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Author: Jim Lepkowski Microchip Technology Inc.

AN894
A list of the sensors that can be used to feedback information to a microcontroller are listed below: • Current sensors - Shunt resistor - Current-sensing transformer - Hall effect current sensor • Speed/position sensors - Quadrature encoder - Hall efect tachometer • Back EMF/Sensorless control method

Motor Control Sensor FeedbackCircuits

INTRODUCTION
Sensors are a critical component in a motor control system. They are used to sense the current, position, speed and direction of the rotating motor. Recent advancements in sensor technology have improved the accuracy and reliability of sensors, while reducing the cost. Many sensors are now available that integrate the sensor and signal-conditioning circuitry into a singlepackage. In most motor control systems, several sensors are used to provide feedback information on the motor. These sensors are used in the control loop and to improve the reliability by detecting fault conditions that may damage the motor. As an example, Figure 1 provides a block diagram of a DC motor control system to show the sensor feedback provided for a typical motor control.

PowerManagement

Torque PICmicro® Speed Input Microcontroller Driver Motor

Direction

Current Sensor

Feedback

Sensors * Speed * Shaft Position * Rotation Direction

FIGURE 1:

Typical DC Motor Block Diagram.

 2003 Microchip Technology Inc.

DS00894A-page 1

AN894
CURRENT SENSORS
The three most popular current sensors in motor control applications are: • Shunt resistors • Halleffect sensors • Current transformers Shunt resistors are popular current sensors because they provide an accurate measurement at a low cost. Hall effect current sensors are widely used because they provide a non-intrusive measurement and are available in a small IC package that combines the sensor and signal-conditioning circuit. Current-sensing transformers are also a popular sensor technology,especially in high-current or AC line-monitoring applications. A summary of the advantages and disadvantages of each of the current sensors is provided in Table 1. Figure 2 shows an example of an AC motor powered by a three-phase inverter bridge circuit. This example shows that the composite current of all three Insulated Gate Bipolar Transistor (IGBT) circuit legs can be measured with a single shuntresistor, or that the current in each individual leg can be determined with three shunt resistors. Figure 2 shows a system that uses shunt resistors. However, Hall effect and currentsensing transformers can also be used to provide the current measurement.

TABLE 1:
Accuracy

COMPARISON OF CURRENT SENSING METHODS
Shunt Resistor Good Good Low No Poor Yes No High Yes Both Hall Effect Good PoorHigh Yes Good No Yes Low No Both
Current Measurement with Three Shunt Resistors

Current Sensing Method Accuracy vs.Temperature Cost Isolation High Current-Measuring Capability DC Offset Problem Saturation/Hysteresis Problem Power Consumption Intrusive Measurement AC/DC Measurements
Current Measurement with a Single Shunt Resistor

Current Sensing Transformer Medium Good Medium Yes Good NoYes Low No Only AC

AC Motor

AC Motor

VDC

VDC

IA I = IA+ IB + IC

IB

IC VOUT R SENSE_A

IA

IB VOUT_A VOUT_B

IC

VOUT_C

RSENSE

RSENSE_B

RSENSE_C

FIGURE 2:

AC Motor Current Measurement.
 2003 Microchip Technology Inc.

DS00894A-page 2

AN894
Shunt Resistors
Shunt resistors are a popular current-sensing sensor because of their low cost and goodaccuracy. The voltage drop across a known low value resistor is monitored in order to determine the current flowing through the load. If the resistor is small in magnitude, the voltage drop will be small and the measurement will not have a major effect on the motor circuit. The power dissipation of the resistance makes current shunts impractical for measurements of more than approximately 20...
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