Proximity Sensors

Proximity sensors include a wide variety of transducer types de scribed in previous sections: magnetic and inductive transducers (Section 9), variable-differential transformers (Section 10), magnetic pickups (Section 11), and photoelectric devices (Section 12).

Other types are based on pure sensors, including sonic sensors, solid-state proximity switches, and fluid devices. At the other extreme, the probe used may be a simple contacting type.

Solid-state proximity switches are based on a sensory element and associated electronics in a single package. Because they have no moving parts, they are particularly suitable for aircraft, ship, and vehicle applications when sensory information is required. This can apply from test and research information down to everyday applications—all met by specially designed devices.

Sensors of this type are somewhat similar in principle to magnetic pickups but with considerably greater sensitivity given by more sophisticated electronic circuitry. They work on the principle of detecting the interruptions to a generated electromagnetic field produced by an oscillator when a metal object comes within the range of the field. The best known type is undoubtedly the portable metal detector, some of which can detect metal objects buried three feet or more in the ground. Other detectors of similar type can even distinguish between different metals.

EDDY URRENT PROXIMITY PROBE

An eddy-current proximity probe is one of the various types of non-contact pickups used for sensing displacement. It consists of a probe or core wound with a coil connected by a coaxial cable to a driver. The driver produces a high-frequency signal that is fed to the coil, which generates a magnetic field surrounding the coil tip (ill. 25-1).

ill. 25-1. An eddy-current proximity probe. A nearby ferrous object disturbs the magnetic field around the coil, producing eddy currents and changing the impedance of the coil.

When a conductive material (of any metal) is approached by the probe tip, eddy currents are produced in the material, which has the effect of absorbing some of the field strength of the probe. The closer the tip to the object the greater the power absorbed.

Under such conditions the driver circuit measures the resulting field strength and compares it with the original to arrive at a difference signal, representing the distance between the probe tip and the surface of the object, which is converted into a standard calibrated output.

Probes of this type are capable of very sensitive and accurate measurement because they can produce an output of the order of 200 mV per thousandth of an inch. Thus, a change in relative position of probe and surface of, say, 0.0025 in. would produce a change in output voltage of 0.5 V. Response is also very rapid, so that the displacement of a vibrating surface having frequencies of vibration up to as high as 3500 Hz can readily be measured. e.g., a probe of this type can readily measure vibration of a rotating shaft.

Its sensitivity can, in fact, work against it in some applications. If it's being used to measure shaft vibration, for instance, it will also pick up and “measure” any shaft imperfections such as scratches, cracks, or dents, and even differences in plating thicknesses on a plated shaft. it's also affected by the proximity of strong electric currents or magnetic properties in the material being sensed.

SIMPLE CONTACTING PROBES

The working principle of a simple contacting probe is fairly obvious. If the object concerned is conductive, e.g., connecting a metal probe and object in a simple series circuit will result in the circuit being switched on whenever the probe contacts the object (ill. 25-2). This condition can be indicated by a motor, a light being turned on, or other ways, as required. Object and probe work just like a simple switch, and it does not matter which moves towards the other. Neither does the form of probe matter as long as it's capable of making contact with the object.

ill. 25-2. A simple contact probe.

This working principle is so simple that it's often ignored. Yet it can provide an easy answer to many proximity indicating requirements. To sense when a door is closed, e.g., the “object” could be one small metal plate mounted on the door contacting a spring plate on the door frame when the door is closed. (Making one plate in the form of a leaf spring can ensure possible contact by suitable bending.)

CAPACITIVE PROXIMITY PROBE

A similar principle can be employed in the noncontacting proximity probe, again where a conductive object is involved. This time the probe is in the form of a plate that, on approaching the object face to face, becomes effectively a capacitor with air dielectric (ill. 25-3). However, this is not so simple.

To be able to devise a signal indicating the near-pressure of the object, we must now detect the level of capacitance in the near- contact position; this will be very low. For a capacitor probe area of 1 cm e.g., within 1 mm of the object, the effective capacitance produced will only be about 1 micromicrofarad (uu1) Thus, noncontacting proximity sensors of this type don't lend themselves to simple construction. The same applies to most non- contacting proximity probes. They normally need fairly complex (but not necessarily sophisticated) electronic circuitry, either capacitor or magnetic types.

ill. 25-3. A simple capacitive proximity probe.

LEVEL DETECTORS

Detection of the height of liquid levels with contacting probes is, however, quite straightforward, provided the liquid is conductive (which most are, including water). The same principle as in ill. 25-2 is used, except that two equal probes are employed, mounted in a suitable position. As the liquid level rises, it eventually touches the ends of the two probes, the resistance between them changing from infinite (open-circuit condition) to some finite value.

A practical circuit for such a type of water-level detection is shown in ill. 25-4. The two probes are of stainless steel wire. Because the path between the probes is only moderately conductive when contacting the water level (the “closed”-circuit condition presents a high resistance between the probes), signal amplification is necessary to provide a usable output to give a reading on a milliammeter. This can be done by using a single-transistor dc amplifier, as shown.

ill. 25-4. A fluid-level-measuring device.

