US3786883A

US3786883A - Electromagnetically-compensated balance having series connected coils

Info

Publication number: US3786883A
Application number: US00287854A
Authority: US
Inventors: P Kunz
Original assignee: Mettler Instrumente AG
Current assignee: Mettler Toledo GmbH Germany
Priority date: 1971-09-22
Filing date: 1972-09-11
Publication date: 1974-01-22
Anticipated expiration: 1991-01-22
Also published as: NL7210824A; DE2233850B2; FR2153235A1; DE2233850A1; JPS5149428B2; JPS4841767A; FR2153235B1; CH529999A

Abstract

An electromagnetically-compensated balance of the type including load and reference compensation coils for returning load and reference assemblies to their neutral positions relative to the balance housing, characterized in that the reference compensation coil is connected in series with reference current regulating means to define a series branch that is connected at one end with one terminal of a direct-current voltage source. Electronic switch means alternately connect the other end of said series branch in series with, and disconnect said other end of said series branch from, said load compensation coil and said voltage source, respectively. The reference current regulating means are operable by a reference control voltage as a function of the displacement of the reference assembly from its neutral position, and the electronic switch means are operable by a load control voltage to effect series connection between said load and reference compensation coils for time periods that correspond with the displacement of the load assembly from its neutral position. Buffer means are provided for effecting a galvanic coupling between the load and reference coil switching circuits, and stabilizing means-such as a dummy load or a catching diodeare provided for stabilizing the switching operation.

Description

[ Jan. 22, 1974 United States Patent 91 Kunz [ ELECTROMAGNETICALLY- COMPENSATED BALANCE HAVING SERIES CONNECTED COILS [75] I Inventor: Peter Kunz, Meilen, Switzerland [73] Assignee: Mettler Instrumente, AG, Zurich,

Switzerland [22] Filed: Sept. 11, 1972 [21] Appl. No.: 287,854

[30] Foreign Application Priority Data Sept. 22, 1971 Switzerland 013867/71 [52] US. Cl 177/164, 177/210, 177/212, 177/25 [51] Int. Cl. G01g 7/04, 601g 23/16 [58] Field of Search 177/164, 210, 212, DIG. 5,

[56] References Cited UNITED STATES PATENTS 3,688,854 9/1972 Strobel 177/210 3,322,222 5/1967 Baur 177/210 3,295,368 l/l967 Nevius 177/210 3,186,504 6/1965 Van Nilgen.. 177/212 3,172,493 3/1965 Koch et al. 177/210 3,133,606 5/1964 Thomson 177/210 7 N 15 1 I 1 GEN.

ile

Primary Examiner-Stephen .1. Tomsky Assistant ExaminerVit W. Miska Attorney, Agent, or Firm-Lawrence E. Laubscher [5 7] ABSTRACT An electromagnetically-compensated balance of the type including load and reference compensation coils for returning load and reference assemblies to their neutral positions relative to the balance housing, characterized in that the reference compensation coil is connected in series with reference current regulating means to define a series branch that is connected at one end with one terminal of a direct-current voltage source. Electronic switch means alternately connect the other end of said series branch in series with, and disconnect said other end of said series branch from, said load compensation coil and said voltage source, respectively. The reference current regulating means are operable by a reference control voltage as a function of the displacement of the reference assembly from its neutral position, and the electronic switch means are operable by a load control voltage to effect series connection between said load and reference compensation coils for time periods that correspond with the displacement of the load assembly from its neutral position. Buffer means are provided for effecting a galvanic coupling between the load and reference coil switching circuits, and stabilizing meanssuch as a dummy load or a catching diode-are provided to; stabilizing the switching operation.

FORWARD REVERSE LOGIC COUNTER CONTROL FREQUENCY 7 DIVIDERS 4O COUNTER BINARY CODED DECIMAL OUTPUT -Vdc INDICATOR DIFFERENCE SIGNAL GEN.

SIIEEI 2 III 2 25 26 2?: DIFFERENCE CURRENT PULSE SIGNAL LENGTH REGULATOR GENERATOR MOD.

