GB2266378A - Inductively coupled transducer arrangement - Google Patents

Abstract

An arrangement for supplying electrical current to a body 10 rotatable about shaft 11 and receiving a signal corresponding to a measured parameter by modulation of the load taken by the body circuitry includes fixed and rotatable collars 16, 20 respectively, coaxial with the shaft and each forming a single turn. Alternating current injected into fixed collar 16 induces a voltage in rotary collar 20, this voltage being fed to a transformer and rectifier unit 21 to power a V-F converter receiving a signal e.g. from a thermocouple 12. The converter output is used to operate a semiconductor switch 27, varying the load on the transformer secondary winding. The resultant current variation is detected by a current sensor 30 in fixed unit 19. Each collar may be of split construction to facilitate assembly on the shaft, and the arrangement is stated to be less subject to eddy current losses and electrical noise than prior arrangements. A design using concentric cylindrical collars is also described (Fig 2 not shown). <IMAGE>

Description

Inductively Coupled Transducer Arrangement This invention relates to inductively coupled transducer systems for contactless transmission of a measured parameter of a movable body to a fixed station by modulating supply of electrical operating power to the movable body from the fixed station, and in particular to a system for determining at a fixed station a measured value of a parameter of a body rotatable with respect to the fixed station about a shaft.

It is known to have such power and data transfer between what may be called master, or fixed, and slave stations in which the slave station is movable with respect to the master station in the sense of being brought into proximity with the master station for inductive coupling to be effected, furthermore, and for coupling efficiency to be maximised by operating at such supply and modulation frequencies that the inductive elements can be formed as tuned circuits which operate at or near resonance. Examples of such devices in disparate fields are given in patent specifications GB-A-2173623, US-A-4725839, W091/11063 and US-A-4741340. All of the arrangements described therein relate to slave stations which at the time of coupled data transfer are substantially stationary with respect to their master or fixed stations.It is, however, also known to communicate information gathered in continuously moving objects, such as vehicle wheels, as exemplified by US-A4737761 and US-A-4942510 and TJO-A-89/08031, although in such arrangements there is no provision for continuous supply of power, whereby the transmission is not continuous, or supply of power is minimised by the above described use of resonant circuits.

However, when a continuous (or substantially continuous as discussed hereinafter) supply is required to be coupled it has been found that in an environment where there is continuous motion between the rotatable and fixed parts, effected by way of a shaft, continuous coupling is effective only in respect of inductive coupling elements disposed co-axially with the shaft and thus under the influence of bulk ferromagnetic bodies, particularly the shaft, in their immediate vicinity. Large amounts of ferromagnetic material in the vicinity of inductive coupling coils, and indeed threading them when as a common shaft about which they extend, has large eddy currents generated therein which dissipate much of the energy transmitted, particularly at so-called sub-radio frequencies of tens to hundreds of kilohertz.

Notwithstanding the dissipation of energy by such eddy currents, it has also been found that in, at least, a workshop environment, where the shaft is part of an electrical machine operated amongst others, inductive circuits having any significant impedance in operation at sub-radio frequencies are susceptible to picking up stray electrical signals or noise at such levels as to mask information being communicated from the body to the fixed station.

However, it will be appreciated that effecting data transfer at a reasonable rate by means of frequency modulation impressed upon the supply frequency requires such a supply frequency at said sub-radio frequencies that makes arrangements constructed in accordance with known teachings ineffectual.

Furthermore, a need has been identified to assemble and dis-assemble such a system without disrupting the body/shaft relationship, including disposing the coupling elements co-axially with the shaft. Thus even though the prior art discloses techniques in respect of contactless power and data transfer this is not satisfactorily directly applicable to the above outlined situation.

It is an object of the present invention to provide an inductively coupled arrangement, for determining at a fixed station a measured value of a parameter of a body rotatable with respect to said fixed station about a shaft, that overcomes some disadvantages of known arrangements.

According to the present invention an arrangement for determining at a fixed station, a measured value of a parameter of a body rotatable with respect to said fixed station about a shaft comprises at said fixed station, (i) a fixed conductive collar adapted to extend around said shaft to form a low impedance coil, (ii) a supply of alternating current to said fixed conductive collar, and (iii) supply current monitoring and detection means, and rotatable with said body a rotatable station including, (1) a rotatable conductive collar substantially co-axial with said fixed conductive collar and forming a low impedance coil inductively coupled to the fixed conductive collar to have induced therein alternating current at the supply frequency, (2) transformer and rectification means operable to derive from said induced current in the conductive collar a supply for transducer means operable to measure said parameter of the body and produce a modulation signal corresponding to a measured parameter value, and (3) switching means responsive to the modulation signal to effect variations of current drawn from and supplied to said inductively coupled collars, said supply current monitoring and detection means being operable to determine from the said variations in the current supplied to the fixed conductive collar the modulation signal and therefrom the measured parameter value.

