Circuit for controlling a free-piston engine, in particular of a refrigerator compressor

Abstract: A circuit for controlling a free-piston engine, particularly of a refrigerator compressor, comprises a part (1) of H configuration, in the sides (4, 5, 6, 7) of which there are positioned electronically controlled switching means (8), said part of H (17) being provided to control the opening and closure of said electronic switching means (8) in accordance with the measurement of the mains power absorbed by the engine (10). The circuit comprises means (23, 25) for measuring the absorbed mains current traversing the diagonals (4, 7; 5, 6) of the H part (1), said current being proportional to the mains power absorbed by the engine (10), said means (25) feeding a signal to the microprocessor (17) which, on the basis of this signal, generates a further signal which by comparison with a value equal to the current actually absorbed by the engine (10) enables the electronic switching means (8) to be contolled.

Description: This invention relates to a circuit for controlling a free-piston engine, in particular of a refrigerator compressor. As is well known, a free-piston engine of a compressor has to be fed with alternating (sinusoidal) current of variable amplitude and frequency. It is also known that the power generated by the engine as a function of parameters such as the temperature and humidity of the environment in which the refrigerator is located, has to be varied as said parameters vary. A control system comprising an H-bridge type configuration is described in EP-A-196543. Such system is used to control an inductive lead such as an electric motor. Generally, the amplitude of the feed current is fixed for certain particular values of said parameters, whereas the frequency of this current is varied, by discrete values, until the frequency at which the mains power absorbed by the engine and therefore the power generated by the engine are a maximum. Various circuits have already been proposed for monitoring and controlling the feed to an engine of the said type, which in particular enable the frequency of said feed to be varied in accordance with the variation in the environmental parameters of the place in which the refrigerator is located. A known control circuit of the aforesaid type is described in EP 0 266 835 and comprises a circuit part of H configuration (or H bridge) to which the free-piston engine is connected and from which a current having a value proportional to the current absorbed by the engine is led off. At the same time, a voltage signal taken from a parallel-connected transformer is fed to a multiplier, for example of four-quadrant type, to which the said current signal is also fed. An analogous current signal is fed to a differential amplifier. The multiplier provides as output a signal representing the product of the current and voltage signals, this signal therefore representing the power generated by the engine. This power signal, after suitable filtration, is fed to a microprocessor which, using a preset program, evaluates whether the determined power is the optimum or not. Based on this determination, the microprocessor generates a current signal which is used as reference in the differential amplifier. This latter provides as output a signal which is used to control the opening and closure of electronic switches connected into the sides of the H bridge, to allow passage of the engine feed current across the diagonals of these sides. The know circuits have various drawbacks. In particular, circuits using multipliers do not have high operational reliability and are often subject to faults, the repair of which involves high cost. In addition, such circuits are costly to construct, reflecting the cost of the components used. An object of the present invention is therefore to provide a circuit for controlling a free-piston engine which offers high operational reliability, is of low constructional cost and uses components which are simpler than those used in analogous circuits of the state of the art. This and further objects which will be apparent to the expert of the art are attained by a circuit for controlling a free-piston engine, particularly of a refrigerator compressor, having the features reported in the attached claim 1. The present invention will be more apparent from the accompanying drawing, which is provided by way of non-limiting example, and in which: Figure 1 is a block diagram of a circuit formed in accordance with the invention; Figure 2 is an electrical schematic of part of the circuit of Figure 1. With reference to Figure 1, the control circuit of the invention comprises a circuit part 1 of H configuration connected to the mains via feed and return lines 2 and 3. In the sides of the circuit part 1 there are positioned power MOSs 8 operated by usual control circuits 9. The circuits 9 represent static switches working in opposition. Specifically, the MOSs 8 on the sides 5 and 6 are connected to inverting buffer circuits 9, whereas the MOSs on the sides 4 and 7 are connected to non-inverting buffer circuits 9. A free-piston engine 10 is connected to the circuit part 1. This engine, of known type and therefore shown only schematically, is connected to the horizontal side 11 of the circuit part 1 and to the side 5 of this latter. The circuits 9 are connected to a pulse-width modulator 12, which is of the known type generating a pulse-width modulated output signal, the frequency of which is considerably greater than the desired engine drive current frequency. The modulator 12 is connected to a control circuit 13 of known type, such as a usual error amplifier circuit. This latter is connected to a differential amplifier 14 which receives signals from a microprocessor 17 and from a further differential amplifier 18 via respective lines 15 and 16. The microprocessor 17, the operation of which is of known type, is also connected to a usual low pass filter 19. Finally, the amplifier 18 is connected via the lines 20 and 21 to two sensors for determining the current flowing alternately through the two diagonals of the circuit part 1, these diagonals being composed of the sides 4, 11 and 7, and the sides 5, 11 and 6. According to the invention, adding means 25 is connected to said lines 20 and 21. This adding means is shown in greater detail in Figure 2. In said Figure 2, parts corresponding to those of Figure 1 are indicated by the same reference numerals. In the Figures under examination the current sensors 23 are shunt resistors 26 and 27, the adder function of the adding means 25 and the filter function of the filter 19 of Figure 1 being permformed by resistors 28 and 29 connected to the sides 6 and 7 of the circuit part 1 respectively. These resistors are also connected to a line 30 terminating in a plate of a capacitor 31 also connected to the return line 3. The non-inverting input of an amplifier 32 is connected to the line 30, and its inverting input is connected to a trimmer 33 connected to the output 34 of said amplifier and to the return line 3. It will now be assumed that the engine 10 is to be operated. For this purpose the circuit is fed (in known manner) with a constant line voltage. By virtue of the control effected in known manner on the power MOSs 8 by the modulator 12 via the circuits 9, these MOSs are alternately closed in accordance with a known sequence, such as to cause the line current from the line 12 to circulate through the diagonals of said part 1 alternately. The sensors 23 determine the current present in the sides 6 and 7 of the part 1. Signals therefore reach the adding means 25 and the amplifier 18 via the lines 20 and 21 which extend from said sensors. Specifically, the signals which reach said differential amplifier 18 cause it to generate a sinusoidal signal which is proportional to the current absorbed by the engine 10. The sinusoidal signal is generated by the (algebraic) difference between the current signal circulating in one diagonal of the circuit part 1 and the signal (of opposite sign to the first) generated, during the opening of the MOSs positioned in said diagonal, by usual recirculation diodes (not shown) connected in parallel with said MOSs. With regard to the adding means 25, the signals taken from the shunt resistors are fed to the resistors 28 and 29 and to the capacitor 31, to filter said signals before they are fed to the noninverting input of the amplifier 32. The amplifier 32 generates an output signal which is proportional to the mains current absorbed by the circuit part 1, its gain over the input being fixed by the trimmer 33. This current is proportional to the power made available to the engine by the mains. The output signal from the amplifier 32 is then fed to the microprocessor which in known manner, in accordance with a set program, calculates on the basis of the absorbed current the power made available to the engine by the mains. The microprocessor then in known manner compares the obtained signal with that stored in a preceding operating sequence. From this comparison the microprocessor determines whether the engine power is optimum, i.e. whether the resonant circuit associated with it operates at resonance frequency, in which case it generates an analog frequency signal which is fed to the differential amplifier 14. This signal acts as the input reference signal for the differential amplifier 14. This latter compares the signal from the amplifier 18 with said reference signal. This comparison, i.e. the double check loop (i.e. the check loop for the power absorbed by the engine and that for the current passing through this latter) is effected, as is well known, to prevent the engine operating under resonant conditions different from those calculated by the microprocessor on the basis of the signal which it receives from the circuit part 25. Finally, said differential amplifier 14 generates a signal which in known manner is used to control the MOSs 8 to enable the engine 8 to absorb from the mains a power equal to the optimum power on the basis of the values of the parameters of the environment external to the refrigerator.

