Magnetic and Inductive Transducers

The most elementary example of a magnetic transducer is the magnetic compass, which utilizes the force present in the earth’s magnetic field to align a suspended or pivoted magnet (pointer) in a North-South direction. In other words, it uses magnetic force to produce mechanical movement in any misaligned position of the pointer.

Arguably, though, this is not true transducer action but a separate phenomenon. Nevertheless, magnetic devices working on magnetic field principles are widely used as true transducers; they are also used in other applications to produce controlled movement (whether or not you regard this as transducer action).

A simple example of the latter is the magnetic follower. If two bar magnets are mounted on separate spindles and placed close together, they will align themselves with opposite poles facing each other (ill. 9-1). If one magnet is now rotated, the other will follow it. The two rotating systems are magnetically coupled: one being the driving system; the other, the driven system. If the driven system is braked by a load (such as appreciable friction), the driven magnet will lag behind the driving magnet by an amount proportional to the braking load. If this load is too high, the coupling effect will be overcome. The driven system will stall. Reducing the load will make it pick up again.

Magnetic drives of this type have numerous applications and can even work through a separating partition of nonmetallic material. If the driving magnet is faced with a metallic material, how ever, its rotation will induce eddy currents in the metal object. If this object is in the form of a disk mounted on a spindle, these eddy currents will effectively couple the two systems, causing the second to rotate with the first (ill. 9-2). This is eddy-current coupling.

ill. 9-1. Two magnets, placed adjacent to each other, will align with the opposite poles side by side (A). If one magnet is then rotated, the other will follow along (B).

ill. 9-2. Side view (A) and broadside view (B) of an eddy-current-coupled magnetic system.

This principle has been widely used in mechanical speedometer drives where the magnet is driven at “wheel” speed, the follower being an aluminum disc (or, more usually, cup stated) mounted on a spindle and lightly spring loaded to resist rotation.

The coupling thus pulls the follower (disc or cup) into sym pathetic rotation until the eddy-current coupling force is overcome by the spring force. The follower then stays in this position until the driving magnet speed changes. This will increase, or decrease, the eddy-current coupling strength, causing the follower to adjust to a new position. In this respect the follower is a speed sensor.

The eddy-current principle can also be used to provide distance or displacement sensing. In this case an electromagnet fed by alternating current is used. If it's located near a ferrous metal object, eddy currents will be generated in that object that will modify the impedance of the coil. The change in impedance will be directly related to the distance separating the pole of the electromagnet from the object (ill. 9-3).

ill. 9-3. When a coil supplied with alternating current is brought near a ferrous object, the coil impedance is altered.

This principle is used in one type of noncontacting proximity sensor, connected to appropriate circuitry to detect changes in impedance in the coil and to display this in terms of displacement (distance) from the object. Other types of noncontacting proximity sensors work on capacitance effect or interruption of fluid (usually air) flow.

The use of magnets (especially permanent magnets) as sensors/transducers is attractive because they are simple devices. An example of a magnetic flowmeter is given in ill. 9-4. Here two permanent magnets are mounted in the center of a pipe carrying fluid flow (A). Between them is a float containing a further magnet. With the magnet polarities shown in the diagram and no flow, the intermediate magnet will be suspended in the null field (midway between the two fixed magnets if they are of equal strength).

ill. 9-4. A magnetic flowmeter. At A, condition of no fluid flow; at B, method of measuring fluid flow by means of magnetic coupling.

When there is fluid flowing through the pipe, the pressure on the float will lift it upwards away from the null field or static position by an amount proportional to the velocity of the flow. The actual displacement of the float will thus be a measure of the flow velocity.

To measure this movement, you can use a follower magnet mounted outside the pipe (which must be nonmetallic in this case) to move a pointer over a scale, as shown in ill. 9-4B. This principle is used in certain proprietary flowmeters.

The majority of true transducers working in electromagnetic principles employ variable induction to generate the response required: e.g., to render physical displacement in terms of a corresponding electrical signal. Main types are the variable-inductance transducers, the differential transformer (described separately in a later section), and certain digital transducers.

The variable-inductance transducer is based on an ac coil with a movable iron core, armature, or diaphragm (ill. 9-5). Movement of this core (armature or diaphragm) varies the inductance of the coil, converting motion into an electrical signal. If the transducer is used as one element of a bridge, and if the bridge is excited by a fixed-frequency, constant-amplitude signal, any change in inductance in the transducer will upset the balance of the bridge, providing an ac signal output proportional to the amount of displacement of the core.

ill. 9-5. Variable-inductance displacement transducer.

The advantage of variable inductance transducers is that they usually exhibit a high signal-to-noise ratio and provide continuous resolution. They do, however, require ac excitation, and they can provide spurious readings if mounted close to magnetic objects.