Each collar may comprise a single complete ring, the ends of which are secured to each other mechanically but insulated from each other electrically to define the electrical ends of a single turn coil.

Alternatively, either or both of the conductive collars may comprise at least two part-circular segments joined to each other about the shaft axis.

Each conductive collar may comprise an annular component having a radial thickness much greater than its axial thickness and preferably such collars are disposed axially adjacent each other with their radially extending faces substantially parallel.

Alternatively, each conductive collar may comprise an annular component having a radial thickness much less that its axial thickness, that is, comprise in effect a cylinder split electrically along its length. Preferably, in such a case, the collars are of different diameter and overlap each other for at least part of their lengths.

Preferably the conductive collars are formed from aluminium or copper sheet.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a schematic perspective view of a test rig for testing a rotatable body and incorporating an arrangement for determining its temperature in accordance with the present invention, showing one form and disposition of the conductive collars and a block circuit diagram illustrative of operation of the arrangement, Figure 2 is a schematic perspective view similar to Figure 1, showing an alternative form and disposition of conductive collars, but omitting the circuitry detail, and Figure 3 is a sectional elevation through a modified form of conductive collar of Figure 1.

Referring to Figure 1, a body 10 is rotatable about shaft 11 and may form part of the test rig for testing inter alia, the performance of vehicle brakes. One of the parameters involved in such testing may be the temperature reached at at least one point of the body and monitored by a thermocouple device 12 of what may be considered generally as transducer means 13, described more fully below, containing electronic circuitry for deriving a processable signal related to the measured parameter and requiring a source of electric power at a particular supply voltage, commonly 5 volts at a few milliamps.

The Figure is divided by chain broken line 14 such that those items to the left of the line may be considered as at a fixed station 15, that is, they do not rotate about the shaft 11, whereas those to the right of the line are considered as rotatable with the body 10 and comprising a rotatable station.

Thus in accordance with the present invention and to determine at the fixed station 15 the measured value of temperature from the body 10, at the fixed station a "fixed" conductive collar 16 extends around the shaft 11 to form a low impedance coil. The collar 16 comprises a single complete annular ring, the ends 161, 162 of which overlap slightly at 17 and are secured to each other mechanically but insulated from each other electrically to define the electrical ends of a single turn coil. The annular ring is preferably formed of aluminium or copper with a radial thickness R much greater than its axial thickness A to give it a low impedance. The conductive collar 16 may be readily placed around the shaft by separating its electrical ends 161 and 162 in an axial direction equal to the width of the shaft and straddling the shaft with the ends.

Alternatively, the collar may be- formed as at least two part-circular segments, such as 16' and 16" and joined to each other, both mechanically and electrically at 17' and mechanically only at 17, about the shaft axis.

In addition to the fixed conductive collar 16, the fixed station includes a supply 18, of alternating current to the collar, and monitoring and detection means 19 associated with that supply current.

In order to supply a significant current to such a low impedance coil as formed by collar 16, the supply 18 may include an impedance transformer 18'.

Considering now the items rotatable with the body 10, these include a "rotatable" conductive collar 20, substantially identical to the fixed conductive collar 16, disposed substantially co-axially with, and adjacent to, collar 16 so that their radially extending faces are substantially parallel and the low-impedance, single-turn coil, formed by the rotatable conductive collar is inductively coupled to the fixed conductive collar to have induced therein alternating current at the supply frequency.

Also rotatable with the body is transformer and rectification means, indicated generally at 21, operable to derive from the induced current in collar 20 a supply voltage for the aforementioned transducer means 13.

Transformer 22 comprises a multi-turn, multi-ratio impedance transformer the primary winding 22 of which is connected to the ends of the conductive collar 20 and matches its lower impedance and the secondary winding 22 of which delivers induced current at a higher voltage to rectification means 23. The rectification means comprises a bridge rectifier 24 and provides a unidirectional charging current to a voltage source, such as reservoir capacitor 25, from which the transducer means is powered.

The transducer means 13 includes, in addition to thermocouple 12 for measuring temperature and deriving a voltage level representative of the temperature, a voltageto-frequency converter 26 which produces a square wave signal of equal mark-space ratio at a frequency which is related to the temperature measured.

