Patent Description:
Electric motors used in various industrial installations are often controlled to achieve a target position. In this context, a feedback loop is used to modify the actual position of a moving part of the motor such that it corresponds to the target position. The difference between the actual position and the target position is processed by an electronic control unit, which determines a target supply current. The control unit of the motor is therefore adapted to supply the motor (each of its phases) in such a way as to generate a supply current corresponding to the target supply current to achieve high precision positioning of the moving part of the motor.

<CIT> discloses a control circuit for an electric motor, which is configured to generate via a first digital-to-analog converter, a control signal for driving the electric motor. The control circuit comprises a low-voltage zone and a high-voltage zone. The low-voltage zone comprises a control unit while the high-voltage zone comprises a current sensor for measuring the current supply to the electric motor and an analog-to-digital converter (ADC) to convert the analog signal into a corresponding digital signal which is sent to the control unit. A galvanic separating element is provided between the low and high-voltage zones for security reasons.

In order to achieve the required performance, the noise level on the output current Ioutput is crucial and is usually expressed in µArms. The noise level is directly linked to the current measurement's least significant bit (LSB) resolution. The smaller the LSB resolution, the better is the noise. The LSB resolution may be reduced in three different ways:.

However, large currents are not required through the whole operating range of the motor. When the motor is at standstill or running at constant speed, the current intensities are significantly smaller than the maximum drive current usually required during acceleration phases of the motor. Different driving modes may therefore be implemented according to the operating range of the motor.

<CIT> discloses for example a control circuit for feedback-based control of motion and positioning of a motor. The control circuit comprises a current measurement device configured to obtain a measurement of the current in the phase load of the motor to provide feedback. The intensity of the current in the phase load varies within an operating range which is made up of a relatively large current range for the acceleration phase of the motor and of relatively smaller current range when the motor is at standstill or running at constant speed. The current measurement device has a first coarse sensor optimized for measuring the relatively large current range and a second fine sensor optimized for measuring the relatively smaller current range, thereby maximizing feedback accuracy when the motor is at standstill or running at constant speed.

<CIT> discloses a motor driving control apparatus and a motor driving method.

USJP5556353 discloses a motor control detection IC and a motor control device using the same.

An aim of the present invention is to provide an alternative control circuit, for driving an electric motor, which optimizes the current feedback as a function of the operating range of the electric motor.

Another aim of the present invention is to provide a control circuit with high and low voltage subcircuit separated by a galvanic isolation and adapted to use an isolator with a limited number of channels to pass control data across the galvanic isolation barrier for selecting a low or high gain driving modes.

Using an isolator with limited number of channels has the advantage to be cost-effective and compact.

The invention is defined in the independent apparatus claim <NUM> and the independent method claim <NUM>. The preferred embodiments are defined in the dependent claims.

This aim is achieved by a control circuit for an electric motor comprising at least one phase. The control circuit comprises a low voltage subcircuit, a high voltage subcircuit and an isolation barrier between the low voltage and high voltage subcircuits. The low voltage subcircuit comprises a current controller adapted to generate a driving signal, and a feedback loop whose output is fed back to the input of the current controller. The high voltage subcircuit comprises a power bridge to output a current for driving the at least one phase of the motor, a current sensor for measuring the current in the at least one phase, an analog front-end and an analog-to-digital converter (ADC). The isolation barrier comprises an isolator adapted to pass the output signal of the ADC across the isolation barrier from the high voltage subcircuit to the low voltage subcircuit. The analog front-end is adapted to apply a first gain or a second gain that is higher than the first gain as a function of the current in the at least one phase measured by the current sensor.

The isolator comprises a first and a second channel to pass respectively a clock signal and a control signal from the low voltage subcircuit to the high voltage subcircuit in order to select the first or second gain of the analog front-end. The isolator comprises a third and a fourth channel to pass respectively the output data of the ADC and a replica of the clock signal from the high voltage subcircuit to the low voltage subcircuit.

In an embodiment, the high voltage subcircuit further comprises a D flip-flop. The clock input of the D flip-flop is arranged to receive the control signal and the input of the D flip-flop is arranged to receive the clock signal. The output of the D flip-flop is used for selection of either first or second gain of the analog front-end as a function of the clock signal.

In an embodiment, the high voltage subcircuit further comprises a multiplexer comprising two data inputs and a selector input, wherein one of the two data inputs is arranged to receive the clock signal, the other of the two data inputs is arranged to receive the output of the D flip-flop. The selector input is arranged to receive the control signal such that either the selected gain or the replica of the clock signal may be passed via the fourth channel of the isolator based on the state of the control signal.

