Emerson 3000/ITR manual Control Types, Proportional Control, PI Control

Operation

Evaluate the start operation of the unit (with pump down).

1.Close LLSV

2.Wait for LPS to make or ignore for WSK time

If LPS is made before WSK has expired, allow the compressor to continue running, if not then stop compressor and alarm low pressure alarm.

3.Once LPS makes start compressor.

4.Verify the alarm phrase for the WSK and the low pressure switch.

5.2Control Types

5.2.1Proportional Control

This is a standard control method that maintains the room at a temperature proportional to the load. The temperature maintained increases as the room load increases. At full load the room would be con- trolled at a temperature equal to the temperature setpoint plus the temperature sensitivity.

5.2.2PI Control

The PI control combines two individual terms to determine the control output for a given set of condi- tions. Note that PI control is used only for temperature. If PI control is selected, humidity will con- tinue to use proportional control.

The proportional (P term) is determined by the difference between the current temperature and the control setpoint. This term is expressed in % cooling (heating) desired for each degree above (below) the setpoint. It is adjustable from 0% to 100% per degree. The purpose of this term is to adjust the control output for any deviation between the current temperature and the control setpoint.

The integral (I term) is determined by two things: the difference between the temperature and control setpoint and the amount of time this difference has existed. This term is expressed in % cooling (heat- ing) desired for each minute and degree above (below) the setpoint. It is adjustable from 0% to 100% per degree-minute. The purpose of this term is to force the control to maintain the temperature around the setpoint by slowly but continuously adding (subtracting) a small amount of cooling (heat- ing) to the total control output until the temperature is at the setpoint.

A suggested tuning procedure is as follows:

1.Initially adjust the integral setting to 0%.

2.Starting with 20%, adjust the proportional setting in small increments (10% steps) until the control sustains a constant hunting action (the temperature swings are approximately the same amplitude from one peak to the next).

3.Note the time in minutes between peaks of adjacent temperature swings and the amplitude of the temperature swing (degrees above the setpoint).

4.Adjust the proportional control setting to about 1/2 the value obtained in Step 2.

5.Adjust the integral setting to a value calculated by the following equation:

Approximate room load (in % full load)

Time between peaks x peak amplitude x 4

NOTE

If this calculation results in a value of less than 1%, then set the integral to 1%.

The tuning procedure above is only an approximation for an initial set of adjustments and are based on the “average” room characteristics. Your particular settings may need to be further adjusted for optimum PI control performance. Some suggestions for additional tuning are as follows:

•If cooling output overshoot is occurring on load changes, decrease the proportional setting.

•If system hunting occurs with constant room load, decrease the integral setting.

•If the control responds too slowly, resulting in large temperature excursions on a load change, increase the proportional setting.

•If a constant temperature deviation exists between the temperature and setpoint, increase the integral setting.

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