Switching means 27, conveniently a semi-conductor device, is connected to the voltage-to-frequency converter 26 to receive this frequency and be alternately opened and closed to conduction thereby. The switching means is connected to provide, when closed, a low impedance shunt path 28 across the secondary winding 222 of transformer 22 in order to increase the current drawn from the rotatable conductive collar. However, to operate such a path with unidirectional current flow (for a semi-conductor device) and to prevent such shunt path from discharging the reservoir capacitor 25, the rectification means 23 comprises a modified bridge in which one branch 24' is common to both the charging circuit of capacitor 25 and the low impedance shunt path, whereas branches 24" and 24"' are unique to, and isolate, the charging circuit and shunt path respectively.

When the switching means 27 is closed and opened alternately at a frequency, much less than that of the induced supply current, representing the measured temperature, the intermittently increased load on the rotatable conductive collar results in a corresponding intermittent increase in the level of current drawn from the supply.

The magnitude of such increase is not important, it is its frequency of occurrence, as a representation of modulation frequency, that is of interest. That is, the turns ratio or efficiency of any of the transformers or of the coupled collars which affects such supply current magnitude change is immaterial. Not withstanding this, it will be appreciated that due to the poor coupling between the collars the changes in supply current effected by the influence of the rotatable collar loading on the fixed collar will be small.

The monitoring and detection means 19 comprises a current sensing device 30, for example a low value resistor across which a potential difference is developed.

It will be appreciated that such a potential difference will be small, as is consistent with supplying a significant current to the collar, and that variations thereof due to the impressed modulation effects will be correspondingly smaller.

To detect such effects the voltage signal output from this device is fed to a voltage comparator 31 both directly and by way of an averaging arrangement 32, such as a low-pass or integrating filter having a time constant in excess of the modulation period.

The comparator 31 thus responds to the small changes to supply current due to the modulation, producing an output represented as a reconstituted square wave having the same repetition frequency as the modulation signal produced by transducer means 13. Conversion means 33, such as a resettable counter, may be included to convert this repetition frequency into an analogue signal representative of the measured temperature or into a suitable digital form for processing by a computer.

It will be appreciated that detail changes may be made to the supply means 18, monitoring and detection means 19 and transducer means 13 without affecting how the invention functions.

It will be appreciated that the transducer means 13 may measure any parameter of the rotatable body other than temperature and may even measure a plurality of parameters.

The transducer means may then include multiplexing means to send samples of each measured parameter in. turn with suitable demultiplexing at the fixed station. In the absence of simple means to synchronise such multiplexing and demultiplexing the modulation signal for each different parameter may be given unique characteristic or be prefaced by a parameter-identifying value.

It will be appreciated that by producing a modulation signal in the form of square wave of equal mark:space ratio the average direct current which flows through the low impedance shunt path is constant for all modulation frequencies, that is, is independent of the temperature (or other parameter) measured, so that any change in such current detected at the fixed station is representative only of the parameter being measured and not of the measurement or communication technique.

However, bearing in mind such potential influence of the modulation on the detection, it is possible to effect variations upon a simple correlation between single temperature value and single modulation frequency. The measured temperature value may be expressed numerically as a multi-bit word, each bit thereof being transmitted in sequence as one or other of two possible modulation frequencies. Clearly the time taken for the transmission of each value is increased or conversely the response rate of the sensor is reduced.

Alternatively, where the temperature value is known to vary only over long periods, such that for even a lengthy measurement operation it can be assumed constant and the averaging or filtering time constant in the detection means can be made long enough, the modulation to transmit binary values may be effected by pulse position modulation. The occurrence of each pulse will be detected (from which any relative change in position or time of occurrance with respect to a known datum can be extracted) although the total current drawn by the shunt path will vary with the temperature value. However, as long as the short-lived changes in current, as reflected in the supply current, are separable from the average current associated with any temperature or other parameter maintained over a longer interval, such average is effectively filtered by the detection means.

Although it is convenient to produce the fixed and rotatable collars as annular rings which can be assembled face to face in close axial proximity, it will be appreciated that the collars may take the form as illustrated at 35 and 36 in Figure 2, which have less thickness R' in the radial direction than A' in an axial direction, appearing as longitudinally split cylinders in which the edges may overlap (as shown) or be flanged (not shown) and mechanically secured for strength but electrically insulated. Furthermore the cylindrical collars may also be made of segments assembled about the shaft or may have a degree of flexibility to enable them to be "sprung" open to pass the shaft.Such cylindrical collars may be mounted end-to-end or preferably, as shown, overlapping to improve on coupling efficiency and compactness, although this may affect the ability of the rotatable body to be moved with respect to the fixed station about any other axis.

As described above the fixed and rotatable collars, whether as rings or cylinders, comprise a single turn coil with the benefits of ease of assembly. It will be appreciated that each collar may be constructed to effect a plurality of turns with the ends having the electrical connections.

