Switch control apparatus and method

Abstract: A method of controlling a switch (3) in a series circuit which also includes a power supply (6) and a load (7). The method comprises: a) closing the switch (3); b) monitoring current flowing in the circuit; c) comparing the monitored current with a reference which varies with time corresponding to the variation expected when an acceptable load is attached between the terminals; and, d) opening the switch (3) if the monitored current varies from the reference in an unacceptable manner.

Description: The invention relates to a method of controlling a switch in a series circuit which also includes a power supply and a load, and to switch control apparatus for carrying out such a method. There are many applications where a switch is provided which is operated in response to the sensing of a condition or the like so as to open or close accordingly. Closure of the switch will cause current to flow through the switch in a circuit in which the switch is positioned, the current flow causing activation of a load which will provide an indication that the switch has been closed. Typical loads include optical devices such as a tungsten filament and relays/contractors. Commercially, the switch will be provided internally of processing unit and will be connected to terminals provided for connection of a load and a supply by the user. A problem arises in that the user can inadvertently connect the supply across the terminals and omit the load altogether or may use a load which is incorrectly rated. In either event, when the switch is closed, the load and/or the unit can be damaged or even destroyed. EP 0 862 261 describes an overcurrent protection circuit. US 5 347 864 describes a liquid level measuring apparatus incorporating an error connection means which is controlled by the activation and deactivation of reed switches. The switches are wired together in parallel and are controlled by a magnet which rises and falls with the water level. US 4 742 422 describes a self monitoring safety circuit. A current is fed into an additional winding of a total current converter, and the pulse voltage created is output to an electronic limit value circuit. The output of the electronic limit value circuit is out put to an evaluation circuit which causes the monitored circuit to be switched off when no periodic impulses arrive. US 5 559 438 describes a composite safety switch. The description is directed to a mathematical discussion based on the arrangement of 2 to 4 switches in a either a parallel or series circuit. Failure detection of a switch occurs when a current is flowing through a circuit and a signal CS is in a state indicating that the switch should be closed. DE 196 53 727 relates to the switching off of a drive motor. EP 0 111 028 relates to a power transistor switch. US 4, 771, 357 discloses switch control apparatus as set out in the preamble of claim 14. The present invention provides method and apparatus as defined in claims 1 and 14 respectively. Preferred features of the invention are set out in dependent claims 2 to 13 and 15 to 10. In preferred embodiments of this invention, we provide a method and apparatus in which the omission of a load or the use of an incorrectly rated load can be automatically detected and compensated for. Thus, if set (c) notes that the monitored current varies unacceptably from the threshold, this will indicate either the omission of the load or the use of an incorrectly rated load and the switch will be immediately opened preventing any damage. The current could be monitored directly, but preferably step b) comprises generating a voltage equivalent to the monitored current, step c) comprising generating an equivalent reference voltage. It is possible that due to noise effects of fluctuating power supplies and the like, step (c) is carried out at a time when the reference is only transiently exceeded. Consequently, the method preferably further comprises e) repeating steps a) - d) after a predetermined interval. Preferably, the predetermined interval is relatively long if the monitored current varies unacceptably from the reference current within a first predetermined period after step a). This first predetermined period is typically 3ms. This will correspond to the absence of a load. The predetermined interval will typically be several orders of magnitude greater than the predetermined period eg 1000 times, a preferred interval being 15 secs. In addition, or alternatively, the predetermined interval may be relatively short if the monitored current varies unacceptably from the reference current within a second predetermined period after step a). Typically the second predetermined period commences upon completion of the first predetermined period. The method will typically be carried out on each occasion on which the switch is closed. Preferably, the method is carried out for a certain test period and is then terminated. The test period may be, for example, 100ms. The switch will remain closed assuming this is its current condition as determined by the controller to which it is attached, if the last few comparisons were satisfactory or at least one of them was satisfactory. The fact that there is a satisfactory comparison indicates that unsatisfactory comparisons are probably due to noise. The unacceptable variation between the monitored current and the reference may occur if the current exceeds the reference. However, other relationships are possible. For example, when the current approaches within a certain range of the reference. As will be apparent from the introduction, the invention is able to determine whether the user has connected an incorrect