Magnetic and Eddy Current Type Transducers for use in Industrial Electronics-Zwe.pdf

MAGNETIC AND EDDY CURRENT TYPE TRANSDUCERS

FOR USE IN INDUSTRIAL ELECTRONICS

David L. Elam

Electro Products Laboratories, Inc.

Chicago, Illinois

are discussed with regard to practical applica- shift takes place. The angular flux shift is af-

tion. The principles of operation are discussed fected by several factors. Ths mass of the ex-

briefly and the output voltages illustrated are citing metal will determine the amount of angular

taken under actual operating conditions at various displacement of the flux lines and thereby the

speeds. The mechanical construction is covered in instantaneous rate of speed of the shift. The

progressive steps. Various models for both low speed at which the exciting metal moves through

and high temperature applications are described. the field produces a direct change of the speed

of the angular flux shift, and the clearance be-

A brief description of the principle of op- tween the pickup and the exciting means has about

eration of the eddy current or proximity pickup is the same effect as varying the mass as explained

presented. The operating clearance range is given above. From a practical standpoint, the output

for various models. A description is given of the of the magnetic pickup tends to vary directly

use of proximity pickups for counting work parts, with the speed of the exciting means and as an

for controlling feed and for initiating steps of inverse square of the clearance. With the excep-

operation in automated factories. tion of our extra sensitive low speed model, the

resonant frequency of the magnetic pickup is very

Introduction high -- in the order of 300 kc. The capacity of

long cables will reduce this value. The writer

This presentation is given to familiarize ha.s excited them at 50 kc, and they have been

you with the general construction and use of mag- used with turbines at 70,000 to 80,000 rpm. In

netic and eddy current type pickups. Since the these instances, the pulse rise and fall time be-

basic theories involved here are well covered in comes exceedingly fast.

electrical and radio text books and are well

known, the writer shall discuss mainly their mech- The simplest way to make a magnetic pickup

anical and applicational aspects. is to wind a coil around one end of a permanent

magnet. Such a pickup would work very well and

Mnetic Pickups produce excellent power output. It would, how-

ever, be quite bulky and would require quite a

Magnetic pickups are self-generating devices large actuating mass and, therefore, find only

which require no external power supply or auxili- limited applications. We might use a very small

ary equipment. They are excited by mounting them magnet diameter, but we would sacrifice most of

so that some ordinary iron or steel piece is made our magneto-motive force and get very little out-

to move in and out of the pickup's magnetic field. put from the pickup.

The actuating member may be the teeth of a gear,

vanes of a turbine or any moving device having A good compromise is to use a relatively

steel projections such as screw heads or steel large magnet with a small diameter soft iron pole

pins. piece attached to one end. This supplies a high-

ly concentrated magnetic field in a small diame-

The voltage output of magnetic pickups is ter and makes possible a small pickup with high

tremendous when compared with other types of output, which may be excited by a relatively

transducers, which usually generate a few milli- small mass. This is the basic design of all of

volts. As we shall show later, the output of

magnetic pickups, in some instances, may be in

excess of 100 volts. The magnetic pickup is so Magnetic pickups will produce the greatest

positive in its action, rugged and fool-proof, output when the dimensions of the exciting member

that it is the obvious choice as a linkage be- equals or exceeds the diameter of the pickup ex-

tween any mechanical device and electrical in- ceeds the diameter of the pickup pole piece and

strumentation or control system. the air gap between exciting member exceeds two

times the pole piece diameter. For most of our

The magnetic pickups consist of a cylindrical pickups these conditions are met by exciting with

shaped coil of wire within a stray magnetic a gear having 'teeth of 10 diametral pitch. There

field having flux lines extending radially is usually more than adequate output at medium

through the coil. When a body of magnetic steel and high speeds with gears as small as 48 pitch.

is placed near the end of this coil, the flux When smaller teeth are to be used, the resolution

lines will tend to shift from a radial to an can be improved by filing the pickup pole piece to

axial direction and thereby induce a current in a dull or truncated chisel edge, or in some cases

the turns of the coil. to a conical shape.

The mlagnetic pickup is a rate-of-change gen- We Shall now Show the progressive steps in

erator. The voltage produced tends to vary di- assemibling them. Figure 1 shows the bobbin and

our magnetic pickups.

