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ATI A15/66 Residual Sulfite Monitor

O&M Manual

Rev-W (8/10)

PULSE WIDTH MODULATION (PWM) CONTROL

Relays A and B can also be used for control of chemical feed systems using a pulse width control

mode. In pulse width control, the relay will close for a variable time period that is linearly proportional to

the deviation of the measured value from your setpoint. This type of output is not as common as PFM,

but can be used when controlling a solenoid feed valve or for running a pump for variable time periods.

To use PWM control, the relay must be configured for PWM control in steps 14 or 16 of the configuration

menu. For chemical feed control, you will normally want to set the alarm relays to low in steps 15 and 17.

To set up a control system using PWM control, you must program a setpoint, a proportional band,

and a frequency range. The setpoint is the maximum value that you would want to maintain. When the

measured value reaches the setpoint, the output duration will be at a minimum. To set the setpoint, press

the MODE key until the display reads “SA X.XX PPM W L”. Then press the CAL key once. The

display will read “CAL SA S: X.XX”. Use the UP and DOWN keys to select the desired setpoint. Then

press ENTER.

The next setting is the proportional band setting. After pressing ENTER in the previous section,

the display will indicate “CAL SA BP: X.XX”. The proportional band is the amount of deviation from the

setpoint required for the duration of the relay closure to reach a maximum. Use the UP and DOWN keys

to select the proportional band and then press ENTER.

The final setting is the maximum pulse duration (time) required to control the feed device. This

duration will be dependent on the type of equipment you are trying to control. The monitor can be

adjusted for pulse durations of 0-99.0 seconds. When you press ENTER in the previous step, the display

will change to “CAL SA T: 20.0 s” This is the default setting of 0-20 seconds. Left at this value, the relay

will close for 20 seconds when the measured value at maximum deviation from the setpoint and will not

close at all when the measured value reaches the setpoint. Enter the value you want for the maximum

pulses per minute and press ENTER. The display will flash UPDATE and the relay will begin to operate

in accordance with the programming entered.

As an example, assume that you want to use relay B to control a solenoid valve feeding sodium

bisulfite into a tank. You would like to maintain a sulfite value between 0.5 and 1.0 PPM and your

variation in feed time is estimated at 0-15 seconds. Programming would then be as follows.

7. In the configuration menu, step 16, select PWM.

8. In the configuration menu, step 17, select LO

9. Go to the B relay display (display D5, page 20) and press CAL.

10. Adjust the setpoint value to 1.00 and press ENTER.

11. Adjust the proportional band value to 0.50 and press ENTER

12. Adjust the time to 15.0 and press ENTER.

With the programming just entered, relay B will close for 15 seconds when the measured sulfite

value is below 0.50 PPM. When the value reaches 0.75 PPM, the relay will close for 7.5 seconds (out of

a 15 second cycle time). When the value reaches 1.00 PPM, the relay will stop remain open.

You can verify the operation of the relay by placing the monitor into simulate (SIM) mode of

operation. By manually increasing or decreasing the displayed value, you will hear the change in the

relay pulses. Note that the “” symbol indicating the relay condition will change from open to closed

indicating the state of the relay.

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