load or has connected the supply without a load. It is also possible that the remainder of the switch control apparatus could suffer a fault and the invention enables a distinction to be drawn between faults caused by the user and faults within the apparatus which a user cannot correct. Thus, if, following a method according to the invention, a fault is determined then a suitable indicator can be activated different from an indicator which is activated when another type of fault occurs. Conveniently, however, the same indicator is used to indicate both types of fault, the method comprising activating the indicator in a first manner if the switch is opened in step d) as the result of step c); and activating the indicator in a second, different manner as a result of another type of fault developing. These different manners are conveniently turning the indicator device ON and OFF at respective different frequencies. The indicating device could also be used for indicating that the apparatus is switched ON or switched OFF. An example of switch control apparatus and a method according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a circuit diagram of the apparatus; and, Figure 2 is a graph illustrating exponential decaying current with time. The circuit shown in Figure 1 is connected in use to a liquid level sensor (not shown) which provides an output signal on a line 1 which is fed in this example to a microprocessor 2. The liquid level sensor may have any conventional form and generates an output signal which varies depending upon whether or not a liquid has been sensed by the sensor. The microprocessor 2 is programmed to respond to this input to operate a switch 3 in the form of a FET so that an indication can be provided to the user. The switch 3 is actuated via a port PB1 on the microprocessor 2 and interface circuits 8 of conventional form. The circuit shown in solid lines in Figure 1 is provided as a unit to the user and terminates in a pair of terminals 4,5 presented to the user and to which the user is intended to connect a power supply 6 and a load, such as a tungsten filament 7. The power supply could take other forms, for example a transformer or the like, as could the load 7. As explained above, a problem can arise if the user connects the power supply 6 directly between the terminals 4,5 or uses a load 7 with the wrong rating. In order to detect a problem in the components attached to the terminals 4,5, the current flowing through the series circuit defined by the terminals 4,5 and the switch 3 is monitored by monitoring the voltage drop across a resistor 9 when the switch 3 is closed by the microprocessor 2 in response to a corresponding signal from the liquid level sensor e.g. indicating that a liquid is sensed. This voltage drop is passed to the inverting input of a comparator 10, to the other input of which is fed a reference voltage supplied from the port PA1 of the microprocessor 2. The reference signal supplied to the non-inverting input of the comparator 10 is controlled so as to take the form of the maximum signal which would be expected from monitoring the voltage drop across the resistor 9 when an acceptable load 7 is connected between the terminals 4,5. Typically, in the case of a tungsten lamp forming the load, this monitored signal will take the form shown by a solid line 11 in Figure 2, that is a rapid increase to a peak followed by an approximately exponential decay. Consequently, the reference generated by the microprocessor 2 and a network 14 will also have an approximately exponential form as shown by the dashed line 12 in Figure 2. If the load current exceeds the exponentially decaying reference voltage 12, the output of the comparator 10 will send an overload signal to the port PB2 of the microprocessor 2. The microprocessor 2 monitors the output of comparator 10 at regular, short time intervals to determine if there is an overload and if such an overload exists, it will turn OFF the switch 3. The action taken by the microprocessor 2 will vary depending upon the time at which an overload is sensed. Thus, for example, if an overload is sensed in a first predetermined period which is relatively soon after initiation (i.e. the time at which the switch is turned ON), for example about 3ms, then this is likely to indicate that no load 7 has been connected between the terminals 4,5 and the power source 6 has been connected directly between the terminals. In that event, the microprocessor 2 will turn OFF the switch 3 for a relatively long predetermined interval (for example 15 seconds) and will then repeat the monitoring process. If an overload is not sensed in the first 100ms after start-up, the overload testing function will no longer operate until the switch 3 is next closed in accordance with the status of the liquid level sensor. If the power supply is a DC supply, then once the overload condition is sensed, no further testing will be carried out until the 15 second time period as been exceeded. However, if an AC power supply is used (as shown) then the microprocessor 2 can carry out a more sophisticated process which can take account of the need for the tungsten load 7 to stabilize. Typically, if no overload is sensed in the first period of 3ms, then the microprocessor 2 will continue to monitor the voltage drop across the resistor 9 over the remaining 97ms (while the switch 3 remains ON) and if it detects an overload, it will turn OFF the switch 3 but turn it on again during the next half-cycle of the power supply 6. This process of successive tests is then continued with the switch being selectively turned OFF if an overload is sensed until completion of the 100ms period from