The present invention relates to an apparatus including an inverter switching arrangement and a method for driving said inverter switches. In particular, the invention relates to determining whether the inverter can be operated in a safe torque off mode.

BACKGROUND OF THE INVENTION

Figure 1 is a block diagram of a system, indicated generally by the reference numeral 1, including an adjustable speed drive (ASD). The system 1 comprises an AC power supply 2, an ASD 4 and a load 6 (such as a three- phase motor). The ASD 4 includes a rectifier 8 (often a diode-based rectifier, as shown in Figure 1, although alternatives, such as advanced front end rectifiers are known), a DC link capacitor 10, an inverter 12 and a control module 14.

The output of the AC power source 2 is connected to the input of the rectifier 8. The output of the rectifier 8 provides DC power to the inverter 12. As described further below, the inverter 12 includes a switching module used to convert the DC voltage into an AC voltage having a frequency and phase dependent on gate control signals. The gate control signals are typically provided by the control module 14. In this way, the frequency and phase of each input to the load 6 can be controlled.

The inverter 12 is typically in two-way communication with the control module 14. The inverter 12 may monitor currents and voltages in each of the three connections to the load 6 (assuming a three-phase load is being driven) and may provide current and voltage data to the control module 14 (although the use of both current and voltage sensors is by no means essential). The control module 14 may make use of the current and/or voltage data (where available) when generating the gate control signals required to operate the load as desired; another arrangement is to estimate the currents from the drawn voltages and the switching patterns - other control arrangements also exist.

A safe torque off (STO) function is a known safety function used in motor drives and other inverter systems. The STO function typically controls the inverter 12 such that the motor 6 does not generate torque. A motor drive system may be required to have the ability to enter a safe torque off state at any time. Furthermore, a system may be required to check that it is able to enter a safe torque off state if requested. The present invention seeks to provide alternatives to existing safe torque off methodologies.

SUMMARY OF THE INVENTION

The present invention provides a method comprising: driving a high-sided switching element and a low-sided switching element of an inverter using first and second gate driver signals respectively such that a first one of said switching elements is turned off before the other one of said switching elements is turned on such that a blanking period exists during a state transition during which neither of said switching elements is turned on, (wherein the high-sided switching element is typically configured to connect an output of the inverter to a positive voltage and the low-sided switching element is typically configured to connect the output of the inverter to a negative voltage sided); monitoring a current and a voltage of the output of the inverter (e.g. the polarity of the current and voltage of the output of the inverter); and determining whether the currents and voltages of the output of the inverter during said blanking period are indicative of the inverter being able to enter a safe torque off mode (and typically providing an output indicative of whether the inverter is able to enter the safe torque mode accordingly).

The present invention also provides an apparatus comprising: a high side- switching element of an inverter configured to connect an output of the inverter to a positive voltage in response to a first gate driver signal; a low- sided switching element of the inverter configured to connect the output of the inverter to a negative voltage in response to a second gate driver signal; and a diagnostic module configured to identify a blanking period during which the output of the inverter is disconnected from both the positive voltage and the negative voltage and to monitor a current and voltage of the output of the inverter in order to determine whether the current and voltage are indicative of the inverter being able to enter a safe torque off mode (and typically providing an output accordingly).

The present invention further comprises a computer program (or a computer program product), configured to: monitor a current and a voltage of an output of an inverter, wherein the inverter comprises a high-sided switching element and a low-sided switching element that are driven using first and second gate driver signals respectively such that a first one of said switching elements is turned off before the other one of said switching elements is turned on such that a blanking period exists during a state transition during which neither of said switching elements is turned on; and determine whether the currents and voltages of the output of the inverter during said blanking period are indicative of the inverter being able to enter a safe torque off mode (and to typically provide an output or take some other action accordingly). The computer program (or computer program product) may be further configured to drive the high-sided and low-sided switching elements of the inverter. The present invention yet further provides a computer program product comprising instructions which when executed by a computer causes the computer to carry out the following steps: monitoring a current and a voltage of an output of an inverter, wherein the inverter comprises a high- sided switching element and a low-sided switching element that are driven using first and second gate driver signals respectively such that a first one of said switching elements is turned off before the other one of said switching elements is turned on such that a blanking period exists during a state transition during which neither of said switching elements is turned on; and determining whether the currents and voltages of the output of the inverter during said blanking period are indicative of the inverter being able to enter a safe torque off mode.

In some forms of the invention, the diagnostic module provides an output indicating that the inverter is not able to enter the safe torque off mode in the event that, during a transition from a first mode in which the high-sided switching element is turned on and the low-sided switching element is turned off to a second mode in which the high-sided switching element is turned off and the low-sided switching element is turned on, the output current detected at the diagnostic module does not remain positive and/or the output voltage detected at the diagnostic module does not transition from positive to negative during the blanking period.

In some forms of the invention, the diagnostic module provides an output indicating that the inverter is not able to enter the safe torque off mode in the event that, during a transition from a second mode in which the high- sided switching element is turned off and the low-sided switching element is turned on to a first mode in which the high-sided switching element is turned on and the low-sided switching element is turned off, the output current detected at the diagnostic module does not remain negative and/or the output voltage detected at the diagnostic module does not transition from negative to positive.

