PID - Reverse Acting
Convert microblock
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PID (Proportional, Integral, Derivative) loops use industry standard algorithms to calculate an appropriate response for controlling a physical output, based on the equipment's setpoint and the actual temperature.
The PID - Reverse Acting microblock calculates three values: a proportional value, integral value, and derivative value. These three values, together with the bias, create an output percentage that increases as the temperature falls below the setpoint.
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Properties
Property
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Notes
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RefName
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Use the default reference name unless you want a more descriptive name for this microblock for graphics, source tree rules, or network links. All reference names in a control program must be unique.
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Interval
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The Interval setting indicates how often the microblock calculates its output value. When the microblock's go input is on, the output value is calculated only once each interval.
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Hold I error
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The Hold I error setting, when enabled, retains the last calculated integral value when the microblock's go input is off.
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Bias
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The Bias value is added to the proportional, integral, and derivative values calculated by the microblock to create the final output value. The bias can be viewed as a starting point for the calculation or as an offset to the final value. When the go input is off, the microblock's output defaults to the Bias value.
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Gain
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The Gain section of the microblock dialog indicates the values used by the PID microblock to calculate each portion of the PID routine. Proper adjustment of these values results in the most efficient use of the equipment. Automated Logic Corporation provides tools to assist you in tuning PID loops. These tools are available in the ALC File Library, which can be accessed at http://www.automagedlogic.com/techsupport.
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Proportional
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The Proportional gain (P-gain) value is used to calculate the proportional component of the routine. This component increases in direct proportion to the difference between the setpoint and the current temperature. If you define only the P-gain portion of the PID microblock, the microblock's output value could cause the temperature to oscillate around the setpoint or possibly to never reach the setpoint (if the P-gain is too low). The proportional value is calculated using the following formula:P = (Input - Setpoint) x P-gainFor example, if the P-gain is 20, the setpoint is 65, and the current temperature is 67, the proportional value is 40.
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Integral
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The Integral gain (I-gain) value is used to calculate the integral component of the PID routine. The integral value accounts for the amount of time that the temperature and the setpoint have been different. The longer the temperature and setpoint are different, the larger the integral value becomes. The integral value is calculated as follows:I = Previous I value + [(Input - Setpoint) x I-gain]For example, if the I-gain is 2, the setpoint is 65, and the current temperature is 67, the integral value for the first interval will be 4. If at the end of the second interval the temperature is still 67, the integral value will increase to 8.
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Derivative
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The Derivative gain (D-gain) value attempts to control the rate at which the temperature is brought to setpoint in order to prevent the setpoint from being exceeded. This value uses information from the current and previous intervals and is calculated using the following formula:D = [(Input - Setpoint)current - (Input - Setpoint)previous] x D-gainFor example, if the D-gain is 3, the setpoint is 65, and the current temperature is 67, the derivative value for the first interval is 6 ([2 - 0] x 3 = 6). If at the end of the second interval the temperature is still 67, the derivative value will decrease to 0 ([2 - 2] x 3 = 0).The microblock's output percentage uses all three components as follows:Output (%) = Bias + P + I + DThe Bias is set on the microblock dialog or the Properties page. Using the examples listed for each component above and assuming a bias value of 0, the output value of the PID microblock for the first interval would be 50; for the second interval the value would be 48.
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Property Page Text
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Enter a meaningful description of the microblock for use on the Properties page in WebCTRL.
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Editing Privilege
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Preset (the default Editing Privilege) indicates that each property in a microblock has an appropriate privilege assigned to it.
If you change the Editing Privilege, all the properties in that microblock are assigned to that one editing privilege, which is not always desirable.
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NOTE Alt+click any value in WebCTRL to view property details, including its expression and editing privilege.
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