I v 1 l EF. T 23x I I W v '2';- V D T PAIENTEI] JAN22I974 BUFFER 22 DIFFERENCE CURRENT SIGNAL GENERATOR REGULATOR Fig. 4

ELECTROMAGNETICALLY-COMPENSATED BALANCE HAVING SERIES CONNECTED COILS This invention relates to load-compensation balances including load and reference assemblies independently suspended from a stationary part of the balance, each of the assemblies being provided with a compensation coil arranged in a common magnetic field. Detector means are provided for generating load and reference control signals the magnitudes of which are a function of the displacement of the load and reference assemblies from their initial neutral positions relative to the housing, respectively, said control signals being operable to regulate the currents flowing in said compensation coils, respectively. Such balances are of the type disclosed in the U.S. patents to Baur No. 3,322,222 and Stroebel No. 3,688,854. In these prior arrangements, both the compensation coil of the reference assembly and also the compensation coil of the load assembly are fed by separate direct currents. In the first mentioned balance the result if determined by the difference between the two currents being related to the current in the reference coil (as determined, for example, by a balanced-bridge network). In the second arrangement, the reference assembly movement is compensated by a constant direct current, position detector means being provided for detecting the position of the reference assembly to influence by way of correction coils the magnitude of the common magnetic field and thus indirectly the magnitude of the compensation current in the load coil.

With these arrangements it is possible to eliminate errors arising from the acceleration due to gravity, which varies according to location, and errors arising from the balance being set in a slightly inclined position. Further, the arrangement of the compensation coils of the load assembly and the reference assembly in the same magnetic field eliminates errors arising from fluctuations in the magnetic field as a result of aging or temperature influences. Degrees of precision of possibly even 10-, can be attained by eliminating these sources of error.

These advantages of the above proposedarrang ments are opposed, however, by the disadvantage that expensive components are required for effectively achieving the degrees of precision which are possibly on the basis of the concept of the arrangement. The balance according to U.S. Pat. No. 3,688,854 requires a high-precision reference voltage source for the very precise constant-current generator that produces the compensation current for the reference assembly, as the degree of any magnetic field correction, and thus the precision of measurement, depends on the precision or constancy of that compensation current.

In the arrangement according to U.S. Pat. No. 3,322,222, inter alia high precision resistors are necessary. This involves careful balancing both in assembly and also during the working life of the balances, as a certain aging of the resistors (long duration drift) prevents a constant degree of precision of better than The present invention was developed to avoid the above and other drawbacks of the known electromagnetically-compensated balances.

A primary object of the present invention is to provide an electromagnetically compensated balance including a reference compensation coil connected in series with reference current regulating means to define a series branch, and electronic switch means operable to alternately connect and disconnect this series branch, respectively, in series with a load compensation coil and a direct-current voltage source. In accordance with a characterizing feature of the invention, the reference current regulating means are operable by a reference control voltage as a function of the displacement of the reference assembly from its normal neutral position, and the electronic switch means are operable by a load control voltage to effect series connection between said load and reference compensation coils for time periods that correspond with the displacement of the load assembly from its neutral position.

A more specific object of the present invention is to provide an electromagnetically compensated balance of the type described above including buffer means arranged between the load and references compensation coils to form a galvanic coupling the amplification ratio of which is preferably lzl This arrangement has the advantage that the analog action control circuit of the reference assembly cannot be influenced by interference pulses from the circuit of the load assembly.

A further object of the invention is to provide apparatus of the type described above including means for stabilizing the reference compensation coil circuit during the switching operation. In one embodiment, the stabilizing means includes dummy load means to which the reference compensation coil circuit is connected during the periods when it is disconnected from the load compensation coil. In a second embodiment, the stabilizing means comprises a catching diode that connects the reference compensation coil with ground during the periods the reference compensation coil is disengaged from the load compensation coil.

In accordance with another object of the invention, the electronic switch means connects the load coil in series with the reference coil for time periods that correspond with the magnitude of the control voltage developed by the load position detector means, which control voltage is a function of the displacement of the load assembly from the neutral position. The same oscillator means that develops a sawtooth waveform for operating a pulse length modulator that controls the operation of the electronic switch means provides also counting pulses that are supplied to a counter during the time period that the load coil is connected in series with the reference coil.

According to a more specific object of the invention, the balance includes counter and indicator means which during the duration of the flow of compensation current in the load coil utilizes high frequency pulses of constant frequency for digital indication or evaluation. This method permits rapid and precise determination of the measuring value, with virtually any degree of resolution. Preferably the arrangement is such that only one oscillator is used for supplying the counting pulses and also for controlling the pulse length modulator and the switching means. Thus, the requirements made as to the stability of frequency of the oscillator do not have to be high, as fluctuations act in the same manner on the respective predetermined counting time as on the number of counted pulses.