In some forms of the invention, the diagnostic module provides an output indicating that the inverter is able to enter the safe torque off mode in the event that: during a transition from a first mode in which the high-sided switching element is turned on and the low-sided switching element is turned off to a second mode in which the high-sided switching element is turned off and the low-sided switching element is turned on, the output current detected at the diagnostic module remains positive and the output voltage detected at the diagnostic module transitions from positive to negative during the blanking period; and/or during a transition from the second mode to the first mode, the output current detected at the diagnostic module remains negative and the output voltage detected at the diagnostic module transitions from negative to positive during the blanking period.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the following schematic drawings, in which:

Figure 1 shows an exemplary motor drive system;

Figure 2 shows a system in accordance with an aspect of the present invention; Figure 3 shows signals demonstrating part of the functionality of the circuit of Figure 2.

Figure 4 is a table demonstrating part of the functionality of the circuit of Figure 2;

Figure 5 is a flow chart demonstrating an aspect of the functionality of the circuit of Figure 2; and

Figure 6 shows an exemplary 3-phase inverter that may be used in embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

Figure 2 shows a motor drive system, indicated general by the reference numeral 20, in accordance with an aspect of the present invention.

The system 20 comprises an inverter having a high-sided switching element (T1) and a low-sided switching element (T2), each having a free-wheeling diode (D1, D2) associated therewith. The system 20 further comprises a gate driver 22, a load 24 (such as a motor), a comparator 26 and a logic circuit 28. The high-sided switching element T1 and the low-side switching element T2 may be implemented using insulated gate bipolar transistors (IGBTs); alternative implementations will be apparent to persons skilled in the art.

As shown in Figure 2, the mid-point of the high-side and low-side switching elements (the output of the inverter) is connected to the load 24 and is also connected to one input of the comparator 26. The other input of the comparator 26 is connected to ground potential, so that the output of the comparator is indicative of whether the inverter output voltage is positive or negative. The output of the comparator 26 is provided to a logic circuit 28. A current sensor is provided to provide the logic circuit 28 with an indication of the current output of the inverter. As discussed further below, the direction of the current is relevant to the determination to be made by the logic circuit 28.

The logic circuit 28 also receives inputs from the gate driver 22. Thus, the logic circuit is informed of the times at which the switching elements T1 and T2 are instructed to turn on and off and can determine the impact of these instructions on the polarities of the current and voltage at the inverter output. On the basis of these data, the logic circuit 28 provides a diagnosis output indicating whether or not the system 20 is able to enter a safe torque off mode. (The logic circuit 28 may be referred to as a diagnostic module.)

The gate driver 22 provides independent gate drive signals to the gate of the high-sided and low-sided switching elements (T1 , T2). Clearly, if the high- sided switching element (T1) is on and the low-sided switching element (T2) is off, then the output as detected by the comparator 26 will be positive. Similarly, if the high-sided switching element (T1) is off and the low-sided switching element (T2) is on, then the output as detected by the comparator 26 will be negative. As discussed further below, whether or not the system 20 is able to enter a safe torque off mode is indicated by the output of the inverter when both the high-sided switching element (T1) and the low-sided switching element (T2) are switched off (referred to herein as a "blanking period") .

Figure 3 shows gate drive signals g1 and g2 demonstrating part of the functionality of the circuit of Figure 2. The first gate drive signal g1 is provided by the gate driver 22 to the gate of the high-sided switching element T1. The second gate drive signal g2 is provided by the gate driver 22 to the gate of the low-sided switching element T2. Figure 4 is a table demonstrating part of the functionality of the circuit of Figure 2 in response to the gate drive signals shown in Figure 3. As shown in Figure 3, initially the first gate drive signal g1 is high and the second gate drive signal g2 is low. In this state, the high-sided switching device is on and the low-sided switching device is off. As indicated in first line of the table of Figure 4, in this state, the output of the inverter of Figure 2 is connected to the positive DC supply such that the voltage output of the comparator 26 is high (positive). Similarly, a positive current flows from the positive DC supply through the inverter to the load 24 such that the current output of the inverter is positive.

Next, the first gate drive signal g1 is turned off so that both the first and second gate drive signals are off. Thus, the high-sided switching element T1 and the low-sided switching element T2 are both switched off for a short period (referred to herein as a blanking period). In this state, the positive current flowing to the load 24 continues to flow, but now, since the upper switching element T1 is turned off, this current flows instead through the anti-parallel diode D2 of the lower switching element T2. Thus, the current flows from the negative DC supply. The effect of this (as stated in Figure 4) is that the current detected at the output of the inverter remains positive, but the voltage detected by the comparator 26 is negative. At the end of the blanking period, the second gate drive signal g2 is turned on (with the first gate drive signal g1 still off) so that the high-sided switching device is off and the low-sided switching device is on. As indicated in the table of Figure 4, in this state, the output of the inverter of Figure 2 is connected to the negative DC supply such that the voltage output of the comparator 26 is low (negative). Similarly, a current flows from the load to the negative DC supply through the inverter such that the current output of the inverter is negative. Next, the first gate drive signal g2 is turned off so that both the first and second gate drive signals are off. Thus, the high-sided switching element T1 and the low-sided switching element T2 are both switched off for a short period. Contrary to the situation described above, in this state, the negative current flowing at the load 24 continues to flow, but now, this current flows instead through the anti-parallel diode D1 of the upper switching element T1. Thus, the current flows to the positive DC supply. The effect of this (as stated in Figure 4) is that the current detected at the output of the inverter remains negative, but the voltage detected by the comparator 26 is positive. The logic circuit 28 receives the gate drive signals g1 , g2 and data

