GB2192064A - Electromagnetic non-destructive sensing/inspection device incorporating a balancing network - Google Patents

Abstract

A sensing device comprises a primary coil P fed from a generator G and a secondary coil S inductively coupled to the primary coil and having two halves connected into a balancing network. Means T1 detects any signal flowing in a null point of the network and means EC compares the phase and/or amplitude of this signal with the generator signal to produce an error signal. Means SP, CD provide an output signal in accordance with the error signal, and impedance means (eg a light dependent resistor LDR1 acted on by a light emitting diode D1) connected in the balancing network responds to the error signal so as to return the network to a balanced condition. <IMAGE>

Description

SPECIFICATION Sensing device This invention relates to a sensing device having a multiplicity of possible uses, one particular use being as an inspection device.

Inspection devices are known which comprise a primary or excitation coil, and a secondary or search coil which has two oppositely wound halves so as to provide a balanced output normaliy. The arrangement becomes unbalanced when an item is present that upsets the coupling between the coils, for example if the item is a metal object (in which the excitation coil induces eddy currents) having a flaw for example. The imbalance is detected in order to indicate the presence of the flaw etc.

We have now devised a sensing device which, in one of its uses, provides for an improved inspection device. In accordance with this invention, there is provided a sensing device comprising a primary coil fed from an excitation generator, a secondary coil coupled to the primary coil and having two halves connected into a balancing network, means for detecting any signal flowing in a null point of the network, means for comparing the phase of this signal relative to the signal generated by the excitation generator in order to produce an error signal, means for providing an output signal in accordance with the error signal, and impedance means connected in the balancing network for responding to the error signal so as to return the network to a balanced condition.

The balancing network may comprise a light dependent resistor positioned adjacent a light emitting device which is driven in accordance with the error signal. Two such resistors may be included in opposed arms of the network, responding to respective light emitting devices which are driven in accordance with the magnitude and sense of the error signal.

The sensing device maybe used to respond to any medium which interferes with the coupling between the primary and secondary coils. For example, the device may be used as an inspecting device for metal items, the primary coil inducing eddy currents in the item and the search coil becoming unbalanced if there is a local flaw or a local change in dimension, material or hardness of the item.

The device may instead be used for non-contact gauging and it may also be constructed as a linear voltage displacement transformer.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIGURE 1 is a schematic block diagram of an eddy current inspection device; and FIGURE 2 is a section through the sensing head of a modified device arranged for inspecting e.g. a tubular item.

Referring to Figure 1 of the drawing, there is shown a sensing device comprising a primary or excitation coil P and a centre-tapped secondary or search coil S. The centre tap is connected to ground and the opposite termi nals of the search coil are connected respectively by a resistor R1 and a light dependent resistor LDR1 to one terminal of a transformer primary T1, the other terminal of which is connected to ground. The search coil is thus connected into a network intended to provide a balanced path through the transformer primary T1, the light dependent resistor LDR1 providing a means for returning the network to a balanced condition if an imbalance is detected, as will be described below.

The secondary winding of transformer T1 is connected to a first amplifier and filter Al which in turn is connected to a main amplifier MA, comprising a series of low gain amplifiers that ensure low noise and minimal frequency shift. The output of amplifier MA is fed to a limiter L which converts the amplified signal to a square wave. This square wave is fed to one input of an error voltage conditioner EC, the other input of which receives a synchronising signal from an excitation voltage generator G which is provided for the excitation coil P. The error voltage conditioner EC serves to provide an output voltage which is dependent upon the phase shift between the signals applied to its two inputs, i.e. the shift between the signal picked off by the transformer T1 and the signal driving the excitation coil P.

The output of the error voltage conditioner is applied to a zero error voltage generator EG which serves to generate a null output when there is residual coupling between the excitation and search coil, or otherwise an output voltage dependent upon the shift between the excitation signal and the signal picked off by the transformer T1. The output from the zero error voltage generator EG is fed to an integrator 1 which is followed by a clamp C and then a driver D for a light emitting diode D1 adjacent the light dependent resistor LDR 1.

There is thus provided a feedback arrangement such that any imbalance in the search coil network is sensed in order that the light emitting diode D1 will act on the light dependent resistor LDR1 to bring the network back into balance, and return to a null output from the zero error voltage generator EG. However the voltage at the output of the zero error voltage generator EG is monitored, being fed via a dc signal processing circuit SP to a custom display CD which may take any desired form.

