How can I prevent the PLC scan rate from affecting batching accuracy?

On the HI 2151/30WC, HI 2160RC, HI 3010, HI 3030, HI 4050, and the HI 4060, the PLC scan rate can be prevented from adversely affecting batching accuracy by allowing the weight controller to perform the material flow cut-off.

When a single PLC is required to control batching of several ingredients and/or several batching processes simultaneously, optimum accuracy is obtained when the PLC controls ingredient selection and starts the product flow. Your Hardy weight controller should be used to perform the flow cut-off.

The attached document (Figure #1) is used for the following logic example:

Steps:

1. Down load set points to the weight controller. Example: Relay #1, 95 lbs; Relay #2 100 lbs.
2. Route the output signal to the correct solenoid. Example: For ingredient #1 close PLC relays #1 and #6.
3. Start product flow. Example: Close relay #16
4. Monitor status of weight controller relays #1 and #2 via Remote I/O.
5. When weight has been reached and weight controller relays #1 and #2 have changed state, open all PLC relays.

NOTE: Your wait/scan time is not predictable and the weight reading will be delayed and inconsistent from batch to batch. The internal relays are predictable and repeatable. The worst problem would be a delay in discharge or starting the next ingredient.  That duration would be the refresh cycle of your PLC and should not be a problem.

6. Return to step #1 for next ingredient.