A RELAY TEST DEVICE

Abstract: A relay test device (24) for testing the state of a relay switch (39) is described. The relay test device comprises: a sensor (9) for testing the presence of an electromagnetic field emitted from a switch-controlling element (37) of a relay; and an indicating device (21) for indicating the presence of the electromagnetic field, in response to a sensor output signal. The test device (24) may be used in-situ within electrical systems whilst the relays are connected to other parts of the system.

Description: A RELAY TEST DEVICE This invention relates to a relay test device, and to a method of testing relays. In particular, the invention relates to a relay test device for, and method of, testing relays which are located in-situ on a circuit board or within an electrical system. In its simplest form, a relay may be regarded as an electrically-controlled switch, with the switch state being determined by the magnetic effect caused by current flowing through a coil located adjacent to the switch within the relay. Relays are used in many electrical applications. For example, in aircraft, many banks of relays are used to switch on and switch off electrical systems which perform particular operations once the aircraft has landed. Upon landing, a signal is sent from the aircraft landing gear to indicate that a so-called "squat" has taken place. Once received, this signal causes the relay coils to become energised, thus closing the relay switches and turning-on the required electrical systems. In such critical applications, it is necessary that the relays be tested regularly to ensure they are switching correctly. Whilst it is possible to do this by detecting the audible switching action of the relays, the relays are generally packed together in closely-spaced relay banks, and problems can arise in determining exactly which relay has switched. Accordingly, most conventional testing techniques require the relays to be removed from their circuit boards for bench testing by technicians. However, this process can be time consuming and can disturb the overall operation of the circuit of which the relay forms an integral part. There is also a risk of so-called "Foreign Object Damage" (FOD) occurring due to unwanted objects such as washers, screws etc. becoming loose and falling behind the circuitry during the relay removal and re-insertion process. According to a first aspect of the present invention, there is provided a relay test device for testing the state of a relay switch, the device comprising: a sensor for sensing the presence of an electromagnetic field emitted from a switch-controlling element of a relay; and an indicating device for indicating the presence of the electromagnetic field in response to a sensor output signal. Accordingly, it is possible to provide a portable test device which may be used in-situ within electrical systems with the relays connected to other circuitry. The electromagnetic field emitted from the switch-controlling element of the relay, which causes the switch to close (or vice versa), is sensed by the sensor of the device, the sensor providing a sensor output signal for use by some indication means to indicate the actual state of the switch. Preferably, the sensor is arranged to provide a sensor output signal only in the presence of an electromagnetic field emitted from a position adjacent to the sensor. The sensor may further be arranged to provide an output signal only when the electromagnetic field source is within a predetermined distance from the sensor. This ensures that the test device will only indicate the state of the relay switch located next to it, and not that of other relays. This is particularly important where relays are closely packed together in relay banks, and wherein electromagnetic fields from other components may be inadvertently sensed. In the preferred embodiment, the sensor is arranged to provide a sensor output signal only in the presence of an electromagnetic field emitted from a substantially unidirectional position relative to the sensor. According to a second aspect of the present invention, there is provided a method of testing the state of a relay switch, the method comprising positioning an electromagnetic field sensor adjacent to a relay incorporating the relay switch, such that the presence of an electromagnetic field emitted from a relay switch-controlling element produces a sensor output signal, the sensor output signal thereby providing an indication of the state of the relay switch. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a circuit diagram for use in a relay test device, in accordance with a first preferred embodiment of the invention; Figure 2 shows an exploded view of the relay test device, including a printed circuit board, and device housing members; Figure 3 shows, in perspective, the relay testing device positioned adjacent to a relay under test; Figure 4 shows a side view of the relay test device and the relay under test; and Figure 5 shows a circuit diagram of a relay test device in accordance with a second embodiment of the present invention. Referring to Figure 1, a relay test circuit 1 is shown. The test circuit comprises two identical sub-circuits, each being capable of testing different relays. The first sub- circuit comprises a battery 3, a reed switch 9, a resistor 13 and a light emitting diode (L.E.D.) 21. The second sub-circuit comprises the battery 3, a reed switch 11, a resistor 15 and the L.E.D. 23. The required sub-circuit is selected using a circuit switch 7, having a common terminal and two switched terminals labelled Si and S2. The first sub-circuit is used when the switch 7 is connected to terminal Si, and the second is used when the switch 7 is connected to S2. The circuit power source of the relay test circuit 1 is provided by a three- volt lithium battery 3, the positive terminal of which is connected to the common terminal of a mode switch 5. The resistors 13, 15 are each of value 560 ohms. The mode switch 5 provides a means for testing the circuit components. The