TORQUE CONTROL APPARATUS FOR ROTARY MACHINE

Abstract: A torque control apparatus for a compressor (21) driven by a motor (20) through a main shaft (4), in which variations of the rotation speed of the main shaft due to torque variations are detected at selected angular positions of rotation of the main shaft, current values to be supplied to the motor for eliminating the rotation speed variations depending on the angular positions of rotation of the main shaft are computed and stored during at least one revolution of the main shaft, and, in the next revolution of the main shaft, that is, when the main shaft rotates then to a 360° advanced angular position, the motor current is control­ led on the basis of the current values supplied at least one revolution of the main shaft, thereby eliminating variations of the rotation speed of the main shaft.

Description: This invention relates to a torque control apparatus which is generally applicable to any one of various rotary machines, for example, a compressor in which a load element is driven by a motor, and more particularly to a torque control apparatus suitable for application to a rotary machine in which load torque varies periodically. EP-A-0145841 (corresponding to JP-A-60-60286 published on April 6, 1985) and US-A-4,604,036 (issued on August 5, 1986) disclose a torque control apparatus in which, in order to minimize the difference (a difference torque) between a load torque of a compressor of a refrigerator and an output torque of a motor driving the compressor through a main shaft, an attempt is made so as to cause the motor to produce an output torque having a pattern which varies in synchronism with variation of the load torque depending on the angular position in one revolution. In the disclosed apparatus, however, the waveform representing variation of the load torque is obtained on the basis of a signal, such as pressure variation in a compressor, derived from a machine effecting the refrigeration cycle. Accordingly, the waveform change thus obtained is delayed relative to the actually occurring change in the load torque waveform, and the resultant control of the output torque of the motor cannot completely eliminate vibration of the compressor housing and its supporting system in the direction of rotation of the main shaft. EP-A-0145841 discloses a torque-controlled rotary apparatus on which a motor element and a compression element are connected together by a shaft. The load torque applied to the compression element is detected and the output torque of the motor element is controlled so that it is equal to the load torque. The load torque is detected at one point in the rotation of the compression element and the load torque immediately calculated. This document corresponds to the pre-characterising part of claim 1. In a rotary machine such as a closed-type compressor, variation of rotating speed of the main shaft in one revolution in the rotary side, and the vibration of compressor housing in the rotating direction of the main shaft in the stationary side are derived from the difference between a load torque of a compression element and an electro-magnetic torque of a motor element during one revolution of the main shaft. Fig. 12 illustrates schematically the reason why vibration occurs in a rotary compressor, by way of example. A compression element and a motor element are each composed of a rotating side such as rotors 14, 15 and a main shaft 16 and a stationary side such as stators 17, 18 and a housing 19. In the compression and motor elements, a gas compression torque T G and an electromagnetic torque T M act in directions as shown in Fig. 12 respectively. In Fig. 12, the torques T G and T M are positive when the rotating direction is clockwise and negative when the rotating direction is counter-clockwise. The equations of motion of the rotating system and stationary system shown in Fig. 12 are given by the following equations respectively: In the equations (1) and (2), J R and J S are the moments of inertia and are the inertia torques of the rotating side and stationary side, respectively, T G - T M corresponds to the difference torque, K is the spring constant of springs supporting the compressor,  R and  S are the respective rotation angles of the rotating and stationary sides and the angular velocity ω of rotation of the rotating side is expressed as Fig. 13 shows torque variations occurring during rotation of the main shaft 16 in the rotary compressor. It will be seen in Fig. 13 that the load torque varies greatly depending on the angular position in rotation of the main shaft 16. Whereas the electromagnetic torque produced by the conventional motor element does not relatively vary. It will be apparent from the equation (1) that, in the range where T G >T M in Fig. 13, dω / dt > 0, and the rotation speed tends to increase. On the other hand, in the range where T G < T M , dω / dt < 0, and the rotation speed tends to decrease. Thus, a variation occurs in the rotation speed of the main shaft 16 in the rotating system because the difference torque T G - T M varies with time. As will be apparent from the equation (2), this rotation speed variation acts as an exciting force in the stationary side, resulting in vibration of the housing 19 in the direction of rotation of the main shaft 16. Therefore, when the electromagnetic torque T M produced by the motor element is so controlled as to maintain the relation T M = T G at all the angular positions of rotation of the main shaft 16, the value of dω / dt is dω / dt = 0, and no rotation speed variation occurs. In the stationary system too, the exciting force appearing as a result of the torque force appearing as a result of the torque variation is cancelled to eliminate the vibration of the housing 19 in the direction of rotation of the main shaft 16. The present invention seeks to improve on the apparatus of EP-A-0145841. The present invention proposes torque-controlled rotary apparatus comprising a motor element, a load element connected to