Field of Invention

The present invention relates to a drive circuit for a resonant reciprocating motor. Background Art

A resonant reciprocating motor is an electric motor wherein the moving part reciprocates either in a straight line or in an arc, or some other path, reversing the direction of its motion at each stroke. The energy in the moving part changes back and forth from kinetic energy to potential energy. The motion is encouraged to reciprocate by means of spring suspension or some method such as compressed gas having the same effect as a spring. A loudspeaker (electromagnetic type such as cone speakers) is a well known example of such a motor.

Some applications require the maximum amount of mechanical effort to be done, while the frequency of operation is less critical, such as in a reciprocating compressor.

It is desirable to apply an optimum electrical waveform and amplitude to such a motor so as to :

♦ Maximise the electromechanical efficiency of the motor; ♦ Make the efficiency substantially independent of the input power source voltage and frequency;

♦ Make the system insensitive to mechanical variations in the motor whether production or load induced; and

♦ Maximise reliability of the system.

The prior art reveals many examples of drive arrangements for generally similar applications to the present invention. Some, for example US patent No 5032772, disclose circuits allowing for control of the frequency of the drive frequency and of the amplitude. There is no disclosure of a relatively simple arrangement concerned principally with optimising electromechanical efficiency of the system, rather than with varying output to control temperature.

Other known means of driving such motors are:

(i) With a line frequency transformer directly from the AC utility line - that is, at the line frequency, or direct from the AC line without a transformer. In this case, the motor must be manufactured to resonate at the line frequency of the country of ultimate sale and use. (ii) From a DC source with a square wave excitation preset at the average motor resonant frequency.

(iii) as above, but with control of amplitude or frequency or both with reference to output pressure of the pump, motor power, or temperature difference signals. None of these approaches are specifically concerned with maximising the electromechanical efficiency of the motor. Summary of Invention

According to one aspect the present invention comprises a drive circuit for a reciprocating resonant electric motor, said motor operatively driving a compressor for a refrigeration system, said circuit comprising means for providing a drive signal having a substantially constant amplitude, said amplitude being selected so as to provide optimum drive efficiency for the motor.

The drive circuit may also provide means for selecting the drive frequency so as to substantially match the resonant frequency of the system.

Preferably the drive circuit further provides means for determining whether the motor is being driven at its resonant frequency, and means for dynamically varying the frequency of the drive signal so as to match the resonant frequency of the motor.

Preferably the amplitude is controlled by pulse width modulation techniques. Preferably the drive circuit adjusts the output frequency to the resonant frequency of the motor.

Preferably the drive signal is phase locked to the resonant frequency of the motor.

Motors of this type are, by their design, resonant. Therefore, to minimise the amount of wasted energy, it is desirable to drive the system at its resonant frequency. The most electrically efficient manner in which to achieve this is with a square wave, or switched, excitation.

The mechanically resilient component is non-linear in its response.and so at some maximum amplitude, energy is wasted. Therefore the amplitude of the excitation driver must be controlled, preferably such that it is independent of the input voltage to the system from the power source. If the motor is to be driven from an alternating current power source, such as from the local power supply, then a different motor would have to be made for different world markets, according to the power line frequency of the country. Certain applications , for instance portable coolers, require portable operation, or operation from battery power. Therefore, it is preferable that an electronic circuit comprising an oscillator and drive circuit is provided.

In the production of the motor, or in the nature of its load, some variation in the self resonant frequency is to be expected. Observations show that the efficiency of the motor is very sensitive to the drive circuit being tuned precisely to the self resonance. Accordingly, a means to adjust the frequency to the resonance on a continual dynamic basis is desirable, but not essential according to the present invention. As a compromise, the frequency may be set at the resonant frequency of the system. While this will vary over time and with load variation, an compromise value may be preset- Two factors relating to power conversion can be used to enhance the motor and driver system reliability. Driving the motor ( or the motor / compressor mechanical system) at its self resonance reduces mechanical stresses on the mechanical elements of the moving part of the motor and its load. Controlling the amplitude of the motor drive, irrespective of input voltage, protects the motor from overdrive and the mechanical consequences thereof, and moreover avoids inserting mechanical energy into the system which cannot be transformed into useful work. Brief Description of Drawings

One embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram showing a first embodiment of the present invention;

Figure 2 is a block diagram showing a second embodiment of the

present invention;

Figure 3 is a graphical illustration showing the voltages and waveforms associated with the first embodiment;

Figure 4 is a graphical illustration showing the voltages and waveforms associated with the second embodiment;

Figure 5 is a more detailed schematic diagram of one implementation of the first embodiment;

Figure 6 is a more detailed schematic diagram of one implementation of the second embodiment; and Figures 7A to 7D show a schematic diagram of a practical circuit implementing the first embodiment. Detailed Description First embodiment

Referring to Figure 1 , a Direct Current power source is connected to the input terminals of the DC-DC CONVERTER, which delivers a regulated DC voltage to the DRIVER circuit, such DC voltage being chosen so as to provide the optimum drive amplitude when the voltage is switched by the DRIVER circuit. Any suitable conventional converter may be used as will be apparent to the skilled reader. The switching of the DRIVE circuit is controlled by the DRIVER CONTROL so that a preset frequency is adjusted so as to continuously be synchronous with the resonant frequency of the motor.

Figure 3 shows the voltages, currents and waveforms associated with Figure 1. Note that V _m is the potential difference across the motor, and l|_ is the input current to the motor.

Figure 5 shows in more detail a typical implementation of the principles shown by figure 1. The DC-DC converter U1 is a conventional non-isolated boost converter, the output of which is set to the predetermined optimum for the motor. The frequency of the drive controller U2 is adjusted automatically so as to seek a frequency at which the average current reaches a minimum, preset to be a typical current for an optimally tuned circuit. While this predetermined current may be slightly higher than what could be achieved by a more complex automatic

tuning circuit, the circuit simplicity provides a cost effective automatic tuning facility. The high side driver circuit is preferably of the type described in the applicant\'s co-pending provisional application No. PL 3640, and the embodiment shown in figure 7B incorporates such a circuit. Figures 7A to 7D show in detail a practical embodiment of the present invention, using PWM techniques.

Referring to figure 7A, this illustrates a conventional current mode controlled boost converter providing a preset optimum DC drive voltage to Drive power, shown in Figure 7B. This voltage is selected so as to provide optimum drive to the motor after allowing for small losses in the four field effect transistors forming a conventional H-bridge drive circuit, shown in figure 7C. The H-bridge is driven by the 3525 IC shown in figure 7B.

The current waveform to the motor is sensed by the 0.2 ohm resistor and fed to the comparator LM 324 ( figure 7A). This signal is then processed by the circuit shown in figure 7C to adjust the frequency of the 3525 IC so as to be phase locked to the current waveform caused by the resonance of the motor.

Figure 7D shows three circuit elements which provide ancillary functions enabling control of the driver from an external temperature control circuit, logic elements to control a fan, and an under-voltage lockout circuit to prevent excessive discharge of an external battery powering the unit.

Second Embodiment

In the block diagram of Figure 2, compared to that of Figure 1 , the DC-DC CONVERTER stage is eliminated. The amplitude of the voltage delivered to the motor is controlled by means of pulse width modulating techniques.

The driver circuit consists of the basic square wave drive at the motor resonant frequency, but this low frequency square wave is comprised of a train of high frequency pulses. The duty cycle of these pulses is controlled so that the average during the period of the low frequency on time is the desired voltage. Figure 4 shows the voltages, currents and waveforms associated with

Figure 2.

Figure 6 shows details of a means to implement the principles shown in

Figure 2. The industry standard controller U2 is modulated by the PWM controller U1 at a frequency about 100 times higher than the motor resonance. The motor acts as an integrator for the higher frequency. The pulse width of controller U1 is controlled by detecting the amplitude of the waveform delivered to the motor. It will be appreciated that variations and additions are possible within the spirit and scope of the invention.