In an embodiment, the feedback loop of the low voltage subcircuit comprises a deserializer arranged to receive serial data from the ADC, representing the current value measured by the current sensor, as an input to convert the serial data into a single value encoded on M bits. The feedback loop also comprises a register shifter for changing the value of said M bits if the selected first or second gain differs from the gain applied to said current value.

In an embodiment, the feedback loop further comprises an accumulator arranged to add up different samples of currents measured by the current sensor, a decimator filter for sampling N current samples and a division element for outputting a moving average of the current measurements.

In an embodiment, the feedback loop further comprises a clock generator arranged to generate a first or a second clock signal as a function of the selected first or second gain. The selection of the first or second gain is based on the current measured by the current sensor.

In an embodiment, the clock generator is arranged to adapt the first and second clock signals, when the current measured by the current sensor is between a first and a second current thresholds, such that the moving average of the current measurement outputted by the division element is made up of current samples measured with the first gain and current samples measured with the second gain during a transition phase from a low to a high gain driving mode or from a high to a low gain driving mode.

Another aspect of the invention relates to an electric motor comprising the control circuit as described above.

A further aspect of the invention relates to a method for controlling the control circuit for an electric motor, comprising the steps of.

In an embodiment, the transition phase is divided into N sub-transition phases. A first sub-transition phase is the sub-transition phase just above the first current threshold value. A last sub-transition phase is the sub-transition phase just below the second current threshold value. At least <NUM>% of <NUM>/N %, and preferably around <NUM>/N % of the current measurement samples are taken during the first sub-transition phase with the second gain while the rest of the current measurement samples are taken during the first sub-transition phase with the first gain. At least <NUM>% of <NUM>/N %, and preferably around <NUM>/N % of the current measurement samples are taken during the last sub-transition phase with the first gain while the rest of the current measurement samples are taken during the last sub-transition phase with the second gain.

The invention will be better understood with the aid of the description of several embodiments given by way of examples and illustrated by the figures, in which:.

<FIG> shows a simplified block diagram of a control circuit <NUM> for an electric motor having at least one phase according to an embodiment. The control circuit <NUM> comprises a low voltage subcircuit 12a, a high voltage subcircuit 12b and an isolation barrier <NUM> between the low voltage and high voltage subcircuits 12a, 12b. The low voltage subcircuit 12a has a digital signal processor <NUM> comprising a current controller <NUM>. The digital signal processor can optionally be replaced by a programmable logic element such as a field programmable gate array (FPGA). The current controller <NUM> may comprise for example a PI controller <NUM> and a comparator <NUM> to output a PWM signal. The PI controller <NUM> is adapted to correct an error value based on a reference signal Iref and a feedback measurement value. A PID controller may however be used instead of the PI controller in an alternative embodiment.

The high voltage subcircuit 12b comprises a power bridge <NUM> to control the electric motor, a current sensor <NUM> for measuring the current flowing in the at least one phase of the electric motor when operating, an analog-front-end <NUM> for signal conditioning of the current signal to exploit the full input range of an analog-digital converter (ADC) <NUM>. The current sensor <NUM> is preferably a shunt resistance Rsh (<FIG>). The current builds up a voltage across the shunt resistance Rsh. This voltage is then amplified by the analog front-end <NUM> and digitally converted by the ADC <NUM>.

The isolation barrier <NUM> is an essential element for safety reasons. The isolation barrier comprises a gate driver <NUM> to produce a high-current drive input for the gate of the transistors of the power bridge <NUM> and an isolator <NUM> so that the serial data outputted by the ADC <NUM> may cross the isolation barrier <NUM>. The gate driver <NUM> and isolator <NUM> may comprise for example transformers, optocouplers or capacitive couplers to create the isolation barrier between the low and high voltage subcircuits 12a, 12b.

With reference to <FIG>, the isolator <NUM> comprises only four channels which has the advantage of being cost effective and having a reduced footprint in comparison with other isolators with higher numbers of channels. Two of the four channels are used to pass a clock signal CLK and a control signal ICS from the low to the high voltage subcircuits 12a, 12b. The clock signal CLK and the control signal ICS are generated respectively by a clock generator <NUM> and a control signal generator <NUM> of the digital signal processor <NUM> as shown in <FIG>.

The other two channels are used to pass a serial data signal SD outputted by the ADC <NUM> and a clock signal CLK", which is a replica of the above clock signal CLK, back to the low voltage circuit 12a to synchronize the clock signal in the low voltage side of the control circuit with the serial data outputted by the ADC <NUM> in order to avoid the risk of desynchronisation between the data and the clock due to isolation barrier delays.