In use of the sensing device as an eddy current inspection device residual coupling will occur between the excitation and search coils when either there is no item under test adjacent the coils, or there is an item adjacent the coils but the coupling is constant: equal but opposite currents will flow from the two halves of the search coil and the output from generator EG will be zero. However, if the item under test exhibits a flaw or a dimensional, hardness or material change for example, there will be non-residual coupling between the excitation and primary coils and unequal voltages will be applied to the transformer primary T1 from the respective halves of the search coil. An output is thus picked up from the transformer and is compared for phase with the excitation signal to produce a voltage at the output of generator EG dependent upon this phase shift.The integrator I integrates according to the changes in the output voltage from generator EG and the integrator output is fed via the clamp to drive the light emitting diode D1, to act on the light dependent resistor LDR 1 and alter its value so as to bring the network back into balance and return the error voltage generator EG to its null output.

Although the device shown in Figure 1 monitors only the phase shift variations, it may additionally monitor variations in amplitude occuring at the output of the amplifier MA. The device may then be arranged to respond differently according to whether a phase shift greater/or less than a predetermined value is detected, or whether an amplitude greater/or less than a predetermined value is detected, or whether both phase shift and amplitude pass respective threshold values.

The sensing device may be modified as shown in Figure 2 for detecting continuous uniform defects or dimensional, material or hardness variations. In this modification, there are two search coil, each with its two halves on opposite sides of the item under test: in the example shown in Figure 2 the item under test is a steel tube 10 excitation coil P is provided, through which the test item is passed longitudinally of itself. One search coil S1 is aligned with the weld, and the other search coil S2 is offset and serves as a reference, and the two search coils are connected into respective balancing networks as shown in Figure 1. Because each search coil has its two halves on opposite sides of the item under test, the search coil will not be affected by any displacements of the item along the axis A of the coil.The two balancing networks are monitored and their outputs compared, a change in the offset indicating a change in the longitudinal weld. In general, any number of search coils can be provided, arranged with their two halves on opposite sides of an item to be tested: for example sufficient coils suitably arranged can be used for inspecting a rail track. Also there may be a plurality of excitation coils, for example one for each search coil.

Because, in the device shown in the drawings, the feedback provision serves to restore the balancing network to its balanced condition, there is no need for the two halves of the search coil to be precisely equal (as in prior art devices), so that a less expensive search coil can be employed. The device is found to exhibit a high degree of sensitivity and readily adapts to different items under test. Moreover it is relatively immune to "liftoff", i.e. movements of the test item towards and away from the coupled coils, and does not therefore require filters (as employed in prior art devices) to reject "liftoff" components in the output signals.

Whilst in Figure 1 the balancing network has a single variable device for restoring it to balance, two such variable devices (e.g. light dependent resistors) may be included, one in each side of the network (i.e. at R1 as well as at LDR1), serving to respond to error signals of opposite senses in order to maintain the network balanced.

In general, the device may be used to respond to any medium which interferes with the coupling between the excitation and secondary coils. For example the device may be used, instead of as an eddy current inspection device, for non-contact gauging. Also, it may be constructed as a linear voltage displacement transformer.

The device may include a self-test facility comprising for example a push-button serving to apply an artificial drive to the light emitting diode D1, thus unbalancing the network and generating an error signal from generator EG: this error signal can be observed as an indication that the sensing head is operating correctly.

Instead of light dependant resistors in the balancing network, other light dependant devices such as FET's may be used. Instead of an optical coupling from the feed-back path to the balancing network, a direct electrical coupling maybe provided to an appropriate variable impedance device included in the network.

Instead of the transformer coupling from the balancing let of the network to the amplifier and filter Al, a resistive coupling may be used (i.e. the transformer is removed and a direct connection is made to the amplifier from the junction between the two arms LDRi,R1 of the network).

Claims (7)

1. A sensing device comprising a primary coil fed from an excitation generator, a secondary coil coupled to the primary coil and having two halves connected into a balancing network, means for detecting any signal flowing in a null point of the network, means for comparing the phase of this signal relative to the signal generated by the excitation generator in order to produce an error signal, means for providing an output signal in accordance with the error signal, and impedance means connected in the balancing network for respond ing to the error signal so as to return the network to a balanced condition.

2. A sensing device as claimed in Claim 1, comprising an integrator connected to receive said error signal in the feedback path to said impedance means.

3. A sensing device as claimed in Claim 1 or 2, in which said impedance means comprises a light dependent device and the feedback path includes an optical coupling to said device.

4. A sensing device as claimed in Claim 1 or 2 in which the feedback path includes a direct electrical coupling to said impedance means.

5. A sensing device as claimed in any preceding claim, in which the balancing network comprises two variable impedances in respective legs, which are controlled according to error signals of opposite senses.

6. A sensing device as claimed in any preceding claim, further including a self-testing means operable to drive the variable impedance artificially to unbalance the network and generate an observable error signal.

7. A sensing device substantially as herein described with reference to the accompanying drawings.