HALL-EFFECT SENSORS

When a conductor carrying a current is located in a magnetic field, a difference in potential is generated between the opposed edges of the conductor in a direction perpendicular to both the cur rent and the magnetic field. This is known as Hall Effect, and the potential difference generated is known as the Hall voltage. This principle is used in magnetically operated position sensors and cur rent sensors. Hall-effect sensors, operated by a magnet in a plunger, are also used in mechanically operated solid-state switches.

Hall-effect sensors are used in a variety of instruments and equipment, including musical instruments, computer peripherals, home appliances, medical instruments, telephone equipment, office machines, farm machinery, copy machines, laboratory equipment, vending machines, and electronic games.

Other applications of Hall effect include cam, lever, or shaft positioning; cylinder positioning; length measurement; linear or rotary motion detection; sorting; current sensing; ignition timing; limit sensing; potentiometer and tachometer sensing.

A typical Hall-effect sensor looks like an electrical component with three or four ends. It may be unipolar or bipolar, the latter having a plus (South Pole) maximum operating point and a minus (North pole) minimum release point. It may also incorporate a flux concentrator, concentrating flux in the sensory area.

Three-lead devices provide one output, and four-lead devices two outputs: that is, one output increasing with an increase in gauss, and one output decreasing with an increase in gauss (differential outputs). Positive gauss separates the South Pole of the magnet facing the sensory area. Negative gauss represents the North Pole of the magnet facing the sensory area. Leads may be identified by numbers; e.g.,:

1 = - supply

2 = output

3 = + supply

Or, for differential outputs:

1 = - supply

2 = 02

3 = 01

4 = + supply

Or they may be color coded, such as:

red = + supply

black = — supply (ground)

gray = output

A typical circuit in which a Hall-effect sensor is used is shown in ill. 9-6, where the supply voltage is about 4-18 V, depending on the construction of the individual device.

ill. 9-6. A circuit for detecting position, using a Hall-effect sensor.

To use Hall-effect sensors, you must know their characteristics in order to know their operating point and release point over their intended working range (also the supply voltage required). Thus, for a Honeywell 513SS16, e.g., the maximum operating point is specified as 330 gauss and the release point as 85-305 gauss over a working temperature range of -40 to 1000 C.

In this example, to ensure reliable operation, at least 330 gauss must be presented to the sensor. The gauss level must then be reduced to less than 85 gauss to guarantee that the sensor will release.

Therefore, when selecting a Hall-effect sensor, you must know the flux density (in gauss) measured at the chip to be able to select a device with the best characteristics for a particular application and to select the best magnet to use with it.

- Head-on—The target is centered over the point of maximum sensitivity and is moved “head-on” to the sensor, then backed off.

- Slide-by—The target is moved across the face of the sensor at a specified distance.

- Rotary—A rotating target, such as a ring magnet, provides an alternating pattern of on-off actuation.

- Vane—The target, a ferrous metal vane, slides through (or rotates through) the gap between magnet and sensor.

OUTPUT CIRCUITS

Two types of output circuits are common to solid-state circuitry: current sinking and current sourcing. The names are derived from the location of the load in the output circuit.

- Current sourcing (open emitter): The load is connected between the output and ground. The load is isolated from the supply voltage when the sensor is off. When the sensor turns on, current flows from the power supply, through the sensor, and into the load. The sensor supplies a “source” of power to the load. A current- sourcing output is normally low, but it goes high when the sensor is on.

- Current sinking (open collector): The load is connected between the power supply and the sensor. In this circuit the load is isolated from ground when the sensor is off. When the sensor turns on, the circuit is complete, and current flows from the supply, through the load, through the sensor, and then to ground. The sensor switches or “sinks” the output current to ground. A current- sinking output is normally high, but it goes low when the sensor is on.

MAGNETIC VELOCITY TRANSDUCERS

Magnetic/inductive-type transducers can be used to measure linear velocity. The usual form of such a transducer is two series- connected coils wound over a long, thin tube of nonmagnetic material. A free-floating permanent magnet lightly fitting the core of the tube is used as an armature. Motion of the armature in either direction produces a signal directly proportional to velocity as the magnet traverses the length of the coils. This signal is self- generated (that is, no external electrical supply is required) and can be read off by a standard high-impedance voltmeter having sufficiently high speed of response.

To design a simple project find a suitable cylindrical magnet of small diameter and reasonably long length. Look for an Alnico 5 magnet of suitable proportions. Then find a rigid plastic tube that is just large enough to let the magnet slide easily through it. Alternatively, wind a tube from gumstrip or similar material around a suitable size of mandrel (e.g. doweling). Make the tube length about 1.5 times the magnet length. Wind the two coils about each side of the midpoint of the tube, using an equal number of turns for each coil. Connect the two coils in series (end of coil 1 to beginning of coil 2). Connect the beginning of coil 1 and the end of coil 2 to a high-impedance voltmeter.

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Fundamentals of Transducers (all articles)