Manually Tuning The Control - Baldor VS1SD Installation & Operating Manual

8.1 Manually Tuning the Control

Explanation of Closed Loop Block Diagrams
Control systems are usually represented by a series of interconnected blocks. The blocks represent the individual functions
of the system. The blocks are interconnected by a series of lines, which represent the variable or quantity involved with
directional arrows showing the direction of information fl ow. See Figure 8-1.
Input Command
(Desired Value)
Any closed loop system can be divided into four basic operations:
1. Measurement of the controlled variable. The controlled variable can be velocity, torque, etc. This measuring means is
accomplished using a sensor that converts the variable to an electrical signal that is compatible with the control inputs,
usually voltage or current. This signal now represents the controlled variable (Feedback).
2. Determination of the error. The summing junction compares the measured value of the controlled variable (Feedback
Input) with the Input Command (desired value) and generates an error signal. The operation is a simple mathematical
subtraction operation as follows:
Error Signal (
3. The error signal is then used by the control to change the motor speed or torque.
4. The motor speed or torque is then used to reduce the error signal by driving the control, and the fi nal controlled variable,
so that the actual value of the controlled variable approaches the Input Command value or desired value. It should be
noted that closed loop control systems are error actuated. In other words, an error must be present before the system
will try to correct for it.
Defi nition of Input Command (Desired Value)
The Input Command is the input signal set by the operator. This can represent speed or torque level.
Defi nition of Feedback
Feedback is the signal which represents the actual measured value from the controlled variable. This can represent a
pressure, fl ow, speed, torque, level or temperature sensor. This input is usually a sensor voltage or current representing the
measured value.
Defi nition of Error
Error is the result of subtracting the Input Command and Feedback signals.
Error is mathematically defi ned as:
Error Signal (
MN766
Figure 8-1 Block Diagram of a Closed Loop System
Summing Junction
(error detector)
Error
2
Signal
3
Output
1

) = Input Command - Feedback

) = Input Command - Feedback
Customizing Your Application
Power Input Load Disturbance
Control
Feedback
Measuring
Means
Controlled Variable
Chapter 8
4
Controlled
Variable
Motor
Customizing Your Application 8-1