Beijer Electronics BFI-H2 V1.12 - Manual (Page 42)

User Guide Revision 1.12
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8. PID Control Applications
8.1. Overview
The PID Controller is a mathematical function designed to automate adjustments within a system and to eliminate the need for the machine
operator to continuously pay attention to machine operation and to make manual adjustments. For a drive this generally means adjusting the
motor speed automatically to try and maintain a specific measured value from a measurement sensor in the system, with the set-point being
provided directly to the drive. For example, when the drive is controlling a pump it might be required to maintain a pressure which is
proportional to the speed the drive runs the motor. The required pressure (known as the set-point) is provided to the drive. The measurement
sensor is connected to the drive analog input and provides a measurement (known as the feedback) of the current system pressure. The PID
function in the drive compares the set-point and feedback and changes motor speed in order to increase or decrease the feedback to match
the set-point. Should the set-point change then the drive will react by again changing motor speed is order to match the feedback signal to
the new set-point value.
The difference between the set-point and feedback signals in real time is known as the PID error. PID represents P -Proportional, I - Integral, D
– Derivative and describes the three basic mathematical functions applied to the error signal, using the calculated sum as the reference for
controlling the motor speed. By adjusting values associated with the P, I, and D functions the programmer can configure how dynamically the
drive responds to the PID error and how stable the system output (motor speed) is able to be maintained. Achieving best possible dynamic
response and maintaining system stability by adjusting the values used by the P, I, and D functions is known as ‘tuning the PID control’.
Caution: Adjusting values for the PID controller can result in dynamic response from the motor or introduce instability into the
motor speed control. Tuning of the PID controller should only be attempted by experienced engineers.
The drive has a full 3 term PID controller function for control of motor speed. The PID Set-point can be a digital or analog reference provided to
the drive. Feedback is via one of the two analog inputs contained with the drive standard control terminals. All values are treated as %
internally by the drive to assist in simple set up. PID control is enabled when P1-12 = 3. A block diagram of the Drive internal PID control
function is shown below.
8.2. PID Function Set-up
8.2.1. PID Set-point (Reference) Selection
The set-point for the PID controller can be a fixed digital or a variable analog signal. Set-point selection is set by parameter P3-05 (PID
Reference Source Select). Either analog input 1 (control terminal 6) or input 2 (terminal 10) can be configured to provided the set-point. The
format for the analog reference can be configured within the drive with all standard formats included. P2-30 configures the signal format for
analog input 1 and P2-33 configures the signal format for analog input 2.
A digital reference can also be provided in parameter P3-06 (PID digital reference) and P3-05 set to reference this value (P3-05=0). A second
digital reference is provided by P3-15 (PID digital reference 2) and a digital input configured to switch between the two digital references (see
P1-13 and section 10.1 – digital input functions). When no digital selection is configured then the PID digital reference is always provided by
P3-06. The reference value for the PID controller can be viewed in the read only parameter P0-08 – User PID reference.
The digital references for the PID function (P3-06 and P3-15) can provided fixed set-points to the PID function or could be manipulated through
serial communication or via the drive PLC functions.
8.2.2. PID Feedback Selection
The feedback for the PID controller can be configured to either variable analog input signal. Clearly, if an analog reference is used to provide
the PID Set-point then it can’t be used for feedback. Selection for PID feedback is set by parameter P3-10 (PID Feedback Signal Source Select).
Either analog input 1 (control terminal 6) or input 2 (terminal 10) can be configured to provided the Feedback. The format for the analog
feedback can be configured within the drive to match the feedback sensor with all standard formats included. P2-30 configures the signal
format for analog input 1 and P2-33 configures the signal format for analog input 2.