start-up. The last few tests are then reviewed (typically the tests are carried out each half-cycle). For example if in the last four tests, at least one was satisfactory, then the switch 3 is maintained ON (assuming this is still the appropriate setting according to the liquid level sensor) on the basis that any unsatisfactory tests will have been due to transient effects. Although Figure 2 shows an approximately exponentially decaying waveform 12, the reference waveform can be chosen depending upon the load expected between the terminals 4,5 and the switching capacity of the switch 3. In addition, the microprocessor 2 could be replaced by other circuitry as desired. A conventional switch unit will include an LED to indicate whether the unit is ON or OFF. In this new unit as shown in Figure 1, however, an LED 13 is provided which not only provides the standard indication but provides other information. As can be seen, the LED 13 operates under the control of the microprocessor via port PB4 and is activated in the following manner: (i) When the unit is operating normally and the load is ON, the LED is continuously lit. (ii) When the unit is operating normally and the load is OFF, the LED flashes at one pulse per second. (iii) When the load is short circuited or the unit is drawing too much power (customer wiring/site problem) the LED flashes at one pulse every four seconds. (iv) If there is an internal electronics failure indicating that the unit should be returned to the manufacturer, the LED flashes at three pulses per second. (v) If the device has not been calibrated, the LED flashes at one pulse every two seconds. It will be appreciated that the flash rate can be chosen as desired. This mode of operation makes use of the fact that a distinction can be made by the microprocessor 2 between a customer wiring/site problem (state iii) and an internal problem requiring the manufacturer to repair (state iv).

Claims: A method of controlling a switch (3) in a series circuit which also includes a power supply (6) and a load (7), the method comprising: a) closing the switch (3); b) monitoring current flowing in the circuit; c) comparing the monitored current with a reference which varies with time corresponding to the variation expected when an acceptable load is connected and, d) opening the switch if the monitored current varies from the reference in an unacceptable manner, wherein the reference is independent of the opening of the switch. A method according to claim 1, wherein step b) comprises generating a voltage equivalent to the monitored current, step c) comprising generating an equivalent reference voltage. A method according to claim 1 or claim 2, further comprising: e) repeating steps a) - d) after a predetermined interval. A method according to claim 3, wherein the predetermined interval is relatively long if the monitored current varies unacceptably from the reference current within a first predetermined period after step a). A method according to claim 4, wherein the first predetermined period is substantially three milliseconds. A method according to claim 3 or claim 4, wherein the predetermined interval is relatively short if the monitored current varies unacceptably from the reference current within a second predetermined period after step a). A method according to claim 6, wherein the second predetermined period commences upon completion of the first predetermined period. A method according to any of the preceding claims, further comprising terminating the method after completion of a test period. A method according to claim 8, wherein the test period is substantially 100 milliseconds. A method according to any of the preceding claims, wherein the unacceptable variation occurs when the monitored current exceeds the reference. A method according to any of the preceding claims, further comprising activating an indicator in a first manner if the switch is opened in step d) as the result of step c); and activating the indicator in a second, different manner as a result of another type of fault developing. A method according to claim 11, wherein the indicator is an optical device, the first and second manners comprising turning the indicating device ON and OFF at respective different frequencies. A method according to any preceding claim wherein the reference takes the form of a rapid increase to a peak followed by an approximately exponential decay. Switch control apparatus for connection to a switch (3) connected in a series with a power supply (6) and a load (7), the apparatus comprising means (2, 9, 10) for monitoring current flowing in the circuit; a reference generator (2) for generating a reference which varies with time corresponding to the variation expected when a load is connected; a comparator (10) for comparing the monitored current with the reference; and a control system (2) responsive to the comparator (10) to open the switch (3) if the monitored current varies from the reference in an unacceptable manner, characterised in that the reference generator is independent of the control system. Switch control apparatus according to claim 14, adapted to carry out a method according to any of claims 1 to 13. Switch control apparatus according to claim 14 or claim 15 for use with a level sensor, the switch being operated normally in response to conditions sensed by the level sensor.

Inventors: DA SILVA RIBEIRO COELHO LUIS F, GUNBY DAVID, SMALLWOOD TIMOTHY JOHN, TURNER ROY COLIN, DA SILVA RIBEIRO COELHO, LUIS FILIPE, GUNBY, DAVID, SMALLWOOD, TIMOTHY JOHN, TURNER, ROY COLIN
Applicants: SOLARTRON MOBREY LTD, SOLARTRON MOBREY LIMITED
Application year: 1999
Main classification: G05B9
IPC codes: G05B9