Switch detection system

Abstract

A switch detection system (10) comprising a first enabling switch (S1) to switch a second multi-position switch (S2) between an enabled and a disabled state. The second multi-position switch (S2) is m

Description

The present invention relates to a switch detection system, in particular to a switch detection system for determining the position of a multi-position switch used to control a safety-critical system such as a high integrity motor control system. Known safety-critical systems are controlled by switches. For high integrity motor control systems, the switch is used to move a device that is physically connected to the motor. It is critical to both detect the position of the switch, and therefore know which direction the device is being moved, and also if the switch has a fault and therefore not able to move the device. Typical faults include the existence of an open-circuited switch, a short-circuited switch, an incorrect wiring connection, or a wiring disconnection. If undetected, such faults can lead to components in the system being damaged, and possible failure of the system itself which is clearly undesirable in a safety-critical system. By detecting a fault, the system can be returned to a system safe state. Known switch detection systems rely on measuring DC voltage levels to determine the position of the switch, with the use of complementary outputs to provide a level of fault detection. For example, if the multi-position switch is a two-way (or position) toggle switch (such as a Double-Throw, Double-Pole type, DPDT), a complementary logic definition can be used where: In position 1, Pole A = HI &Pole B = LO In position 2, Pole A = LO &Pole B = HI Whilst such a system works adequately for a two-way switch, extending the system to a three-way switch typically required in safety-critical systems adds significant complexity, even more so if the three-way switch is enabled and disabled by a connected enabling switch. If the enabling switch is in an "enabled" position, then the three-way switch can control the system to which it is connected. If the enabled switch is in a “disabled” position, then any position of the three-way switch will not be able to control the system to which it is connected. The enabling switch adds further complexity in determining the position of the three-way switch. Even with the added complexity, relying on complementary outputs to determine the switch position does not enable the detection of faults within the system within which the switch is operating, for example, should a wire go open or short circuit then the voltage level would go low which is an allowable state and would not be detected as a fault. Such as system is therefore not suitable for a switch operating in a low-reliability environment where faults are likely to occur. An object of the present invention is to provide a simpler and more reliable switch detection system for a multi-position switch. Thus, according to the present invention, there is provided a switch detection system comprising a first enabling switch and a second multi-position switch, the first enabling switch movable between an enabled position and a disabled position to switch the second multi-position switch between an enabled and a disabled state, the second multi-position switch movable between multiple positions, in which the switch detection system is configured to apply a plurality of frequencies to the first enabling switch, measure a frequency at each pole of the second multi-position switch, compare the frequency at each pole of the second multi-position switch to an expected frequency at each pole associated with each position of the second multi-position switch, and at least one of determine the position of the second multi-position switch, determine the position of the first enabling switch, or determine a fault condition based on the comparison between the expected frequency and the measured frequency. Advantageously, a system which relies on the detection of frequencies at the output of the multi-position switch enables the more reliable detection of a fault condition in the system compared to a system which relies on DC voltage levels. Specifically, the use of frequency enables the detecting of errors in the circuit or switch where should a line go short or open the frequency is no longer present and a fault can be detected. In contrast, in a system relying on DC voltage levels, should a line go short or open, the voltage level would go low which is an allowable state and would not be detected as a fault. Furthermore, the system relying on frequency detection can be used to detect the position of multiple position switch with more than two positions. Preferably, the first enabling switch is a four-pole two-position switch having a first enabling switch first pole, a first enabling switch second pole, a first enabling switch third pole, and a first enabling switch fourth pole. Preferably, the second multi-position switch is a double-pole three-position switch have a second multi-position switch first pole and a second multi-position switch second pole, the second multi-position switch movable between an up, a centre and a down position. Preferably, the first enabling switch first pole is connected to the up position of the first pole of the second multi-position switch, the first enabling switch second pole is connected to the centre position and the down position of the first pole of the second multi-position switch, the first enabling switch third pole is connected to the down position of the second pole of