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SHMy -- The characteristics of magnetic pickups rectly with the speed at which this angular flux

coil in place on the pole piece. In Fig. 2 the devices. The output curve for our Model 3010-A,

ends of the coil wires have been attached to a which we use as our standard, is shown in Fig. 11.

pair of heavy leads which are taped to the magnet. Curve B shows the voltage for surface speeds up to

Figure 3 is an exploded view of the complete pick- 100 inches per second. Note that at 3,400 inches

up showing the coil-magnet assembly, the pickup 140 volt output is obtained.

shell and the connector. The coil assembly is

placed in the shell and the shell is filled to the Figure 12 is the curve for our 3015 or minia-

back end of the magnet with an epoxy potting res- ture pickup. This has about one-third the output

in. When the resin is cured, it completely in- of the standard unit. The output data for our ex-

capsulates the coil winding and renders it imper- tra sensitive Model 3030 is shown in Fig. 13.

vious to moisture, oils and many chemicals. The This pickup is recommended for very slow speeds.

epoxy resin also forms a solid, rigid mechanical Note that at 10 inches per second there is a 3

support having mechanical properties similar to volt output.

those of bakelite.

The next step is to insert the connector and can't be used because of space tations, or

fasten the leads to the connector pins. Figure 4 other reasons. In such casea, special pickups are

slhows the completed pickup after the pole piece provided to fit the aplication.

has been cut to length and any escaped resin

cleaned off. Figure 14 shows two of these.specials. The

We have two basic type magnetic pickups -- one on the left was made for military aircraft

In many instances, the regular stock pickup

the low tevperature one, which we used for our over speed control for the turbine driving the

construction views, and one made for high temper- cabin pressurizer in one of the new Jet air liners

use. and the one on the right will actuate an

atures. The low temperature models use bakelite cabin pesrider in oeoft n etil r

bobbins and regular magnet wire. The connections

are made with conventional soft solder. These

pickups will stand temperatures of about 2500 F.

The high temperature models use teflon bobbins

and teflon-insulated magnet wire. The connections

r m i

are made by silver brazing or high temEperature ordinary concept of transducers. Transducers, as

soldering. These may be operated up to 5000 F. a rule, produce a voltage as an analog of some

The proximity pickup is entirely outside the

function. The proximity system closes a relay be-

cause of the presence of a mass of metal. It is

Figure 5 shows a group of pick~ups which are atrebn obialedvc.TreP 0

standard models. The smallest one on the left

a true binary or bistable device. There are no

mounts in a i inch by 40 threaded hole. The next intermediary states.

two have 3/8 inch by 24 threads, and the rest are

provided with 5/8 inch by 18 mounting threads. Tro ofim ica sequence or feed cntrl

Alpickfup shells are made of type #416 stainless control of mechanical sequence or feed control,

The proximity pickup is most useful in the

steel and are rust-resistant. The ones with the

white ends are the teflon-insuliated high tempera- protective device by shutting off the power when

ture models.

and for counting parts. It has been used as a

used to start feed devices when a feed rack is

Figure 6 shows our subminiature pickup along- empty, and stop the feed when the rack is fuill.

side of a match and a dime for comparison. This It is being used in. automatic packaging to place

pickup is 1 inch in diameter and 1 inch long and an exact quantity of items in each package, and

weighs less than 4 grams. This pickup_is intended it is being used to count production where workers

tor aircraft and missile use. Our goal here was are payed by the piece rates.

to produce the smallest possible pickup with a

practical voltage output. The proximity pickup is an eddy current loss

device. The sensing unit is a coil which radiates

The output voltage of this pickup is shown in a high frequency magnetic field. When any con-

Fig. 7. The output is plotted against the surface ducting body comes within the field of this coil,

speed of a 20 pitch gear which is used to excite eddy currents are induced in it and it takes en-

the pickup. A new power type pickup is shown in ergy from the coil. This loss of energy stalls

Fig. 8. This pickup will generate enough power to the high frequency oscillator. A small part of

operate a tube plate type relay without any amli- the high frequency voltage frbm this oscillator

fication. Figure 9 shows the output power of this is used to keep a relay tube biased to cutoff.

pickup at varying excitation speeds.

a machine becomes jammed or stalled. It is being

When the oscillator stops, the bias is removed

from the relay tube and it closes the relay.

