### PID - Reverse Acting The information below provides a FULL description of this microblock and all of its properties. What information you see and what you can do with it depends on your license and the application you are in, and in some cases, a controller's driver version.
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What it does | 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 input. The PID - Reverse Acting microblock calculates three values: a proportional value, integral value, and derivative value. These 3 values, together with the bias, create an output percentage that increases as the input falls below the setpoint.
#### Properties TIPS Alt+click any value in the WebCTRL® or Field Assistant interface to view property details, including its editing privilege and expression (location path) for use on graphics. You can right-click some properties in the EIKON® Property Editor and select Make editable or Make read-only to determine that property's functionality in your system.
Reference Name RefName | Use the default reference name unless you want a more descriptive name for graphics or network links. Limitations: lower case only limited to 40 characters cannot begin with a number must be unique within a control program
Editing Privilege | Preset - Each microblock property has an appropriate privilege or role assigned to it. You can use Global Modify in the WebCTRL® interface to find out what the actual privilege is. CAUTION If you change the Editing Privilege from Preset, the privilege you select will be used for all properties of this microblock, which is not always desirable.
Gain
Proportional | You can define P-gain by entering a value in this field as described below or by entering a value in the Input range field. Entering a value in either field automatically sets the other field. The Proportional gain (P-gain) value is used to calculate the proportional component of the routine. This component increases in reverse proportion to the difference between the setpoint and the input. When the input goes up, the proportional component goes down. If you define only the P-gain portion of the PID microblock, the microblock's output value could cause the input 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 = (Setpoint - Input) x P-gain. For example, if the P-gain is 20, the setpoint is 65, and the current temperature is 67, the proportional value is 40. If the P-gain is 20, the setpoint is 65, and the current temperature is 70 or higher, the proportional value is 100%. So the proportional output increases from 0 to 100% as the input changes from 0 to 5 degrees away from setpoint. The EIKON® application sets the Input range field to 100/20 = 5.
Input range | You can define P-gain by entering a value in this field as described below or by entering a value in the Proportional field. Entering a value in either field automatically sets the other field. In this field, specify the range for the input that will cause the P output to vary from 20% to 100%. For example, if you need to maintain temperatures ±2 from setpoint, enter 2. The EIKON® application sets the Proportional field to 100/2 = 50.
Integral | 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 input and the setpoint have been different. The longer the input and setpoint are different, the larger the integral value becomes. The integral value is calculated as follows: I = Previous I value + [(Setpoint - Input) 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.
Derivative | The Derivative gain (D-gain) value attempts to control the rate at which the input 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 = [(Setpoint - Input)current - (Setpoint - Input)previous] x D-gain. For 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 + D. The 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.
Loop
Bias | 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. When the go input is off, the microblock's output defaults to 0.
Interval | How often (in seconds) the microblock calculates its output value. When the microblock's Go input is on, the output value is calculated once each interval.
Hold I Error | When checked, this setting retains the last calculated integral value when the microblock's go input is off.
Property Page Text
Show Property Page Text | Check to show this microblock's value on the equipment's Properties page.
Property Page Text | You can edit the microblock description that appears on the Properties page. See "Editing Properties page text using special characters" and "Formatting a microblock property" in EIKON® Help.