[0001] This invention relates to telemetry apparatus comprising a strain gauge mounted on
a first of two parts and providing an alternating signal to a detector circuit
4nounted on the second part by way of a capacitive coupling between the two parts.
[0002] It is often necessary in evaluating the dynamics of a system to collect strain data.
For example, one may wish to obtain torque and bending strain data from a vehicle
drive shaft. A conventional method to collect strain data from a vehicle drive shaft
under operating conditions is to mount a bridge circuit, of which one element is the
strain gauge, strategically on the shaft. The problem then exists of transferring
the data from the rotating shaft, i.e. the aforementioned first part, to the body
of the vehicle, i.e. the second part. One solution (e.g. US-PS 3 824 845) is to use
slip rings but slip rings are a source of trouble in that the electrical connection
provided thereby tends to deteriorate. Another solution (US-PS 3 668 673) applies
the output from the strain gauge bridge to a sub-carrier oscillator (SCO) whose deviation
from its centre frequency is proportional to the strain imposed on the strain bridge.
The output from the SC0 is used to modulate a radio frequency oscillator (RFO) whose
output is transmitted to a receiver. The use of a radio frequency signal can create
disturbance to other vehicle electrical components. Also, the transmitted radio frequency
can be reflected off metal vehicle components to create noise problems in the received
signal.
[0003] In another solution, which is in accordance with the introductory part of claim 1,
(US-PS 3 224 257) the strain gauge circuit is energised by an alternating signal from
an oscillator mounted on the second part and capacitatively coupled to the strain
gauge circuit on the first part. Such an arrangement requires two capacitive couplings,
each involving two capacitors, to couple the energising alternating signal to the
strain gauge circuit and to couple the output alternating signal to the detector circuit.
Four capacitors in all are thus required, each with one plate on the first part and
one plate on the second shaft. This makes the apparatus complicated.
[0004] An arragement is also known CUS-PS 3 303 701) which requires only two coupling capacitors
for the case of a temperature measurement, employing a resonant circuit on the first
part. The frequency of the resonant circuit varies in dependence upon a temperature
sensitive element therein and the resonant circuit is coupled by the capacitors to
an amplifier circuit on the second part to form an oscillator whose frequency indicates
the temperature. However the relationship between frequency and temperature is nonlinear.
[0005] The object of this invention is to provided an apparatus according to the introductory
part of claim 1 which only requires two coupling capacitors but which can provide
a linear representation of the measured strain and this object is met by the characterising
features according to which the strain gauge circuit includes a voltage to frequency
converter providing the alternating signal in response to a voltage proportional to
the strain.
[0006] In the preferred practice of the invention, a bridge network is secured on a part
subjected to strain, such as a shaft. The output from the bridge network, which is
proportional to element strain, is delivered to a voltage to frequency converter.
The output from the frequency converter is in the form of a square wave with a frequency
proportional to the strain. The output from the frequency converter is delivered to
a first set of capacitance rings placed about the part for which strain readings are
desired. A second set of stationary capacitance rings placed around the first rings
provide capacitive coupling utilizing air as the dielectric. The received signal is
in the form of a differentiated square wave and is delivered to a squaring amplifier
and thereafter to a frequency to voltage converter, wherefrom the signal is delivered
by a readout device.
[0007] The invention will be described in more detail, by way of example, with reference
to the accompanying drawings in which:
Fig 1 is a block diagram of a telemetry apparatus embodying the invention,
Fig 2 is a diagram of a square wave having a frequency proportional to element strain,
Fig 3 is a diagram of a differentiated square wave having a frequency proportional
to strain.
[0008] Referring to Fig 1, a voltage source shown as a battery supplies a voltage to an
adjustable voltage regulator 11 of conventional design which delivers a voltage to
a bridge network 13. The bridge network 13 is strategically located on the vehicle
drive shaft (not shown) in a conventional manner and includes as one resistor thereof
a strain gauge. The bridge produces an output voltage drop across the bridge network
13 proportional to shaft strain. The output from the bridge network 13 is delivered
to a voltage to frequency converter 15, such as manufactured by Analog Devices, Model
AD537. The output from the voltage to frequency converter 15 is in the form of a square
wave having frequency proportional to shaft strain (illustratively shown in Fig 2).
The output from the frequency converter 15 is delivered to a capacitive coupling system
17.
[0009] The capacitive coupling system 17 includes first and second capacitance rings 19
and 21.secured around the periphery of the shaft in a conventional manner. All of
items 11, 13, 15, 19 and 21 are mounted on the shaft. Third and fourth capacitance
rings 23 and 25 are fixed in close proximity to the associated rings 19 and 21 respectively.
The frequency modulated voltage across rings 19 and 21 as received from the voltage
to frequency converter 15 is transmitted to rings 23 and 25 using air as a dielectric.
The signal output from rings 23 and 25 is in the general form of a differentiated
square wave (Fig 3). The signal from rings 23 and 25 can then be delivered to an input
amplifier 27 to increase its level. The signal can be processed.further by passing
it to a squaring amplifier 29, frequency to voltage converter 31 and then to a recorder
or direct readout device 33. All of items 23, 25, 27, 29, 31 and 33 are mounted on
the vehicle body.
1. Telemetry apparatus comprising a strain gauge circuit mounted on a first of two
parts and providing an alternating signal to a detector circuit mounted on the. second
part by way of a capacitive coupling between the two parts, characterised in that
the strain gauge circuit (11, 13, 15) includes a voltage to frequency converter (15)
providing the alternating signal in response to a voltage proportional to the strain.
2. Telemetry apparatus according to claim 1, characterised in that the detector circuit
(27, 29, 31) includes a frequency to voltage converter (31) which converts the altenating
signal back to a voltage proportional to the strain.