The single output of such a circuit can be greatly improved by adding an SCR to act as a power switch capable of working a bell, buzzer, or speaker, with a rated voltage of about 20 V less than the supply voltage used. Note that in this case the SCR covers the full output circuit through the alarm device. The transistor merely amplifies the original (much smaller) signal current and triggers the SCR into its switching mode.

A point to note with all immersible liquid-level probes is that if liquids containing dissolved metallic salts are involved, the in- put should be an ac signal to prevent plating deposits being formed in the probe(s).

PNEUMATIC SENSORS

ill. 25-5. A simple back-pressure proximity sensor. At A, no object is near the sensing jet; at B, an obstruction causes back flow.

Extremely sensitive sensors of simple form can be produced by using compressed air. They are noncontacting devices in the sense that the sensor itself never contacts the object it's sensing; only a compressed air jet impinges on the object.

In all cases the output signal devised is air pressure (or a change, in air pressure), which can be used to operate a switching element controlling an appropriate circuit. Normally, in industrial sensors of this type, the switching element used is a transducer converting air pressure (signal) into an electrical signal.

A basic type is the back-pressure proximity sensor shown in ill. 25-5. Compressed air is directed down the straight length of a T shaped tube assembly and normally passes straight through this tube. An object in the way of the emergent jet, however, will pro duce a back-pressure effect, causing flow out of the tee arm. The strength of this output signal is directly related to the proximity of the object to the end of the jet pipe.

This type of sensor produces a relatively weak signal unless the object approaches or passes close to the end of the jet pipe. It will, however, sense movement across the jet in any direction.

Using two separate tubes, aligned axially, with one tube fed with compressed air and the other acting as a receiver, produces a more sensitive sensor (ill. 25-6). In the absence of an object the output signal strength (pressure) is nominally the same as that of the supply. A solid object intruding in the gap reduces the output signal to zero.

ill. 25-6. A direct-pressure proximity sensor. Air flows freely when there is no obstruction (A) but is blocked by any obstruction in the gap (B).

This type of sensor is not critical on gap size, responds extremely rapidly, and is not sensitive to the actual shape in texture of the object, as long as it blocks the gap. Also, it provides essentially one output signal strength virtually corresponding to that of the supply pressure.

A rather more elaborate form of pneumatic proximity sensor is shown in ill. 25-7. Here the receiver tube is T-shaped and connected to an auxiliary supply at the far end at a pressure appreciably lower than that of the supply pressure. The object of this is to pro duce a back pressure that increases the output signal strength and at the same time purges the receiver of any entrained air.

ill. 25-7. A more elaborate form of direct-pressure sensor. At A, no obstruction; at B, obstruction in the gap.

This form of sensor is also used with a second supply tube at right angles to the first and directed at the jet gap (ill. 25-8). This works on a rather different principle. In the absence of any object the jet from supply 2 impinges on the jet across the gap from supply 1, rendering this gap flow turbulent and resulting in a very low-to- nil signal output.

ill. 25-8. A still more elaborate device. When there Is no obstruction (A), the output is essentially zero. The presence of an obstruction (B) produces pressure at the output.

ill. 25-9. A sophisticated back-pressure pneumatic proximity sensor. Air flows freely through the chamber and out of the sensing jet when there is no obstruction (A). The presence of an obstruction generates turbulence in the chamber and results in pressure at the output (B).

If now an object appears to block the jet from supply 2, the gap flow reverts to laminar. This change from turbulent to laminar flow produces a marked change in signal output level, which switches to full system pressure.

Finally, a configuration that combines the merits of the two is shown in ill. 25-9. Here two separate supplies are used, feeding into a larger chamber with a single outlet through which the sensing jet flows. Output is tapped off one of the supply pipes. This is a back-pressure system; the actual back pressure produced depends on the two supply pressures. These can be the same, that is, derived from the same source, in which case a restrictor is fitted in the receiver line to drop the back pressure to a suitable level.

This type of sensor can be extremely sensitive and work well over quite large sensing-gap distances. With a pressure of only 1.5 lb/in. e.g., it will work with a sensing gap of up to 1 in., and over much larger gaps with higher pressures.

Pneumatic proximity sensors make interesting projects to try because they are quite easy to make. Also, they don't need very high compressed air supplies to work, as we have just noted. The main problem lies in designing a suitable method of “reading” the signal output, which is air pressure. With low-pressure air supplies this is normally too low to detect by a simple pressure gauge. How ever, a simple pressure-operated switch is very easy to devise.

An elementary design of this type might consist of strips or leaves of any nonconducting material hinged together along one edge. A simple spring normally holds the two leaves together, like a closed-up hinge. Sandwiched between the two leaves is a thin rubber “balloon” with a neck tube. Contacts are fitted to each leaf so that when normally closed (balloon deflates) they come together and make contact (switch on).

The neck tube of the balloon is connected to the signal output of the pneumatic sensor. With no interruption to the jet this signal is at high level, inflating the balloon to open the leaves and the contacts (switch off). When the output signal falls, the balloon deflates along the hinged leaves to close the contacts. Contacts are finally adjusted to work at the required signal output level.

If necessary, of course, such a pressure-operated switch can be designed to work in the mode—switch off at high pressure and switch on at low pressure. Also, there are several alternatives to using a balloon; e.g., a bellows constructed from metal foil might be employed.

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