mode switch 5 has a common terminal and two switched terminals labelled R and T. When the mode switch 5 is connected to the R terminal, one of the relay test sub-circuits will operate in the relay test mode, depending on the position of the circuit switch 7. When mode switch 5 is connected to the T terminal, the components of the sub-circuits are under test. In this case, a current path is formed via diodes 17, 19, and the L.E.D. s 21, 23 will glow. If any L.E.D. 21, 23 doesn't glow, the particular sub-circuit of which that L.E.D. is a part may be tested to determine whether there is a problem with that L.E.D. or the resistor. If neither L.E.D. 21, 23 glows, the battery 3 might also be tested. The reed switches 9, 11 act as electromagnetic field sensors. In the absence of an electromagnetic field (or rather, in the presence of an electromagnetic field below a predetermined level), the reed switches will be in an open position. In the presence of an electromagnetic field above a predetermined level, the reed switches 9, 11 will close. Since relays emit some level of electromagnetic field when a current flows through the switch-controlling element of the relay (usually a coil), the reed switches 9, 11 may be used to indicate the state of the relay switch of the relay under test. In the case of the selected sub-circuit shown in Figure 1, the closing of the reed switch 9 in the presence of an electromagnetic field will complete the circuit path, and so current will flow through the resistor 13, and the L.E.D. 21 will glow, thus indicating the presence of an electromagnetic field. As such, the glowing of the L.E.D. 21 will indicate that the switch of a relay under test will be closed. Of course, if the relay under test is of the "normally closed" type (i.e. the relay switch is closed in the absence of a magnetic field from the switch-controlling element, and opened when the coil is energised) then the glowing of the L.E.D. 21 will indicate that the relay switch is open. Referring to Figure 2, the various mechanical components of a relay test device 24 in accordance with a first preferred embodiment are shown. The circuit shown in Figure 1 is constructed on a printed circuit board (p.c.b.) 25, with the reed switches 9, 11 and the respective L.E.D. 's 21, 23 being located at opposing ends of the p.c.b. (note that the other components are not shown). The reed switches 9, 11 are located on the underside of the p.c.b. 25. The mode switch 5 and the circuit switch 7 are provided as slide switches and are positioned as shown in Figure 2. When assembled, the p.c.b. 25 sits towards the bottom of a base enclosure 27, with the reed switches 9, 11 resting a few millimeters from the bottom surface. The circuit switch 7 is arranged to protrude from an aperture 31, with the mode switch 5 protruding from a similar aperture (not shown) on the opposing side. A lid 29 is secured to the base enclosure 27, with holes being provided in the lid to allow the L.E.D. s 21, 23 to protrude. It will be noted that an alignment arrow 33 is provided on the right-hand end-face of the base enclosure 27, this arrow being aligned along the longitudinal extent of the device, with reed switch 9, to enable the test device to be positioned relative to the relay under test. It will be appreciated that there is an identical arrow (not shown) located on the opposing end- face of the base enclosure 27, this arrow being aligned with the reed switch 11. Referring to Figure 3, the assembled relay test device 24 is shown, located above one of a number of relays 35a - 35e in a relay bank. In use, the relay test device 24 is placed on top of . the relay under test, with the base of the relay test device forming an alignment surface with the top face of the relay. Referring to Figure 3, relay 35b is tested by switching the circuit switch 7 to the Si position, and by ensuring that the mode switch 5 is in the R position. Accordingly, the reed switch 9 is selected as the "live sensor". The relay test device 24 is then positioned so that the alignment arrow 33 lines up with a dot 37 provided on the top surface of the relay 35b. As will be described below, this dot 37 is provided so as to ensure that the reed switch 9 is located substantially vertically above a switch-controlling coil of the relay 35b. The presence of an electromagnetic field (above a predetermined threshold value) emitted from a switch-controlling element of the relay 35b, will cause the reed switch 9 to complete the circuit and to cause current to flow through the L.E.D. 21 thus causing the L.E.D. to glow. Referring to Figure 4, a side-sectional view of the relay test device 24, in position on relay 35b is shown. A relay switch 39 and the switch-controlling coil 41 (as mentioned above with reference to Figure 3) are shown located within the relay 35b. It will be appreciated that, in use, a current passing through the relay coil 41 will emit an electromagnetic field causing the relay switch 39 to close. In order for the reed switch 9, within the relay test device 24, to close, it must be located within a predetermined distance from the coil 41 (typically within a few millimetres) so that the electromagnetic field will be strong enough to close it. In fact, the positioning of the reed switch 9 within the base enclosure 27 requires that it be positioned substantially above the relay coil 41 to close the reed switch. In effect, the reed switch 9 will only close in the presence of electromagnetic fields emitted from a substantially unidirectional position relative to the reed switch . This is particularly advantageous, since the relay test device 24 will be used in close relation to other electromagnetic sources, e.g. the adjacent relays in the relay bank. Accordingly, it is important that the reed switches 9, 11. of the relay switch device 24 only cause the L.E.D.s 21, 23 to glow in the presence of fields from the relay under test, and not from adjacent relays. In order to ensure that adjacent fields will not affect the operation of the relay tester, other types of sensors may