This application claims priority from United Kingdom Patent Application No. 0803737.6, filed Feb. 29, 2008, the contents of which is hereby incorporated by reference in its entirety.
The present invention relates to DC motors generally, and more specifically to driving of a brushed DC motor with a PWM input current.
In order to monitor the rotor position in a Brushed DC motor, one applicable method is pulse count. This involves counting the pulses generated in the coil current every half cycle by the power supply brushes short circuiting the power supply. This allows the speed and position of the motor to be reliably estimated.
Pulse counting is however only usually applicable to low current motors because these smaller motors are driven by a fixed voltage supply on which pulses can easily be detected. For higher current brushed DC motors an additional rotor sensor is required, which may typically be an optical sensor or a Hall sensor. Alternatively, one may choose to use a Brushless DC motor instead. All of these are however more expensive solutions and require greater complexity to put into practise.
One way to apply a higher current to a brushed motor more efficiently is by PWM driving which involves driving the motor with a series of pulses rather than a constant input. By varying the duty cycle of the PWM current, essentially the ratio of the time that the current flows to the times when it is switched off, the power supplied can be varied readily and accurately. Unfortunately, to date, pulse count systems have not been compatible with PWM driving because it is very difficult to directly detect both motor pulses in any one cycle, the motor pulses tending to be swamped by PWM pulses.
It is therefore desirable to provide a method of operating a pulse count rotor monitoring method in a brushed DC motor having a PWM supply.
In accordance with a first aspect, there is provided a method of determining the occurrences of pulses for implementing a pulse count monitoring method for a brushed DC motor driven by a pulse width modulated (PWM) driving current comprising the steps of: acquiring the driving current signal; performing peak detection on the acquired signal; identifying the largest peaks and thus determining the occurrence of pulses.
By performing peak detection (peak detection meaning detecting minima, maxima or both), the overall envelope of the current waveform can be regenerated and the highest peaks identified as motor pulses, since these being essentially short circuits will have greater magnitude than the PWM current pluses. This allows pulses to be identified in a PWM driving signal.
In accordance with a second aspect, there is provided a method of driving a brushed DC motor comprising the steps of applying a PWM driving current to the motor and monitoring the motor position by means of pulse counting wherein pulse occurrence is determined according to the method of the first aspect.
The method of the second aspect may incorporate any or all features of the method of the first aspect as are desired or as are appropriate.
In accordance with a third aspect there is provided a driver for a brushed DC motor, the driver operable to control the driving of a brushed DC motor in accordance with the methods of the first and/or second aspects.
The driver of the third aspect may incorporate any or all features of the methods of the first and/or second aspects as are desired or as are appropriate.
In accordance with a fourth aspect there is provided a brushed DC motor incorporating a driver in accordance with the third aspect and/or operated in accordance with the methods of the first and/or second aspects.
The motor of the fourth aspect may incorporate any or all features of the driver of the third aspect or the methods of the first and/or second aspects as are desired or as are appropriate.
The largest peaks may be identified by tracking the detected peaks against time. This provides a good recreation of the overall envelope of the driving current waveform.
The driving current can be provided at any suitable PWM duty cycle and/or at any suitable PWM frequency. The on current PWM pulses may be of any suitable magnitude. The peak detect is operable to detect appropriate peak widths as determined by the chosen PWM frequency, duty cycle and PWM pulse height.
The peaks may be detected by any suitable peak detection method such as fast peak detection. Alternatively other methods of peak detection may be used including and not limited to diode peak detection. Diode peak detection in comparison with fast peak detection however has the disadvantages of slower detection and inaccuracy caused by the diode voltage drop. The fast peak detection may be carried out be appropriate circuitry or by a suitably programmed microcontroller. If the fast peak detection is carried out by dedicated circuitry, the dedicated circuitry may comprise a comparator and one or more amplifiers. The comparator may be operable to compare the present current signal with the output of an operational amplifier in series with the output of the comparator. This allows the output of the operational amplifier to adjust the voltage input to the comparator to therefore act as a suitable reference for the comparator. There may be a suitable combination of resistance and capacitance provided between the comparator and the operational amplifier.
The pulses detected by the above method may be processed in a conventional manner to determine motor speed and/or position. Such processing may include filtering the pulses and/or incrementing one or more counters.
The motor may be driven through a H-bridge arrangement. The current signal for peak detection may be monitored across a shunt resistor connected in series with the H-bridge. The shunt resistor may have any suitable resistance value. In one example, it may have a resistance of the order of 10 mΩ.
In order that the present invention is more clearly understood, one embodiment will now be described further herein, by way of example only, and with reference to the accompanying drawings in which:
Various embodiments relate to the use of pulse detection for monitoring the speed and/or position of a brushed DC motor driven by a PWM driving current. Turning first to
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Performing an oscilloscope peak detect (peak detection meaning detecting minima, maxima or both) on the driving current signal of a motor with a PWM input as shown in
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In
When a positive voltage peak occurs on the input then the Ecl comparator COMP charges the capacitor C1. The resistor R1 discharges capacitor C1 so the next lower or higher peak level can be detected. At the comparator output Ecl, the voltage typically varies between −0.8V and 1.8V. The first operational amplifier is provided to adjust the voltage to match the input signal. This means that the adjustable voltage will play the role of a reference for the input comparator. In order to operate effectively in the present example, the first operational amplifier must have a bandwidth of the order of 50 MHz to follow the input signal.
In the example shown, a second operational amplifier is provided to act as an output buffer.
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While the invention has been described by way of example and in terms of the specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Number | Date | Country | Kind |
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0803737.6 | Feb 2008 | GB | national |