Referring to Figure 4 the fixed collar 37 is of the ring type shown in Figure 1 but comprises a plurality of rings 37'1 37'2 37'3 each coupled to the next electrically at one corresponding point about the shaft to build up a multi-turn helix . Although such an arrangement may be expected to improve coupling efficiency, the inductive impedance of the coil formed thereby, the influence of ferromagnetic object such as the shaft and the greater complexity in assembly may outweigh the single coil arrangements described above.

As described above, the rotatable body is located such that the inductive coupling in respect of charging the voltage source 25 is permanent, and the source may simply comprise a capacitor. It will be appreciated that the source may be a rechargeable battery that is able to power the transducer means with interrupted charging periods.

It will be appreciated that the arrangement may be associated with other than a test rig on which a rotatable body is fixed, and may for example comprise a part of a vehicle of which the rotatable body is a wheel supported by the shaft. Furthermore, the rotatable body, whether on such a rig or on a vehicle, may be steerable from a position at which the rotatable collar couples well with the fixed collar. In such a case the rotatable collar may. be intermittently decoupled from receiving supply current and the use of a rechargeable battery as the voltage source would enable continuity of operation albeit with possible short interruptions in receipt of measured values whilst the collars are temporarily aligned.

Claims (14)

1. An arrangement for determining at a fixed station, a measured value of a parameter of a body rotatable with respect to said fixed station about a shaft, the arrangement comprising, at said fixed station, (i) a fixed conductive collar adapted to extend around said shaft to form a low impedance coil, (ii) a supply of alternating current to said fixed conductive collar, and (iii) supply current monitoring and detection means, and rotatable with said body, a rotatable station including (1) a rotatable conductive collar adapted to be disposed substantially co-axial with said fixed conductive collar and forming a low impedance coil inductively coupled to the fixed conductive collar to have induced therein alternating current at the supply frequency, (2) transformer and rectification means operable to derive from said induced current in the conductive collar a supply for transducer means operable to measure said parameter of the body and produce a modulation signal corresponding to a measured parameter value, and (3) switching means responsive to the modulation signal to effect variations of current drawn from and supplied to said inductively coupled collars, said supply current monitoring and detection means being operable to determine from the said variations in the current supplied to the fixed conductive collar the modulation signal and therefrom the measured parameter value.

2. An arrangement as claimed in claim 1 in which the modulation signal comprises a square wave having an equal mark:space ratio.

3. An arrangement as claimed in claim 1 or claim 2 in which each collar comprises a single complete ring, the ends of which are secured to each other mechanically but insulated from each other electrically to define the electrical ends of a single turn coil.

4. An arrangement as claimed in any one of claims 1 to 3 in which at least one conductive collar comprises at least two part-circular segments joined to each other about the shaft axis.

5. An arrangement as claimed in any one of the preceding claims in which each conductive collar comprises an annular component having a radial thickness much greater that its axial thickness.

6. An arrangement as claimed in claim 5 in which the conductive collars are disposed axially adjacent each other with their radially extending faces substantially parallel.

7. An arrangement as claimed in any one of claims 1 to 4 in which each conductive collar comprises an annular component having a radial thickness much less than its axial thickness.

8. An arrangement as claimed in claim 7 in which the conductive collars are mounted at least partially one within the other.

9. An arrangement as claimed in any one of the preceding claims in which the conductive collars are formed from aluminium or copper sheet.

10. An arrangement as claimed in any one of the preceding claims in which the transformer and rectification means includes an impedance transformer connected to the ends of the coil formed by the rotatable conductive collar in which induced alternating current flows in a secondary winding at a terminal voltage required for powering electronic circuitry of the transducer means, the rectification means is operable to rectify the alternating current in the secondary winding of the transformer to provide a unidirectional charging current to a voltage source of the transducer means, and said switching means is connected to provide, when switched by said modulation signal, a low impedance shunt path across said transformer secondary winding to increase the current drawn from said rotatable conductive collar.

11. An arrangement as claimed in claim 10 in which the low impedance shunt path comprises a unidirectional direct current circuit isolated from the charging circuit of the voltage source to prevent discharge thereof.

12. An arrangement as claimed in claim 11 in which the rectification means comprises a bridge network one arm of which is common to the charging circuit and said low impedance shunt path of the switching means.

13. An arrangement for determining at a fixed station a measured value of a parameter of a body rotatable with respect to said fixed station about a shaft, substantially as herein described with reference to, and as shown by, the accompanying drawings.

14. A vehicle including an axle on which is mounted a wheel and including an arrangement as claimed in any one of the preceding claims in which said shaft comprises the vehicle axle and said rotatable body comprises said wheel.