be used, i.e. sensors having a lower sensitivity to electromagnetic fields. Alternatively, electromagnetic shielding may be incorporated inside the device enclosure 27, with an aperture being left adjacent to where the sensor is located. In use, in order to ensure that the reed switch 9 and the relay coil 41 are substantially aligned under test conditions, the relay test device 24 is positioned with the alignment arrow 33 placed over the dot 37 marked on top of the relay 35b. Since the distance between the arrow 33 and the reed switch 9 is the same as the distance between the dot 37 and the coil 41, alignment of the arrow with the dot will ensure that the reed switch and coil are vertically aligned. The dot 37 may be made, in a one-off procedure, by the person initially testing the relay (provided he has knowledge of the internal construction of that relay type). Alternatively, the distance of the reed switch 9 from the arrow 33 may be made equal to the distance of the coil 41 from a particular manufacturers marking on the relay. It will be understood that the above description of the procedure for testing a relay using the reed switch 9, applies also to the reed switch 11, provided the circuit switch 7 is switched to the S2 position. In this case, an alignment arrow (not shown) is provided on the opposing end face of the base enclosure 27, and the presence of an electromagnetic field emitted from a position substantially underneath the reed switch 11, is indicated by the L.E.D. 23 glowing. It will also be understood that other types of electromagnetic sensors may be used, instead of reed switches. For example, Hall effect devices may be used, wherein simple Hall effect integrated circuit (i.e.) switches may replace the reed switches 9, 11. Over time, such Hall effect switches generally prove more reliable than reed switches, since they are not mechanical switches. A different level of switching sensitivity is also achievable. A simple circuit arrangement incorporating a Hall effect switch, in accordance with a second embodiment of the present invention, is shown in Figure 5. By comparing Figure 5 with Figure 1, it will be observed that the only difference resides in that Hall effect Lc.s 45, 47 have replaced the reed switches 9, 11. In operation, an electromagnetic field above a predetermined level will cause a current to flow through the device, from terminal a to terminal b, and through the L.E.D. 's 21, 23 as before. Note that the power supply required will be higher than the 3 volts used in the first embodiment shown in Figure 1. Typically, a voltage supply of between 4.5 volts and 24 volts will be required. In summary, a relay test device can be provided which enables in-situ testing of relays, without requiring their removal from the circuitry of which they form an integral part. By providing that the sensor being used only senses electromagnetic fields emitted from a position beneath the sensor, fields emitted from other sources have no effect on the test device.

Claims: Claims 1. A relay test device for testing the state of a relay switch, the device comprising: a sensor for sensing the presence of an electromagnetic field emitted from a switch- controlling element of a relay; and an indicating device for indicating the presence of the electromagnetic field, in response to a sensor output signal. 2. A device according to claim 1, wherein the sensor is arranged to output a sensor signal only in the presence of an electromagnetic field emitted from a position adjacent to the sensor. 3. A device according to claim 1 or claim 2, wherein the sensor is arranged to output a sensor signal only in the presence of an electromagnetic field emitted from a position which is within a predetermined distance of the sensor. 4. A device according to any preceding claim, wherein the sensor is arranged to output a sensor signal only in the presence of an electromagnetic field emitted from a substantially unidirectional position relative to the sensor. 5. A device according to any preceding claim, wherein the device further comprises a relay alignment surface for locating the device on a corresponding surface of the relay. 6. A device according to claim 5, wherein the device further comprises an alignment indicator for positioning the device relative to the corresponding surface of the relay. 7. A device according to any preceding claim, further comprising a second sensor and a second indicating means for indicating the presence of an electromagnetic field emitted from a second switch-controlling coil of a relay switch in response to a signal from the second sensor. 8. A device according to any preceding claim, wherein the or each sensor comprises a magnetically operated switch. 9. A device according to claim 8, wherein the or each sensor comprises a reed switch 10. A device according to claim 8, wherein the or each sensor comprises a hall-effect switch. 11. A method of testing the state of a relay switch comprising positioning an electromagnetic field sensor adjacent to a relay incorporating the said relay switch, such that the presence of an electromagnetic field emitted from a relay switch-controlling element produces a sensor output signal, the sensor output signal thereby providing an indication of the state of the relay switch. 12. A method according to claim 11, wherein the electromagnetic field sensor is positioned substantially adjacent to a switch-controlling coil of the relay switch so as to provide a sensor signal. 13. A method according to claim 11 or claim 12, wherein the electromagnetic field sensor is positioned within a predetermined distance of the sensor so as to provide a sensor signal. 14. A test device substantially as hereinbefore described and shown with reference to the accompanying drawings. 15. A method of testing the state of a relay switch substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: WORSLEY IAN DAVID, WORSLEY, IAN, DAVID
Applicants: BAE SYSTEMS PLC, WORSLEY IAN DAVID, WORSLEY, IAN, DAVID
Application year: 2001
Main classification: G01R31
IPC codes: G01R31