said motor element through a rotating shaft to be driven by said motor element and a torque control apparatus for controlling an electromagnetic torque produced by said motor element so as to eliminate the difference between the electromagnetic torque of said motor element and a load torque of said load element wherein each revolution of said rotating shaft is divided into a plurality of angular sections; and the apparatus further includes sensor means for obtaining a plurality of variation values of the rotating speed of said rotating shaft, corresponding to said plurality of angular sections during at least one previous revolutions of said rotating shaft; calculating means for calculating a plurality of control values based on the plurality of variation values of the rotating speed of said rotary shaft obtained at said plurality of angular sections so that each control value corresponds to one angular section; and control means for applying each of said plurality of control values to said motor element at the time when said rotating shaft is rotated to the corresponding angular section during the succeeding revolution of said rotating shaft; thereby to cancel the difference between the electromagnetic torque and the load torque at each of said angular sections. Preferably the variation of the rotating speed of said rotating shaft is obtained during a plurality of revolutions of said rotating shaft. Embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal sectional view of a rotary compressor controlled by an embodiment of the torque control apparatus of the present invention. Fig. 2 shows the waveform of the output signal of the gap sensor detecting the rotation speed of the rotary main shaft shown in Fig. 1. Fig. 3 corresponds to the graph of the rotation speed signal shown in Fig. 2 to illustrate variations of the rotation speed of the main shaft. Fig. 4 is a block diagram showing schematically the structure of the torque control apparatus embodying the present invention. Fig. 5 is a flow chart of torque control by the torque control apparatus shown in Fig. 4. Fig. 6 is a partial modification of the flow chart shown in Fig. 5. Figs. 7 and 8 are partial modifications of the flow chart shown in Fig. 6. Fig. 9 is a circuit diagram of a digital filter preferably used for the statistical processing shown in Fig. 8. Fig. 10 is a block diagram showing the general structure of another embodiment of the torque control apparatus when the present invention is applied to a compressor driven by a brushless DC motor. Fig. 11 is a circuit diagram of the torque controller shown in Fig. 10. Fig. 12 shows torques produced in a rotary machine. Fig. 13 shows torque variations occurring during one revolution of the main shaft shown in Fig. 12. Fig. 1 shows schematically the structure of an inverter-driven type rotary compressor which is one form of a rotary machine driven by a motor element (a DC motor) and to which the present invention is applied. The structure of the compressor shown in Fig. 1 is generally similar to that of the prior art compressor disclosed in U.S. Patent No. 4,604,036 cited hereinbefore. Therefore, parts of the compressor different from those of the prior art compressor will only be described, and description of the same or equivalent parts will be dispensed with. Referring to Fig. 1, a rotor 3 of a motor element 20 is directly coupled by a rotary main shaft 4 to a rotor 8 of a compression element 21, and a gear 22, whose rotation is to be detected, is fixedly mounted on an extention of one end of the main shaft 4, so that the gear 22 rotates together with the main shaft 4. A gap sensor 23 fixed to a compressor housing 1 senses the teeth of the gear 22 and generates a pulse signal indicative of the rotation speed of the main shaft 4. Fig. 2 shows the waveform of the rotation speed detection signal 24 generated from the gap sensor 23. A base line 25 shown in Fig. 2 represents the central output level of the detection signal 24. For example, the signal waveform crosses the base line 25 at time t₀ and then crosses the base line 25 again at time t₁. On the basis of the period of time t between the time t₀ and the time t₁ and the number of teeth m of the gear 22 sensed in this period t, the rotation speed of the main shaft 4 at the time t₁ is determined. Fig. 3 corresponds to the graph shown in Fig. 2 and illustrates variations of the rotation speed 26 of the main shaft 4 with time. In ranges A and B, the rotation speed 26 of the main shaft 4 is higher and lower than the average rotation speed (a speed setting) 27 of the main shaft 4 respectively. The output signal 24 of the gap sensor 23 is applied to a control circuit 28 shown in Fig. 4, and this control circuit 28 which may be a microcomputer controls the electromagnetic torque produced by the motor element 20. Referring to Fig. 4, the speed detection signal 24 from the gap sensor 23 is converted by a wave-shaping preamplifier 29 into a rectangular-wave signal indicative of momentary variations of the rotation speed of the main shaft 4 in each revolution, and this signal is applied through an interface 30 to a CPU 31. On the basis of the momentary variations of the rotation speed of the main shaft 4, the CPU 31 computes current values to be supplied to the motor element 20 of the compressor so that no rotation speed variations occur in the next revolution of the main shaft 4. The data thus obtained are stored in a RAM 32 until they are required. The data written in the RAM 32 are then read out and supplied to the CPU 31, and, from the CPU 31, the data are supplied to a controller 34 which generates a chopper signal to be applied to a base driver 35, which in turn controls an inverter 36 so that a current which varies according to a pattern corresponding to variation of desired output torque of the motor is supplied to the motor from the inverter. At the same time, the CPU 31 applies a timing signal to the base driver 35 through the interface 30, so that the inverter 36 produces the current having the