It must be noted that when the motor is at standstill or operating at constant speed, the current levels are significantly smaller than current usually required during acceleration phases of the motor. In addition, a low noise level on the current outputted by the control circuit <NUM> is required when the motor is at standstill. The control circuit <NUM> is therefore adapted to switch the gain of the analog front-end <NUM> such that the motor is driven either in a low or high gain driving mode as a function of the current measured by the current sensor <NUM>.

The control circuit <NUM> is configured, on the one hand, to set the analog front-end <NUM> in a high gain driving mode when current flowing in one phase of the electric motor measured by the current sensor <NUM> is below a first current threshold in order to reduce the LSB size and, on the other hand, to set the analog front-end <NUM> in a low gain driving mode, when the current flowing in said one phase measured by the current sensor exceeds a second current threshold, in order to reach the maximum current level to achieve optimal dynamic performances.

Therefore, a high or low gain information coded on one bit (G=<NUM> or <NUM>) must also be transferred across the isolation barrier via the isolator <NUM>, despite the constraint in terms of limited channels, in order to set the analog front-end <NUM> in the low or high gain driving mode.

The electronic circuit diagram, illustrated in <FIG>, is adapted to pass such high or low gain information. To this end, the high voltage subcircuit 12b of the control circuit <NUM> of <FIG> further comprises a D flip-flop <NUM> and a multiplexer <NUM> arranged to control the gain of the analog front-end <NUM> as a function of current measurement samples. More particularly, the clock signal CLK and the control signal ICS generated by respectively a clock generator <NUM> and a control signal generator <NUM> of the digital signal processor <NUM> as shown in <FIG>, may cross the isolation barrier <NUM> from the low volage side to the high voltage side of the control circuit <NUM> via respectively the first and the second channel of the isolator <NUM>.

The serial data SD outputted by the ADC <NUM> may cross the isolation barrier <NUM> from the high voltage side to the low voltage side of the control circuit <NUM> via the third channel. The clock input of the D flip-flop <NUM> is arranged to receive the control signal ICS and the input of the D flip-flop <NUM> is arranged to receive the clock signal CLK. The inverted output of the D flip-flop which has been named G, represented in <FIG>, is used for selection of a low gain or a high gain of the analog front-end <NUM> as a function of the current measured by the current sensor <NUM>. In the electronic circuit diagram illustrated in <FIG>, the inverted output of the D flip-flop has been used for the selection of gain, but the non-inverted output could also have been used depending on the circuit design.

In other words, the signals are swapped, in such a way that the control signal ICS is used as the clock input of the D flip-flop, whereas the clock is used as the input to be sampled. Typically, the output of the D flip-flop is in a "zero" state (G=<NUM>) to set the analog front-end <NUM> to the high gain driving mode for low currents and where position stability is crucial, whereas the output of the D flip-flop is in a "one" state (G=<NUM>) to set the analog front-end <NUM> to the low gain driving mode for dynamic acceleration phases of the electric motor.

The electronic circuit diagram of <FIG> further comprises a multiplexer <NUM> comprising two inputs and a selector input SEL. One input of the multiplexer <NUM> is arranged to receive the clock signal CLK while the other input of the multiplexer <NUM> is arranged to receive the output of the D flip-flop <NUM>. The selector input SEL is arranged to receive the control signal ICS such that either the selected gain G or the replicated clock signal CLK" may pass via the fourth channel of the isolator <NUM> from the high voltage side to the low voltage side of the control circuit based on the state of the control signal ICS. Typically, the selected gain G will pass through the multiplexer when the control signal ICS is in high state (ICS=<NUM>), and the replicated clock signal CLK" will pass through the multiplexer when the control signal ICS is in low state (ICS=<NUM>). This allows conditioning of the signal in the low voltage subcircuit 12a as a function of the selected gain G, as described in detail subsequently with reference to <FIG>.

The low and high gain driving modes are used as a function of the intensity of load current of the electric motor to exploit the benefits of both driving modes as already mentioned above. For example, let us assume that a ratio of two has been foreseen by design between the high gain mode and the low gain mode. Indeed, any power of two is optimal to be able to use a simple register shifter <NUM> in the subsequent signal processing. However, in practice, an exact gain of two is never obtained due to the tolerances on the values of electronic components. Therefore, in the current range around the threshold it is necessary to make a smooth transition from the low gain driving mode to the high gain driving mode and vice versa to prevent abrupt changes, as shown in <FIG>, resulting in torque and speed ripples.

