Industrial Motor Control: Proximity Detectors

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  • Describe the operation of proximity detectors.
  • Describe different types of proximity detectors.


Proximity detectors are basically metal detectors. They are used to detect the presence or absence of metal with out physically touching it. This prevents wear on the unit and gives the detector the ability to sense red hot metals. Most proximity detectors are designed to detect ferrous metals only, but there are some units that detect all metals.

Circuit Operation

There are several methods used to make proximity detectors. One method is shown in Fgr. 1. This is a very simple circuit intended to illustrate the principle of operation of a proximity detector. The sensor coil is connected through a series resistor to an oscillator. A voltage detector, in this illustration a voltmeter, is connected across the resistor. Since AC voltage is applied to this circuit, the amount of current flow is determined by the resistance of the resistor and the inductive reactance of the coil. The voltage drop across the resistor is proportional to its resistance and the amount of current flow.

If ferrous metal is placed near the sensor coil, its inductance increases in value. This causes an increase in inductive reactance and a decrease in the amount of current flow through the circuit. When the current flow through the resistor is decreased, the voltage drop across the resistor decreases also (Fgr. 2). The drop in voltage can be used to turn relays or other devices on or off.

Fgr. 1 Simple proximity detector.

Fgr. 2 The presence of metal causes a decrease of voltage drop across the resistor.

Fgr. 3 Tuned tank circuit used to detect metal.

This method of detecting metal does not work well for all conditions. Another method, which is more sensitive to small amounts of metal, is shown in Fgr. 3. This detector uses a tank circuit tuned to the frequency of the oscillator. The sensor head contains two coils instead of one. This type of sensor is a small transformer. When the tank circuit's tuned to the frequency of the oscillator, current flow around the tank loop is high. This causes a high voltage to be induced into the secondary coil of the sensor head.

When ferrous metal is placed near the sensor, as shown in Fgr. 4, the inductance of the coil in creases. When the inductance of the coil changes, the tank circuit no longer resonates to the frequency of the oscillator. This causes the current flow around the loop to decrease significantly. The decrease of current flow through the sensor coil causes the secondary voltage to drop, also.

Notice that both types of circuits depend on a ferrous metal to change the inductance of a coil. If a detector is to be used to detect nonferrous metals, some means other than changing the inductance of the coil must be used. An all-metal detector uses a tank circuit as shown in Fgr. 5.All-metal detectors operate at radio frequencies, and the balance of the tank circuit's used to keep the oscillator running. If the tank circuit becomes unbalanced, the oscillator stops operating. When a non ferrous metal, such as aluminum, copper, or brass, is placed near the sensor coil, eddy currents are induced into the surface of the metal. The induction of eddy currents into the metal causes the tank circuit to become unbalanced and the oscillator to stop operating. When the oscillator stops operating, some other part of the circuit signals an output to turn on or off.

All-metal proximity detectors will sense ferrous metals better than nonferrous. A ferrous metal can be sensed at about three times the distance of a nonferrous metal.

Fgr. 4 The presence of metal detunes the tank circuit.

Fgr. 5 Balance of the tank circuit permits the oscillator to operate.


Some proximity detectors are made as a single unit.

Other detectors use a control unit that can be installed in a relay cabinet and a sensor that's mounted at a remote location. Fgr. 6 shows different types of proximity detectors. Regardless of the type of detector used, care and forethought should be used when mounting the sensor. The sensor must be near enough to the target metal to provide a strong positive signal, but it shouldn't be so near that there is a possibility of the sensor being hit by the metal object. One advantage of the proximity detector is that no physical contact is necessary between the detector and the metal object for the detector to sense the object.

Sensors should be mounted as far away from other metals as possible. This is especially true for sensors used with units designed to detect all types of metals.

In some cases, it may be necessary to mount the sensor unit on a nonmetal surface such as wood or plastic. If proximity detectors are to be used in areas that contain metal shavings or metal dust, an effort should be made to place the sensor in a position that will prevent the shavings or dust from collecting around it. In some installations, it may be necessary to periodically clean the metal shavings or dust away from the sensor.

Fgr. 6 Proximity detector

Fgr. 7 Capacitive proximity detectors.

Capacitive Proximity Detectors

Although proximity detectors are generally equated with metal detectors, there are other types that sense the presence of objects that don't contain metal of any kind.

One type of these detectors operates on a change of capacitance. When an object is brought into the proximity of one of these detectors, a change of capacitance causes the detector to activate. Several different types of capacitive proximity detectors are shown in Fgr. 7.

Since capacitive proximity detectors don't depend on metal to operate, they will sense virtually any material such as wood, glass, concrete, plastic, and sheet rock. They can even be used to sense liquid levels through a sight glass. One disadvantage of capacitive proximity detectors is that they have a very limited range. Most cannot sense objects over approximately one inch or 25 millimeters away. Many capacitive proximity detectors are being used to replace mechanical limit switches since they don't have to make contact with an object to sense its position. Most can be operated with a wide range of voltages such as 2 to 250 volts AC, or 20 to 320 volts DC.

Ultrasonic Proximity Detectors

Another type of proximity detector that does not depend on the presence of metal for operation is the ultrasonic detector. Ultrasonic detectors operate by emitting a pulse of high frequency sound and then detecting the echo when it bounces off an object (Fgr. 8).

These detectors can be used to determine the distance to the object by measuring the time interval between the emission of the pulse and the return of the echo. Many ultrasonic sensors have an analog output of voltage or current, the value of which is determined by the distance to the object. This feature permits them to be used in applications where it's necessary to sense the position of an object (Fgr. 9). An ultrasonic proximity detector is shown in Fgr. 10.

Fgr. 8 Ultrasonic proximity detectors operate by emitting high frequency sound waves. THE DETECTOR EMITS A PULSE OF HIGH FREQUENCY SOUND WAVES. THE ECHO IS DETECTED WHEN IT BOUNCES OFF AN OBJECT.

Fgr. 9 Ultrasonic proximity detectors used as position sensors.

Fgr. 10 Ultrasonic proximity detector.


1. Proximity detectors are basically ___.

2. What is the basic principle of operation used with detectors designed to detect only ferrous metals?

3. What is the basic principle of operation used with detectors designed to detect all types of metals?

4. What type of electric circuit's used to increase the sensitivity of the proximity detector?

5. What type of proximity detector uses an oscillator that operates at radio frequencies?

6. Name two types of proximity detectors that can be used to detect objects not made of metal.

7. What is the maximum range at which most capacitive proximity detectors can be used to sense an object?

8. How is it possible for an ultrasonic proximity detector to measure the distance to an object?

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