Control Loop Setup Screen
Users may view and edit up to eight (8) configured control loops. The settings on this page will differ depending on the type of control configured for the system. The control loops all operate on the principle of a 0-100% calculated output with special considerations for the Digital loop types. Four control loop types are possible. a) Analog and Digital types use a closed loop PID calculation to calculate the output value, in which a process variable is maintained at a desired setpoint with the PID generating a 0-100% corrective action to the process. The ultimate goal of the PID is to reduce the error to zero effectively maintaining the control setpoint (e.g. speed, load, pressure). PID calculations attempt to model the process being controlled by allowing tuning for the dynamics of the process based on the present (Proportional), past (Integral) and future (Derivative) error of the loop. The controller uses the parallel form of the PID equation as follows:
% Output = Kp(Error) + Ki∫(Error)dt + Kd×dErr or∕ dt b) 2 Pulsed Digital type uses a closed loop pulse equation that calculates the On time for either the increase or decrease digital output based on the control error.
% Output = (Error x Kp) + 50%
The ultimate goal of the loop is to reduce the error to zero effectively maintaining the control setpoint (e.g. speed, load, pressure). The control algorithm is centered at 50% output. At 50% output, neither digital output is on. The control loop will either add to 50% or subtract from 50% to pulse the increase or the decrease outputs. Larger deviation from 50% will result in longer output pulses. c) 4-Step Load turns on 4 digital outputs in a 4-step staggered loading scheme with time delays in between the loading steps. The ultimate goal of the loop is to reduce the error to zero while maintaining the control setpoint (e.g. speed, load, pressure). The control loop will turn on the outputs in succession, and the 0-
100% control value will step up from 0 to 20%, 40%, 60% and 80% as the time delays expire. If the deadband is reached, the time delays will reset, and no change in control will occur. d) Open Loop calculates an analog output value using a linear scaling based on a feedback input. In this case, there is no closed feedback and no setpoint to maintain. There are 2 coordinates specified in engineering units for the process being controlled, one for minimum % output and one for maximum % output. A linear scaling function will be applied using these 2 values to range the output between the specified minimum and maximum % settings. e) Common setpoints to Analog/Digital, 2 Pulsed Digital and 4-Step Load loop types. Open Loop Ramp loop type shares only those setpoints marked with (*).
1. Auto/Manual Mode*: Set the loop to Auto to enable the control. Set to Manual to allow user entry for the control output as a 0.00-100.00% value.
2. Setpoint: Assign the desired value that is to be maintained by the loop. Depending on configuration this may be a variable setpoint based on analog input, not user editable from the display.
3. Deadband: Assign a value around the setpoint during which the loop will not take any corrective action.
4. Minimum Output*: Set the minimum limit on the calculated value during auto control
5. Maximum Output*: Set the maximum limit on the calculated value during auto control.
6.
Loop Update time: Loop calculation frequency. This time should be set no shorter than the update rate for the feedback reading.
Section 50
2021-06-21 - 24 -
00-02-1031
7. Max Rate of Decrease/Increase: This is a maximum slew rate setting for the output change per loop update time.
8. Set Output % (Manual) *: This is the control output value data entry for manual mode.
9. Override 1-3 Ramp time*: Set the interval used to modify the calculated output when a configured override signal is active.
10. Override 1-3 Change %: Set the amount of change either positive or negative required to the calculated output while a configured override signal is active.
NOTE: Override settings are only used when there is a configuration for overriding the control loop with another process variable. Each control loop may have up to 3 control loop override settings based on setpoint or digital inputs signals going true and false.
11. Display Loop Bar Graph*: Set to yes to show a bar graph page for the control loop as it operates.
12. Ref Line Select*: Setting to select any value to be shown on the Display Loop Bar Graph in addition to the setpoint, feedback and control output values. This setting is useful for showing the process value of a different analog, thermocouple or speed input that may be affected by changes to the PID output. Select from available analog, thermocouple or speed inputs or None to disable the Reference Line Select feature.
Section 50
2021-06-21 - 25 -
00-02-1031
Control Loop Setup Screen (continued)
a) Analog/Digital PID loop specific setpoints:
1. Proportional: Proportional gain tuning for the control process.
2. Integral: Integral time constant (%/sec) tuning for the control process. Integration adjusts the output value on the accumulated of the error over time.
3. Derivative: Derivative time (% seconds) tuning for the control process. Derivative adjusts the output value based on the rate of change of the error over time. b) 2 Pulse Digital loop specific setpoints:
1. Proportional: Proportional number multiplied by the error to result in the on time for the pulse. Larger numbers here will result in longer on times for the pulse at a given error.
2. Inc/Dec Max On Time 1/20s: Set the maximum on times for the calculated on time of the pulse. This is a clipping value applied to the calculated result.
3. Inc/Dec Sample Time 1/20s: Set the fixed off time between pulses. This should be set long enough to allow the results of the previous pulse to have effect on the process before a new pulse is generated.
4. Inc/Dec Xover On Time 1/20s: Set the fixed on pulse that is generated when the control changes from increasing to decreasing or vice versa. This is optional and typically used to prime hydraulic controls to reverse direction.
5. Inc/Dec Xover Off Time 1/20s: Set the fixed off pulse that is generated when the control changes from increasing to decreasing or vice versa. This is optional and typically used to prime hydraulic controls to reverse direction.
C) 4. Step Load loop specific setpoints:
1. Input For Min % Out: This is the engineering unit value for the loop input that will cause the Minimum % output to be calculated. This value will be a pressure, temperature, RPM or other variable signal that controls the output directly.
2. Input For Max % Out: This is the engineering unit value for the loop input that will cause the Maximum % output to be calculated. This value will be a pressure, temperature, RPM or other variable signal that controls the output directly.
Section 50
2021-06-21 - 26 -
00-02-1031