The possible uses of the balance can be enlarged by means of an arrangement in which the counter is a forward-reverse counter and there is provided a taring means which includes a forward-reverse logic circuit associated with the forward-reverse counter, a tare storage means and a zero detector, and a counter control means for controlling the above mentioned components, and a tare switch.

Other objects and advantages of the invention will become apparent from a study of the following specification when considered in the light of the accompanying drawing, in which:

FIG. 1 is a diagrammatic cross-sectional view a first embodiment of the electromagnetically compensated balance of the present invention, the electrical circuit being illustrated in block diagram form;

FIG. 2 is a voltage versus time diagram illustrating the operation of the pulse length modulator;

FIG. 3 is a detailed schematic diagram of the buffer means of FIG. 1; and

FIG. 4 is an electrical schematic diagram of a second embodiment of the invention.

Referring first to FIG. 1, the balance comprises a stationary portion 1 including a housing 2 which is made of magnetic material and is closed on all sides, and a permanent magnet 3 arranged centrally in the housing 2. The housing 2 is secured by means of an annular flange 4 to a cantilever bracket 5 which in turn is fixedly connected with a column 6 that is carried by a fixed support.

Relative to an inward projection 8 on the housing 2, the middle portion 7 of the permanent magnet 3 forms an annular air gap 9. Arranged in the air gap 9, and vertically movable independently of each other, are the annular load coil 10, which is a component ofa load assembly 12 of the balance, and the annular reference coil 11, which is a component of a reference assembly 13.

The load assembly 12 and the reference assembly 13 are each connected with the column 6, and thus guided parallel to each other, by two pairs of resilient suspension means 14 and 14' and 15 and 15', respectively. A plurality of

apertures

16 and 16 in the upper and lower end portions respectively of the housing 2 permit free vertical movement of the load assembly 12 and the reference assembly 13. Arranged above the suspension means 14 of the load assembly 12, and secured thereto, is a balance pan 17 for carrying the material to be weighed.

Arranged concentrically in pairs above and below the air gap 9 and symmetrically relative thereto, are four stationary annular capacitors 18 and 18', 19 and 19', respectively. The capacitors l8 and 18 act as position detectors in respect of the reference assembly 13.

The negative terminal of the direct-current voltage source S is connected with the input terminal of change-over switch means 23 via current regulator means 21, reference compensation coil 11 and buffer means 22 (which includes at least one field effect transistor), thereby defining a first series branch. The switch means 23 alternately connects this series branch with the positive terminal of the source via dummy load 24 and the load compensation coil 10, respectively.

In operation, if the reference assembly is deflected from its neutral position, the two annular capacitors l9 and 19' connected with difference signal generating means 20 produce a difference signal which in known manner is supplied to regulator means 21 to vary the amplitude of a compensation current supplied by direct current voltage source S to the reference coil 11. By

means of the electromagnetic force produced therein, the reference assembly 13 is returned to the neutral position.

After passing through the reference coil 11, the compensation current is passed by way of buffer means 22 to an electronic switch means 23 from which it can flow alternately to the load coil 10 or to a dummy load 24. The buffer means 22 has a low input impedance and a high output impedance. It has an amplification ratio of 1:1 and forms a galvanic coupling between the switching circuits of the load assembly 12 and the reference assembly 13. This arrangement has the advantage that the analog action control circuit of the reference assembly cannot be influenced by interference pulses from the circuit of the load assembly.

The compensation current in the load coil 10 is controlled as follows. Upon deflection of the load assembly 12 out of the neutral position, for example under the weight of the material to be weighed, the two capacitors l8 and 18' produce a difference signal which is converted by difference signal generating means 25 and passed to current regulating means 26. The control voltage produced therein is applied to a pulse length modulator 27 which in turn is controlled by a quartz oscillator 28 having a frequency divider 29 connected downstream thereof. As shown in FIG. 2, in the pulse length modulator 27, a sawtooth voltage is continuously compared with the control voltage from the regulator 26. One output of the pulse length modulator 27 leads to a switch control means in the form of a flip-flop 30 which, in conjunction with the second input from the oscillator 28, controls the switching times of the switch means 23, to provide a flow of current to the load coil 10 or to the dummy load 24. The operation of the pulse length modulator 27 for varying the lengths of the pulses in accordance with the magnitude of the control voltage Vc (FIG. 2) is disclosed in greater detail in the copending Kunz application Ser. No. 222,960 filed Feb. 2, 1972 and the Naydan et al U.S. Pat. No. 3,028,550.