the second multi-position switch and the down position of the second multi-position switch, and the first enabling switch fourth pole is connected to the centre and up position of the second pole of the second multi-position switch. Preferably, the step of applying a plurality of frequencies (to the first enabling switch comprises applying the first frequency to the enabled position of the first pole and the third pole of the first enabling switch, applying the third frequency to the enabled position of the second pole and the fourth pole of the first enabling switch, and applying the second frequency to the disabled position of the first, second, third and fourth poles of the first enabling switch. Preferably, the step of measuring the frequency at each pole of the second multiposition switch comprises measuring the frequency at the second multi-position switch first pole and measuring the frequency at the second multi-position switch second pole. Preferably, the step of comparing the frequency at each pole of the second multiposition switch comprises comparing the frequency at the second multi-position switch first pole and comparing the frequency at the second multi-position switch second pole with the expected frequency. Preferably, the step of determining the position of the second multi-position switch based on the comparison between the expected frequency and the measured frequency comprises determining the switch is in the up position if the frequency at the second multi-position switch first pole is equal to the first frequency and the frequency at the second multi-position switch second pole is equal to the third frequency, or determining the switch is in the centre position if the frequency at the second multiposition switch first pole and the frequency at the second multi-position switch second pole is equal to the third frequency, or determining the switch is in the down position if the frequency at the second multi-position switch first pole is equal to the third frequency and the frequency at the second multi-position switch second pole is equal to the first frequency. Preferably, the step of determining the position of the first enabling switch based on the comparison between the expected frequency and the measured frequency comprises determining the first enabling switch is in the disabled position if the frequency at the second multi-position switch first pole and the frequency at the second multi-position switch second pole is equal to the second frequency. Preferably, the step of determining a fault condition based on the comparison between the expected frequency and the measured frequency comprises determining a fault condition if the frequency at the second multi-position switch first pole or the frequency at the second multi-position switch second pole is zero, or if the frequency at the second multi-position switch first pole or the frequency at the second multi-position switch second pole is not the first, second or third frequency. Advantageously, by applying different frequencies to the first enabling switch and connecting the first enabling switch to the second multi-position switch in a known logic, and measuring the frequencies at the output poles of the second multi-position switch, it is possible to determine if the first enabling switch is enabled or disabled, the position of the second multi-position switch, or if there is a fault in the system, based on the expected frequency at each pole according to the known logic. According to another aspect of the present invention there is provided a method of determining the position of a second multi-position switch comprising the steps of providing a first enabling switch and second multi-position switch, the first enabling switch movable between an enabled position and a disabled position to switch the second multi-position switch between an enabled and a disabled state, the second multi-position switch movable between multiple positions, applying a plurality of frequencies to the first enabling switch, measuring a frequency at each pole of the second multi-position switch, comparing the frequency at each pole of the second multi-position switch to an expected frequency at each pole associated with each position of the second multi-position switch, and determining at least one of the position of the second multi-position switch, the position of the first enabling switch, or a fault condition based on the comparison between the expected frequency and the measured frequency. According to another aspect of the present invention there is provided a system including the above-mentioned switch detection system. Preferably the system comprises a high integrity motor control system configured to physically move a device. Preferably, the second multi-position switch is movable between the multiple positions to physically move the device between multiple positions. Preferably, the system is returned to a safe state if a fault is detected. According to another aspect of the present invention there is provided a switch detection system comprising a second multi-position switch, the second multi-position switch movable between multiple positions, in which the switch detection system is configured to apply a plurality of frequencies to the second multi-position switch and measure a frequency at each pole of the second multi-position switch, compare the frequency at each pole of the second multi-position switch to an expected frequency at each pole associated with each position of the second multi-position switch, and at least one of determine the position of the second