists during the highest RPM. Reference Notes : For details you will need to refer to eZ-Analyst or eZ-TOMAS documentation, as applicable. A PDF version of each is included on the installation CD. ¾ eZ-Analyst Series Software Reference Manual ¾ eZ-TOMAS User’s Manual 7-8 Tach Channels 917695 ZonicBook/618E
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The following diagrams show typical tach pulses and appropriate comparator threshold trigger settings.
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