The compensation process and digital indication or evaluation will be described with reference to the following numerical example.

The weight, which is constant, of the reference assembly 13 is 200 g., while that of the load assembly 12 (without material to be weighed) is 400 g. The weighing range selected is l,200 g., with an excess range (reserve) of 400 g.; the total possible weight which can be compensated of the load assembly is therefore 2,000 g. The ratio of the coil windings is selected as l:l0, corresponding to the ration of the weight of the reference assembly l3 and the total possible weight of the load assembly 12, therefore the same rated current results in both coils at maximum load.

The frequency of the oscillator 28 is 10 MHz. The full value, which serves as a counting frequency, acts on the flip-flop 30 and also a gate 31 and an auxiliary logic means forming a counter control means 32. The divider 29 reduces the oscillator frequency in the ratio 1120,000; pulse length modulator 27 connected downstream of the divider 29 therefore receives a pulse every 2 ms, which pulse returns the sawtooth voltage to zero and allows it to rise again, and after transmission by way of the flip-flop 30, switches the switch means 23 to pass the compensation current to the load coil 10. At the same time the gate 31 is opened and counting pulses from the oscillator 28 are allowed to pass to act on a counter 33. This switching condition persists until the sawtooth voltage in the pulse length modulator 27 has reached the value of the load dependent control voltage from the regulator 26. At that moment the pulse length modulator 27 closes the gate 31, counting being interrupted, and passes to the flip-flop 30 the signal for switching the switch means 23; thereupon, after the counting pulse which was just arriving has concluded its input, the switch means 23 switches the com pensation current from the load coil to the dummy load 24.

Connection with the divider 29 is a second divider 34 which again reduces the frequency, this time in the ratio 1:100, and thus delivers a pulse to the counter control means 32 every 0.2 s, which pulse causes the condition of the counter 33 to be transmitted to an indication storage means 35 from which it passes to an indicator 36 (and at the same time to a binary coded decimal output 37). After the condition of the counter 33 has been transmitted, the counter 33 is set to zero. The details of the counter and indicator means including the counter control means 32 are set forth in the copending Allenspach US. application Ser. No. 244,054 filed Apr. 14, 1972, and assigned to the assignee of the instant application.

This means that in each case 100 individual counting steps are totalized and the result thereof is indicated. By the totalizing operation, any errors arising for example from vibration are averaged out, so that their influence on the measuring result can be greatly reduced.

From the above numerical example, it is apparent that with a weighing cycle of 100 individual measure ments, between 400,000 and 2,000,00 pulses can be counted and indicated. In order however to keep the indication substantially stable, the lowest place is excluded from the indicated result, especially as a resolution of 10 satisfies virtually all requirements.

ln order to take into account the dead load and tare weights, there is connected in parallel with the indication storage means 35 a tare storage means 38, while there is also provided a forward-reverse logic circuit 39, a manually operable tare switch 40 which is connected to the counter control means 32, and a zero detector 41. When weighing with tare, after the tare switch 40 is actuated the value resulting from the tare weighing operation is also accommodated in the tare storage means 38. In subsequent weighing operations, the value of the tare storage means 38 is introduced into the counter 33 at the beginning of each weighing operation. The forward reverse logic circuit 39 causes the counter 33 to count backwards, that is to say, the counting pulses entering therein are subtracted from the tare weight until the zero detector 41 signals that zero has been reached. This signal passes to the forward-reverse logic circuit 39 which then causes the counter 33 again to count in the forward direction.

If no additional material to be weighed is placed on the weighing pan 17, the number of pulses passing into the counter 33 is equal to the number of pulses stored in the tare storage means 38; at the end of the measuring cycle of 0.2 s, the counter 33 has therefore reached zero, and this result is stored in the indication storage means 35 and is indicated at the indicator 36 and the binary coded decimal output 37.

