Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With rotor
Patent
1991-02-07
1992-06-23
Nguyen, Vinh
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With rotor
324 725, 324 731, 324158R, G01R 2704
Patent
active
051246473
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to electrical circuit testing, more particularly to detecting whether or not a testing probe is in contact with a circuit to be tested.
Various methods are known for testing electrical circuits, using at least one testing probe. For example, at least one such probe may be used in testing automatically a printed circuit board before components are added, by testing conductive tracks for open circuit faults and/or short circuit faults on the basis of capacitance value checks with reference to a ground plane or resistance value checks or a combination of such checks.
However, if a probe does not make proper contact with a conductive track or an end terminal of such a track, then faulty readings will be derived from the probe.
According to the present invention from one aspect, there is provided a method for detecting whether a testing probe is in contact with a circuit to be tested, the method comprising obtaining a measure of an impedance value between the probe and a reference and comparing the measure with a measure of the impedance taken without the probe contacting the circuit.
According to the present invention from another aspect, there is provided apparatus for detecting whether a testing probe is in contact with a circuit to be tested, the apparatus comprising means for obtaining a measure of an impedance value between the probe and a reference and means for comparing the measure with a measure of the impedance taken without the probe contacting the circuit.
The present invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic representation of an embodiment of apparatus according to the present invention.
Referring to the single FIGURE of the accompanying drawing, reference P denotes a testing probe for use in making contact with a circuit under test U for testing the latter. Located close to the probe P is a capacitance measurement circuit M connectable to the probe P via a relay RL1 and isolated from the latter by a capacitor Cy. Relay RL1 has a contact which connects either terminals 1 and 2 or terminals 3 and 2 of the relay. Connected between terminal 3 of relay RL1 and ground (G) is a relay RL2, which normally provides an open circuit between its terminals 1 and 2. The normal electrical path to the probe P for testing purposes is via an input S and terminals 1 and 2 and the contact of relay RL1.
For the purpose of enabling a subsequent check as to whether, in use, the probe P is actually in contact with a circuit under test, first the probe is removed from any circuit which happens to be under test. Then, under the control of circuitry T, relay RL1 is put into a state such that its contact connects its terminals 2 and 3 so that the capacitive measurement circuit M is connected to the probe P via capacitor Cy. The circuit M then, under the control of circuitry T, makes a measurement of capacitance, which measurement will be of the effect of the capacitor Cy and the stray capacitance (Cp) of the probe P and its associated wiring and is stored as a background reading in control circuitry T via an output R.
The probe P is then moved to a circuit under test to make contact with it. A capacitance measurement (Cm) is then made by circuit M, which will be of the combined effect of the capacitor Cy, the stray capacitance Cp of the probe P and its associated wiring and the added electrical effect of making contact to the circuit under test if the latter has occurred. If there is no other direct connection from the circuit under test to ground G, the capacitance added by the circuit under test can be represented solely by Cx, where: ##EQU1##
The background measurement value of the capacitance effect of capacitor Cy and the stray capacitance Cp of the probe P and its associated wiring with the probe not making contact with a circuit under test is then compared automatically with Cm by circuitry T. If they are the same, then it is assumed that the probe P has not contacted the circuit under test. If Cm is different from
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patent: 5006808 (1991-04-01), Watts
patent: 5019771 (1991-05-01), Yang et al.
Bath Scientific Limited
Nguyen Vinh
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