Claims: A circuit for controlling a free-piston engine (10), particularly of a refrigerator compressor, comprising a part (1) of H configuration, in the sides of which there are positioned electronically controlled switching means (8), said part (1) of H configuration being connected to said engine (10), a microprocessor (17) being provided to control the opening and closure of said electronic switching means (8) in accordance with the measurement of the mains power absorbed by the engine (10), the circuit being provided with means (23, 18) for measuring an energising current and for generating a measurement signal which is proportional to the measured energising current, characterised by comprising adding means (25) for feeding a signal proportional to the mains current absorbed by the circuit part (1) to the microprocessor (17) which, on the basis of this signal, calculates the power made available to the engine (10) by the mains and which by comparison with a value equal to the current actually absorbed by the engine (10) enables the electronic switching means (8) to be controlled. A circuit as claimed in Claim 1, characterised in that the means for measuring the absorbed mains current are current sensors (23) connected to the adding means (25). A circuit as claimed in Claim 2, characterised in that the current sensors (23) are shunt resistors (26, 27). A circuit as claimed in Claim 2, characterised in that the adding means (25) comprises resistors (28, 29) and a capacitor (31) which act as a low pass filter. A circuit as claimed in Claim 4, characterised in that the adding means (25) comprises an operational amplifier (32) with feedback to its inverting input via a trimmer (33).

Inventors: VAN DER BROECK HEINZ, VASCONI ENRICO, VAN DER BROECK, HEINZ, VASCONI, ENRICO
Applicants: WHIRLPOOL EUROP, WHIRLPOOL EUROPE B.V.
Application year: 1990
Main classification: G05D19
IPC codes: G05D19, F04B49, F02B71, H02P25, F25B9, F01B11, H02P27