July 2, 2008
Understanding Index Gating Options
The index pulse is also referred to as the reference, marker pulse, or home pulse. This pulse is an individual output channel provided by the encoder that generates a single pulse once per revolution. It simply notes a discrete or fixed position in the mechanical rotation of the unit. Sometimes it is used with a counter to indicate the total number of revolutions the shaft has rotated, counting one pulse per revolution. Often times it is used to reset a counter if the counter needs to be reset to zero at the end of each encoder shaft revolution.
Quite often it is used in servo applications where total system synchronization is required. Once every revolution, if everything agrees with the position feedback, the system knows it is still operating correctly. Or a system can return to a known physical position aligned with the index pulse. EPC defines the index as follows: Once per revolution centered over channel “A”. For the HV output option, it is normally gated to channel “A”, and is 180 electrical degrees wide, or known as “half-cycle gating”. We also have the ability to gate the index pulse to the “B” channel, or to both “A” and “B” channels if required. If it is gated to both channels, it results in what is called “quarter cycle gating”, which is 90 electrical degrees wide, allowing more precise positioning using the index point.
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| Diagram of index pulses for Accu-codersTM that have a positive index following a positive Channel A. |
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Diagram of index pulses for Accu-codersTM that have a positive index following a negative Channel A. |
However, keep it in mind that with a narrower index pulse, comes the possibility of the device the encoder is connected to not seeing the narrow pulse because it happens so quickly. Please note that these comments regarding the index pulse ONLY apply to units with the “R” or “D” in the part number, which calls out A, B, & Z channels. With the “A”, “Q” or “K” in the number of channels spot, there is no index pulse provided. Non-standard gating options must be requested by the customer at the time of ordering.
Most index pulses are related to electrical degrees, but some are mechanically derived. These are comprised of a disc with the index pulse etched onto this disk in addition to the main count channels. This type of index is usually referenced to a particular count channel such as “A”. It is normally not gated, however it can be done on occasion. By making this etched aperture a certain width, the resulting pulse width of the index can be varied.
Remember that the actual pulse “width” is a function of time, and the width will vary as an inverse function of rotational shaft speed. Some uses for lengthened mechanical index pulses are for resetting counters that have long reset time periods. Also by gating to the count channel, the encoder output can be inhibited for a period of time while the index pulse is active. This is commonly used in the packaging industry where counts are not desired while the container is filled and is moving to the next processing station. Mechanically derived index pulses are only available on select Cube Series encoders.
If your application requires a specific index gating option, don’t hesitate to contact a sales engineer to help with your Accu-CoderTM selection.
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