concerning the current and voltage output of the inverter of the circuit 20. Accordingly, all of the data in the table of Figure 4 is available to the logic circuit 28. Thus, each time a transition with a blanking period occurs, the logic circuit can determine whether or not the circuit 20 is operating in accordance with the expected logic shown in Figure 4. If so, the "diagnosis" output of the logic circuit 28 indicates that the circuit is operating normally (and is able to enter a safe torque off mode if required). If not, the diagnosis output of the logic circuit indicates that the circuit is not operating normally (and that it cannot be stated that a safe torque off mode could be entered if required).

Figure 5 is a flow chart showing an algorithm, indicated generally by the reference numeral 50, demonstrating an aspect of the functionality of the circuit of Figure 2. The algorithm 50 starts at step 52, where it is determined whether or not a blanking period has occurred. As noted above, a blanking period occurs when both the high-sided and low-sided switching elements (T1 and T2 in the example above) are switched off in a transition between modes. The existence of a blanking period can be detected from the states of the gate driver signals (g1, g2 in the example above). If a blanking period is detected, the algorithm moves to step 54, otherwise the step 52 is repeated. The step 52 could be implemented as an interrupt, although many

alternative arrangements are possible.

At step 54 of the algorithm 50, the polarity of the current and voltage outputs of the inverter are determined. These data are readily available to the logic circuit 28 from the current sensor monitoring the inverter output current and from the output of the comparator 26.

Next, at step 56, it is determined whether the various data obtained by the logic circuit 28 matches the data in the table described above with reference to Figure 4. Thus, if the blanking period has been entered from a first mode in which the high-sided switching element is turned on and the low-sided switching element is turned off, it is determined in the step 56 whether the output current remains positive and the output voltage transitions from positive to negative during the blanking period and, if so, the algorithm 50 moves to step 58; otherwise the algorithm moves to step 59. Similarly, if the blanking period has been entered from a second mode in which the high- sided switching element is turned off and the low-sided switching element is turned, it is determined in the step 56 whether the output current remains negative and the output voltage transitions from negative to positive during the blanking period and, if so, the algorithm move to step 58; otherwise the algorithm 50 moves to step 59.

At step 58, the diagnosis output of the logic circuit 28 is set to indicate that it is possible for the inverter to enter the safe torque off mode. The algorithm 50 then terminates.

At step 59, the diagnosis output of the logic circuit 28 is set to indicate that it cannot be confirmed that it is possible for the inverter to enter the safe torque off mode. The algorithm 50 then terminates.

The inverter of the system 20 is a one-phase inverter that may implement the inverter 12 in a motor drive system of the form described above with reference to Figure 1. Of course, other arrangements, such as three-phase inverter arrangements, are possible and are within the scope of the present invention. By way of example, Figure 6 shows details of an exemplary implementation of the inverter 12.

As shown in Figure 6, the inverter 12 comprises first, second and third high- sided switching elements (T1 , T2 and T3) and first, second and third low- sided switching elements (T4, T5 and T6). Each switching element may, for example, be an insulated-gate bipolar transistor (IGBT) or a MOSFET transistor. As shown in Figure 6, each of the switching elements (T1 to T6) is associated with a corresponding free-wheeling diode (D1 to D6).

The logic circuit 28 described above with reference to Figure 2 can readily be adapted to function with the three-phase inverter 12. For example, the logic circuit may operate the algorithm 50 separately for each phase of the inverter 12 such that, each time a blanking period occurs in any of the three phases of the inverter, it is determined whether that phase is able to enter the safe torque off mode.

The exemplary inverter 12 is a three-phase inverter generating three outputs: U, V and W. The three phases of the inverter 12 can be used to provide inputs to the three-phases of the load 6 in the system 1 described above with reference to Figure 1. Of course, the inverter 12 could be modified to provide a different number of outputs in order to drive a different load (such as a load with more or fewer than three phases).

The inverter 12 is a 2-level, 6 transistor inverter. As will be apparent to those skilled in the art, the principles of the present invention are applicable to different inverters, such as 3-level inverters and 5-level inverters. The description of the inverter 12 is provided by way of example to help illustrate the principles of the present invention.

The embodiments of the invention described above are provided by way of example only. The skilled person will be aware of many modifications, changes and substitutions that could be made without departing from the scope of the present invention. For example, the principles of the present invention are not limited to use with a motor drive system of the form shown in Figure 1. The claims of the present invention are intended to cover all such modifications, changes and substitutions as fall within the spirit and scope of the invention.