For operating a relay, the pickup output is

rectified as shown in Fig. 9 and connected to the The operation of the proximity pickup does

relay coil. Such a rectifier and relay combina- not depend on motion or speed, and it needs no

tion is shown in Fig. 10. This with the power contact or mechanical linkage. It requires only

pickup makes an excellent self-powered over and the presence of conducting metal in the field of

under speed control. the coil. It-may be any kind of metal, but steel

can be detected at a greater distance than low

All of the other magnetic pickups are voltage resistance metals such as copper or silver. The

genera.tors and intended to drive high imEpedance mass of thickness is of very Little importance,

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Aleel pickup shellsstresimtaDeoftype #416estainls

just so long as the area presented to the coil is hanging up. If the parts are of carbon steel, a

large enough to absorb energy from it. given coil will detect anything having a cross-

section equal to or greater than the coil radius.

These pickups will easily detect thin foil When relatively large parts are to be detected at

wrapper as used on cigarette packs or candy bars. a distance of several inches, large flat or pan-

We have successfully used this system for count- cake coils can be furnished as in Fig. 19. We

ing the element supporting wires of receiving

across and they will operate at a distance approx-

imately equal to the coil radius.

The proximity pickup circuit is shown in Fig.

15. As you can see, it is a basic Hartley oscil- One of the proximity control units is shown

lator. Since there is no grid resistor and con- in Fig. 20. This unit has a relay in it which

denser, the tube operates as a class "A" ampli- will handle up to 5 amperes. These may also be

fier. The resistance in the cathode circuit furnished with a 5 volt dc output or with positive

serves both to maintain proper tube bias and as a and negative pulses exceeding 40 volts.

source of degeneration. The degeneration limits

the gain in the tube and increases the importance Figure 21 is the control unit furnished to

of the coil Q as a factor in maintaining oscilla- machine tool users. It is mounted in a heavy dust,

tor. This renders the oscillator very sensitive oil and leak-proof JIC approved box as required

to any loading of the coil. for mounting on machine tools. An automation set-

up for making automobile transmission gears is

The capacities indicated by the dotted lines shown in Fig. 22.

are the associates distributed capacities of the

pickup coil and cable. These are sufficiently Some 200 proximity controls and pickups are

large as to satisfy the requirements for the res- used in this installation. They are used for

onant circuit.

feed and sequence control as well as for counting

the finished gears. Figute 23 is a closeup of the

Several types of pickups are used with this pickups mounted above the track which conducts the

system. The one in Fig. 16 has a coil Of the

smallest diameter which is practical. The coil is gears along the line.

located just behind the bakelite face at the end

of the pickup. It will detect very small bits of

metal at a distance of 1/16 to 1/8 inch. Figure

17 shows a pickup having a coil about 3/4 inch in Magnetic transducers are used to conmert

diameter. It will detect larger pieces of metal mechanical motion to electrical output. It has

at a distance of about 3/8 inch. been shown that voltage outputs from these devices

are phenomenal as compared to most generally

A hollow coil pickup is shown in Fig. 18. available transducers. Sufficient power can be

This type of pickup is used for counting and con- generated under certain conditions to operate re-

trol work, where it is possible to have the metal lays without amplifiers.

parts pass through the coil. When the hollow coil

is used it is not necessary to position or orient Proximity transducers are really systems con-

the metal parts. They may tumble or roll through sisting of an oscillator controlling a relay and a

without being able to evade the active field of remotely located sensing coil. Motion is not a

the coil. These coils are supplied in a great necessary element of operation, but rather Uprox-

number of diameters to accommodate the size of imity" of metal is the operating factor. It has

the parts involved. For nonferrous or low resis- beeen shown that proximity transducers have prac-

tance materials, the hollow coil must be just tical application in control of mechanical devices

large enough for the parts to pass through without and for counting metallic objects.

Conclusion

Fig. 1 - Magnet, bobbin and coil assembly.

Fig. 2 - Magnet, bobbin, coil and leads..

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type vacuum tubes.

have made this type of coil as large as 18 inches

Fig. 3 - Exploded view of standard magnetic pickup.

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Fig. 4 - Completed mag-

netic pickup.

Fig. 5 - Eight standard stock magnetic pickups.

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.0 'LAU :.'Pe __8E

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Fig. 6 -Model 3M55 sOL

too -q

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51atLrU magnetic-E

S,cp

Fig. 7

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pickup.

_ _ _ a ta,t. zWI WE ez- -

bj. RO v t

NO )A _t L kw-:

- ~~~~I( Wf 11 .

Fig. 8 -Model 3040 power mag- __

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Fig. 9

Fig. 10 - Model 3420 over/under speed control relay.

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_etic pick-up.

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