<FIG> shows an optimized feedback loop <NUM> used in the digital signal processor <NUM> of <FIG> according to an embodiment to prevent the abrupt changes of the output voltage during the transition phases from the low to the high gain driving modes and vice versa. The feedback loop <NUM> comprises a deserializer <NUM>, a register shifter <NUM>, an accumulator <NUM>, a decimator filter <NUM> and a division element <NUM>.

The deserializer <NUM> is arranged to receive serial data SD from the ADC <NUM> as an input. The serial data SD are transmitted one bit at a time in a serial manner and synchronized on the clock CLK". These bits are parallelized to create a single value encoded on M bits.

The register shifter <NUM> is used for changing the value if the current measurement has been performed with a gain different from the low gain in the analog front-end <NUM> to ensure that the values accumulated in the accumulator <NUM> are coherent and in the same range. The register shifter <NUM> is therefore controlled by an input signal G. When G is in "one" state, the register shifter <NUM> is inactive, whereas when G is in "zero" state, the register shifter <NUM> shifts the bits in the register by one or more positions to the right so that the value is coherent with the current measurements. Shifting one bit to the right is equivalent to dividing the value (encoded in binary format) by two. This is why the ratio between the high gain mode and the low gain mode is typically designed to be a power of two, and the same exponent is used as the number of positions to shift the register in the shifter <NUM>.

Oversampling technique is used to increase the smoothness during the transition phases between the low and high gain driving modes and vice versa so as to suppress, or at least reduce, torque and speed ripples that occur during these transition phases as shown in <FIG>. Accordingly, the accumulator <NUM> is arranged to add up different samples of currents measured by the current sensor <NUM>. The decimator filter <NUM> is used for downsampling the current samples (keeping only every Nth sample), and the accumulator <NUM> is reset at this moment. A reset module <NUM> is foreseen to count samples and send a single pulse when the number N is reached. This pulse is used both by the decimator filter <NUM> to latch the current value of the output of the accumulator, and by the accumulator <NUM> to reset its value to zero.

In the exemplary embodiment shown in <FIG>, the reset module <NUM> is set up to count fifty samples and generate the pulse signal (SUM_Ready) for the accumulator in such a way that it adds up <NUM> current measurement samples and the decimator filter <NUM> latches the sum upon completion of the addition of the <NUM> measurement samples. The decimator filter <NUM> transmits the accumulated value to the division element <NUM> for outputting a moving average of the current measurement for each oversampled measurement made of <NUM> current measurement samples.

The clock generator <NUM> of the digital processing unit, as shown in <FIG>, is arranged to generate a first or a second clock signal CLK as a function of the selected gain G which is determined based on the previous current measurement. The control signal generator <NUM> is arranged to generate the control signal ICS so as to initiate an analog to digital conversion in the ADC and select the gain of the analog front-end <NUM> as previously described in relation to <FIG>, <FIG>. The first and second clock signal CLK and control signal ICS are derived from a main cock CLKmain generated by the digital processing unit <NUM>.

<FIG> shows a smoother variation of the output voltage from the low to the high gain driving modes during a first transition phase when the current changes from -<NUM>[A] to -<NUM>[A], as well as a smoother variation of the output voltage from the high to the low gain driving modes during a second transition phase when the current changes from <NUM>[A] to <NUM>[A] when using the feedback loop of <FIG>. During the first and second transition phases, the number of ADC samples that are in low or high gain driving mode depends linearly on the previous current measurement as shown in <FIG>. In this way it is possible to smooth out the transition and to avoid abrupt changes.

More particularly, as illustrated in <FIG>, each current measurement sample having a value ranging from -<NUM>[A] to -<NUM>[A] and from <NUM>[A] to <NUM>[A] has been performed when the analog front-end <NUM> was set to a low gain driving mode while each current measurement sample having a value ranging from -<NUM>[A] to +<NUM>[A] has been performed when the analog front-end <NUM> was set to a high gain driving mode. During the first and second transition phases ranging from -<NUM>[A] to -<NUM>[A] and from <NUM>[A] to <NUM>[A] respectively, a certain percentage of current measurement samples have been performed in the low gain driving mode, while the rest of the current measurement samples have been performed in the high gain driving mode to add up to <NUM>%.

One would indeed see from the first transition phase of <FIG> that the percentage of the current measurement samples made in the low gain driving mode is high at the very beginning of this first transition phase and linearly decreased such that around <NUM>%, <NUM>%, <NUM>% and <NUM>% of the current measurement samples are performed in the low gain driving mode while the respective remaining <NUM>%, <NUM>%, <NUM>% and <NUM>% of the current measurement samples are performed in the high gain driving mode for a current of around -<NUM>[A], -<NUM>[A], -<NUM>[A] and -<NUM>[A] respectively.