If however further material is placed on the pan 17 (or removed therefrom), the number of counting pulses passing into the counter 33 is greater (or lower respectively) than the number of pulses stored in the tare storage means 38. The excess (or deficit respectively) is indicated in the indication storage means 35 at the end of the measuring cycle and for the duration of the next measuring cycle. It should be noted that the above described arrangement causes the result ob tained to appear with the correct sign and with its real value in the indicator 36, that is to say, an addition of 315 g. is indicated as +0315, and a removal of weight of the same amount is not indicated as a complement, as in the case of previously proposed arrangements, but in the form 03l5. 1n the present arrangement, the correct sign is derived from the forward-reverse logic circuit 39.

It is obvious that it is also possible to weigh-in a plurality of components, with indication of the net addition of each component, by repeated actuation of the tare switch 40 whenever a component is added.

The dead load of the balance can also be taken into account, in the above described manner. By actuating the tare switch 40, for example at the beginning of a series of net weighing operations, the dead load is eliminated from the indication. With this arrangement it is also possible, if required at any time, to re-set the zero point of the balance by simply redetermining the dead load by actuating the tare switch 40.

It is clear from the foregoing that the weight of the reference assembly 13 is compensated with direct current which flows continuously through the coil 11 while the weight of the load assembly 12 is compensated with direct current which flows through the coil 10 only intermittently, in the form of pulses of relatively short duration (less than 2ms), the amplitude of the two currents being identical. The overall arrangement is a real mass comparison device of high precision and with direct digital indication of high resolution, but which does not require highly stable reference voltage sources or other expensive components such as digital voltmeters. The frequency of the oscillator 28 also does not have to be particularly stable, as fluctuations act in the same manner both on the length of the compensation current pulses supplied to the load coil 10 and on the measuring time.

Finally it should be noted that, because of its short weighing time and ease of operation (only one or two operating members, namely the main switch and possibly the tare switch 40), the balance is of almost universal application and is particularly suitable for use in series weighing operations in rapid succession.

Various modifications can be made without departing from the scope of the present invention. For example, the above mentioned resolution of indication could be nc eas d om 19- 9 .9-? w tit bs laa sss;

sary for that reason to forego the stability of the readoff image, that stability being achieved by suppressing the last place of the indicated results, by increasing the frequency of the oscillator 28 and also increasing the divider ratio of the divider 34 (for example, a frequency of 20 MHz and a ratio of 111,000).

Referring now to the embodiment of FIG. 4, the electronic switch means 23' (which is illustrated schematically in the form of a single pole switch) merely connects and disconnects from the load compensation coil 10 the series branch including reference current regulator means 21', reference compensation coil 11, and buffer means 22. In order to stabilize the series branch during the switching operation, the connection between buffer means 22' and switch means 23' is connected with ground via catching diode D, whereby a lower leakage power is achieved.

While the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent that various modifications may be made without deviating from the inventive concepts.

What is claimed is:

1. An electromagnetically-compensated balance including, in combination, a stationary housing (2); load (12) and reference (13) assemblies connected for independent movement relative to said housing, said assemblies normally having no-load neutral positions relative to said housing; load and reference (11) compensation coils connected with said load and reference assemblies, respectively; means (3) establishing a magnetic field common to said load and reference compensation coils; and means for energizing said load and reference compensation coils when the assemblies associated therewith are displaced from their neutral positions in such a manner relative to said magnetic field as to return said assemblies to their neutral positions, respectively, said energizing means comprising a. a direct-current voltage source (S) having a first terminal connected with one end of the load compensation coil;

b. a reference current regulator (21) connected in series with the reference compensation coil to define a series branch one end of which is connected with the other terminal of said voltage source;

c. switch means (23) alternately operable between first and second conditions for connecting and disconnecting the other end of said series branch with the other end of said load compensation coil, respectively;

d. reference position detector means (l9, 19', for

generating a reference control voltage that corresponds with the displacement of the reference assembly from its neutral position, said reference position detector means being connected with said reference current regulator means to regulate the current in said series branch as a function of the magnitude of the reference control voltage;

e. load position detector means (18, 18', 25) for generating a load control voltage that corresponds with the displacement of the load assembly from its neutral position; and

. means (27) connected with said load position detector means for controlling the operation of said switch means to cause the periods of time that said switch means is in the first condition to correspond with the magnitude of the load control voltage.

2. Apparatus as defined in claim 1, and further including buffer means (22) connected in series between said series branch and said switch means.

3. Apparatus as defined in claim 2, wherein said buffer means has a 1:1 amplification ratio and forms a galvanic coupling between the load and reference compensation coils.