multi-position switch, or determine a fault condition based on the comparison between the expected frequency and the measured frequency. The invention will now be described with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of the of a switch detection system according to the present invention in an enabled state, Figure 2 is a circuit diagram of the switch detection system of Figure 1 in a disabled state, and Figure 3 is a schematic view of a system incorporating the switch of Figure 1. In Figures 1 to 3, a first enabling switch in the form of a four-pole switch S1 is shown. The four-pole switch has a first pole P1, a second pole P2, a third pole P3 and a fourth pole P4. The first enabling switch S1 is a two-position switch, movable between an enabled position (Figure 1) and a disabled position (Figure 2). The four poles P1 ,P2,P3,P4 of the switch S1 are mechanically fixed to each other by a bar (represented by the dotted line 12) such that when actuated between the enabled and disabled positions, all four poles P1 ,P2,P3,P4 move simultaneously. The four-pole switch S1 is connected to a second multi-position switch, in this embodiment, a three-way switch S2. The three-way switch S2 is movable between an up, a centre and a down position. The three-way switch 10 is used to control a high integrity motor control system 60 which physically moves a safety critical device 70 up and down (Figure 3). In Figures 1 and 2, the switches S1,S2 are connected as described below where the different paths between each pole P1,P2,P3,P4 of the first enabling switch S1 and each position of the second multi-position switch S2 are shown. The first enabling switch first pole P1 is connected to the up position of the first pole A of the second multi-position switch S2. The first enabling switch second pole P2 is connected to the centre position and the down position of the first pole A of the second multi-position switch S2. The first enabling switch third pole P3 is connected to the down position of the second pole B of the second multi-position switch S2 and the down position of the second multi-position switch S2. The first enabling switch fourth pole P4 is connected to the centre and up position of the second pole B of the second multi-position switch S2. The position of the second multi-position switch S2 is determined as follows: Multiple frequencies, in this embodiment, three non-overlapping frequencies F1 ,F2,F3 are applied simultaneously and continuously using a signal generator (not shown) to the four-pole switch S1 in the combinations shown in Figures 1 and 2 and as described below. The frequencies F1,F2,F3 cannot overlap but can be close if the generation and detection technology is sufficiently precise, for example, using a field-programmable gate array (FPGA). The type of frequency depends on the implementation, but in this embodiment, a square wave is used which is easy to generate and detect using an FPGA. A first frequency F1 is applied to the enabled position of the first pole P1 and the third pole P3 of the first enabling switch S1. A third frequency F3 is applied to the enabled position of the second pole P2 and the fourth pole P4 of the first enabling switch S1. A second frequency F2 is applied to the disabled position of the first P1, second P2, third P3 and fourth P4 poles of the first enabling switch S1. The frequency Fa and Fb is measured at the respective poles A and B of the multi (three)-position switch S2. When the three-position switch S2 is enabled (Figure 1), each of the three positions is indicated by the presence of either a frequency F1 or F3 at the pole outputs A and B, as shown in the Table 1 below. Position Expected Frequency Fa Fb Up F1 F3 Centre F3 F3 Down F3 F1 Table 1 When the three-position switch S2 is disabled (Figure 2), only a frequency F2 is present at the pole outputs A and B as shown in the Table 2 below. Position Expected Frequency Fa Fb Up F2 F2 Centre F2 F2 Down F2 F2 Table 2 If a fault condition occurs then no frequency would be present at either pole outputs A or B and thus the fault would be detected, or an unexpected frequency would be detected at one or both of the pole outputs A,B, that is a frequency other than the first F1, second F2 or third F3 frequency. If a fault condition is detected, the system 50 can be returned to a safe state. Accordingly, by applying different frequencies to the first enabling switch S1 and connecting the first enabling switch to the different positions of the three position switch in a known logic, and measuring the frequencies at the output poles of the three position switch, it is possible to determine if the first enabling switch S1 is enabled or disabled, the position of the three position switch S2, or if there is a fault in the system, based on the expected frequency at each pole according to the known logic achieved by configuring the connections of switches S1 ,S2 as described above. In the above embodiments, the first enabling switch has four poles which is one more pole than the number of positions of the three-position switch to enable the three frequencies to be applied to all positions on the three-position switch. It will therefore be understood that one more pole is required on the first enabling switch than the number of positions on the multi-position switch. In an alternative system, if the multi-position switch is always enabled, i.e. there is no first enabling switch in the system, only two frequencies are required to determine the position of the switch for a three-position switch.

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