Likewise, one would see from the second transition phase of <FIG> that the percentage of the current measurement samples made in the low gain driving mode is low at the very beginning of this second transition phase and linearly increased such that around <NUM>%, <NUM>%, <NUM>% and <NUM>% of the current measurement samples are performed in the low gain driving mode while the respective remaining <NUM>%, <NUM>%, <NUM>% and <NUM>% of the current measurement samples are performed in the high gain driving mode for a current of around +<NUM>[A], +<NUM>[A], +<NUM>[A] and +<NUM>[A] respectively.

The order in which the current measurement samples are performed in the first and second high gain driving mode does not impact the output of the control circuit <NUM>. For example, for current measurement samples of around +<NUM>[A], the current measurements samples that made up the <NUM>% of samples taken in the low gain driving mode can be added together by the accumulator <NUM> in a row and the <NUM>% of samples taken in the low gain driving mode can be added together thereafter or they can be mixed together without any predefined order considering that the sequence of order of the low and high driving modes does not have any effect on the moving average of the current measurement calculated by the division element <NUM>.

<FIG> show three different time diagrams for different operating conditions. In <FIG>, each oversampled measurement is made of <NUM> ADC samples S0, S1, S2 [. ] in the high gain driving mode, that is to say, in the particular case shown in <FIG>, when the current load measured by the current sensor is in the range between -<NUM>[A] and +<NUM>[A]. When working in this region, the amplifier noise is minimised, but only low currents can be measured (up to ±<NUM> [A] in the example above).

<FIG> shows the timing diagram for the low gain driving mode. The gain never changes and the control signal G is set to the "one" state. This is the gain driving mode used when the current is inferior to -<NUM>[A] or exceeds +<NUM>[A].

<FIG> shows the timing diagram during the first or second transition phases of <FIG> during which <NUM>% of the current measurement samples are performed in the low gain driving mode and <NUM>% of the current measurement samples are performed in the high gain driving mode for a current of -<NUM>[A] or +<NUM>[A]. In this particular example, the current measurement samples are performed in the low and high gain driving modes in an alternate fashion.

Various modifications and variations to the described embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. For example, the control circuit <NUM> may be adapted to control a single-phase or multiple phase AC motor, for instance a three-phase AC motor. In the latter case, the control circuit may comprise three feedback loops as shown in <FIG> and <FIG> for adjusting the current in each phase of the electric motor.

Claim 1:
A control circuit (<NUM>) for an electric motor comprising at least one phase, the control circuit (<NUM>) comprising a low voltage subcircuit (12a), a high voltage subcircuit (12b) and an isolation barrier (<NUM>) between the low voltage and high voltage subcircuits (12a, 12b), the low voltage subcircuit (12a) comprising a current controller (<NUM>) adapted to generate a driving signal, and a feedback loop (<NUM>) whose output is fed back to the input of the current controller (<NUM>), the high voltage subcircuit (12b) comprising a power bridge (<NUM>) to output a current for driving the at least one phase of the motor, a current sensor (<NUM>) for measuring the current in said at least one phase, an analog front-end (<NUM>) for signal conditioning of the current signal and an analog-to-digital converter-ADC (<NUM>) to digitally convert the conditioned signal, the isolation barrier (<NUM>) comprising an isolator (<NUM>) adapted to pass the output signal of the ADC (<NUM>) across the isolation barrier (<NUM>) from the high voltage subcircuit (12b) to the low voltage subcircuit (12c), wherein the isolator (<NUM>) comprises a first and a second channel to pass respectively a clock signal (CLK) and a control signal (ICS) from the low voltage subcircuit (12a) to the high voltage subcircuit (12b), the isolator (<NUM>) comprising a third channel to pass the output of the ADC (<NUM>) from the high voltage subcircuit (12a) to the low voltage subcircuit (12b)
characterized in that the analog front-end (<NUM>) is adapted to apply a first gain or a second gain that is higher than the first gain as a function of the current in said at least one phase measured by the current sensor (<NUM>),
wherein said clock signal (CLK) and control signal (ICS) are output to select said first or second gain (G) of the analog front-end (<NUM>), and wherein the isolator (<NUM>) comprises a fourth channel to pass a replica of the clock signal (CLK") from the high voltage subcircuit (12a) to the low voltage subcircuit (12b), the high voltage subcircuit (12a) further comprising a D flip-flop (<NUM>), and wherein the clock input of the D flip-flop (<NUM>) is arranged to receive the control signal (ICS) while the input of the D flip-flop (<NUM>) is arranged to receive said clock signal (CLK), the output of the D-flip-flop (<NUM>) being used for selection of said first or second gain of the analog front-end (<NUM>).