4. Apparatus as defined in claim 3, wherein said buffer means includes at least one field effect transistor.

5. Apparatus as defined in claim 1, and further including means for stabilizing the switching characteristie of said switching means when said switching means is in the second condition.

6. Apparatus as defined in claim 5, wherein said stabilizing means comprises a dummy load (24) connected at one end with said first terminal of said voltage source, said switch means being operable in said second condition to connect the other end of said dummy load with said series branch.

7. Apparatus as defined in claim 5, wherein said stabilizing means comprises catching diode means (d) connected at one end between said series branch and said switch means, the other end of said catching diode means being connected with ground.

8. Apparatus as defined in claim 1, wherein said means for controlling the operation of said switch means comprises sawtooth waveform generator means including an oscillator (28), and pulse length modulator means (27) for comparing said sawtooth waveform and said load control voltages to produce switch control voltage pulses the lengths of which are a function of said load control voltage.

9. Apparatus as defined in claim 8, and further including counter means (33) for counting pulses produced by said oscillator means during the time periods of said switch means is in said first condition, and indicator means (36) for indicating as a function of the total count of said counter means the load applied to said load assembly.

10. Apparatus as defined in claim 9, wherein said counter means further includes a forward-reverse counter (33), and taring means for initially subtracting from said counter means counting pulses corresponding with the tare value of the balance, said taring means including tare store means (38) for storing the tare count, forward-reverse logic means (39) connected with said counter means, zero detector means (41) connected between said tare store means and said forward-reverse logic means for reversing the direction of said counter means when the count stored in said tare storage means equals zero, and counter control means (32) for introducing the initial tare count into said tare storage means and for controlling the operation of said forward-reverse logic means, said counter means, and said tare storage means.

11. Apparatus as defined in claim 1, and further including resilient means (14,14 and 15,15) for supporting said load and reference assemblies for movement relative to said housing.

12. Apparatus as defined in claim 11, wherein said housing contains a chamber and wherein said means establishing a magnetic field comprises a permanent magnet (3) mounted within said housing chamber to the axis of said load and reference assemblies.

* k l t I

Claims

1. An electromagnetically-compensated balance including, in combination, a stationary housing (2); load (12) and reference (13) assemblies connected for independent movement relative to said housing, said assemblies normally having no-load neutral positions relative to said housing; load (10) and reference (11) compensation coils connected with said load and reference assemblies, respectively; means (3) establishing a magnetic field common to said load and reference compensation coils; and means for energizing said load and reference compensation coils when the assemblies associated therewith are displaced from their neutral positions in sUch a manner relative to said magnetic field as to return said assemblies to their neutral positions, respectively, said energizing means comprising a. a direct-current voltage source (S) having a first terminal connected with one end of the load compensation coil; b. a reference current regulator (21) connected in series with the reference compensation coil to define a series branch one end of which is connected with the other terminal of said voltage source; c. switch means (23) alternately operable between first and second conditions for connecting and disconnecting the other end of said series branch with the other end of said load compensation coil, respectively; d. reference position detector means (19, 19'', 20) for generating a reference control voltage that corresponds with the displacement of the reference assembly from its neutral position, said reference position detector means being connected with said reference current regulator means to regulate the current in said series branch as a function of the magnitude of the reference control voltage; e. load position detector means (18, 18'', 25) for generating a load control voltage that corresponds with the displacement of the load assembly from its neutral position; and f. means (27) connected with said load position detector means for controlling the operation of said switch means to cause the periods of time that said switch means is in the first condition to correspond with the magnitude of the load control voltage.

5. Apparatus as defined in claim 1, and further including means for stabilizing the switching characteristic of said switching means when said switching means is in the second condition.

11. Apparatus as defined in claim 1, and further including resilient means (14,14'' and 15,15'') for supporting said load and reference assemblies for movement relative to said housing.

12. Apparatus as defined in claim 11, wherein said housing contains a chamber and wherein said means establishing a magnetic field comprises a permanent magnet (3) mounted within said housing chamber to define an annular air gap (9), said load and reference assemblies and said load and reference coils comprising concentrically arranged annular members contained in said annular air gap.

13. Apparatus as defined in claim 12, wherein each of said load and reference detector means comprises a pair of annular capacitors connected with said housing on opposite sides of said load and reference coils, said detector means being colinearly arranged relative to the axis of said load and reference assemblies.