pattern determined by the controller 34 at a timing determined according to the angular position of the motor shaft. The inverter 36 driven by the base driver 35 acts to increase and decrease the amount of current supplied to the motor element 20 so that variations of the rotation speed of the main shaft 4 do not occur at all the angular positions of rotation of the main shaft 4. The rotation speed of the main shaft 4 is continuously controlled so as to limit its variations to less than a predetermined allowable value at all times. The series of such control loops are written in a ROM 33. A flow chart of torque control by the control circuit 28 will be described in detail with reference to Fig. 5. In a step 110 in Fig. 5, one revolution of the main shaft 4 at time k 1 is divided into N sections, and the rotation speed v x of the main shaft 4 in each of the divided sections (1, ---, N) is detected. That is, rotation speed pulses detected by the gap sensor 23 are shaped by the wave-shaping preamplifier 29, and the widths t x of the pulses are computed by a timer T x built in the microcomputer, thereby computing the rotation speed v x of the main shaft 4 in each of the sections (x = 1, ---, N). Then, in a step 120, the rotation speed v x detected in the step 110 for each of the individual sections is compared with a rotation speed command V x applied from the CPU 31 to compute the speed error Δv x = v x - V x , and the step 120 is followed by a step 130. In the step 130, the relative magnitudes of the detected speed v x and the commanded speed V x are judged. When the result of judgement in the step 130 proves that the detected speed v x is lower than the commanded speed V x , that is, when the speed error Δv x (= v x - V x ) < 0, the step 130 is followed by a step 140. In the step 140, a required current increment ΔI x is computed which is to be added to the current value being supplied to the motor element 20 so that the speed error Δv x can be reduced to zero, and the step 140 is followed by a step 150. On the other hand, when the result of judgment in the step 130 proves that the detected speed v x is higher than the commanded speed V x , that is, when the speed error Δv x > 0, the step 130 is followed by a step 142. In the step 142, a required current decrement ΔI x is computed which is to be subtracted from the current value being supplied to the motor element 20 so as to reduce the speed error Δv x to zero, and the step 142 is followed by the step 150. On the other hand, when the result of judgment in the step 130 proves that v x = V x , that is, When the speed error Δv x = 0, the step 130 is directly followed by the step 150. In this step 150, judgment is made as to whether or not x ≧ N, that is, whether or not the speed errors Δv x have been computed for all of the N sections (x = 1 to N). When the result of judgment in the step 150 proves that x < N, the step 150 is followed by a step 160 in which the required current values computed on the basis of the speed errors Δv x for all of the sections x (< N) judged in the step 150 are stored in the RAM 32. Then, x is incremented to x → x + 1, and the program returns to the rotation-speed pulse detection step 110 to repeat the same flow. On the other hand, when the result of judgment in the step 150 proves that x ≧ N, the step 150 is followed by a step 170. In this step 170, the current value information for all the sections x (= 1 to N) stored in the RAM 32 are read out to produce a current pattern corresponding to one revolution (divided into the N sections) of the main shaft 4, thereby controlling the rotation speed of the main shaft 4. In a step 180 following the step 170, the base driver 35 is driven under control of the controller 34 on the basis of the current pattern thus produced, thereby controlling the electromagnetic torque of the motor element 20. Thus, in this step 180, the drive signal driving the base driver 35 is generated. Then, k is incremented to k → k + 1, and the program returns to the step 110 to repeat the same flow. In the flow chart of Fig. 5, the information of rotation speed variations detected in preceding one revolution of the main shaft 4 before the present time are used to compute the amounts of current to be supplied to the motor element 20, thereby controlling the rotation speed of the main shaft 4. In a modification as shown in Fig. 6, information of rotation speed variatons detected in past plural revolutions of the main shaft 4 before the present time are stored and statistically processed so as to determine the amounts of motor current on the basis of the statistical data, thereby controlling the rotation speed of the main shaft 4. In the flow chart of Fig. 6, the control flow from the step 170 to the step 180 differs from that of Fig. 5, and the remaining steps are the same as those of the control flow shown in Fig. 5. Therefore, the difference will only be described with reference to Fig. 6. In the step 170 in Fig. 6, a current pattern corresponding to one revolution of the main shaft 4 at time k y is produced and stored in the RAM 32. Then, in a step 171, whether y is larger or smaller than m is judged. When the result of judgment proves that y < m, k is incremented to k → k + 1, and the program returns to the step 110 so as to produce a current pattern corresponding to one revolution of the main shaft 4 at time k + 1. On the other hand, when the result of judgment in the step 171 proves that y ≧ m, the controller 34 generates a drive signal driving the base driver 35. Successive current patterns produced in the step 170 are synthesized in the CPU 31, and this synthesized current pattern is stored in the RAM 32. The synthesized current pattern stored in the RAM 32, is continually applied to the controller 34, so that, when the controller 34 is actuated by an actuating signal applied thereto as a result of judgment made in the step 171, the controller 34 drives the base driver 35 on the basis of the synthesized current control pattern. According to the above manner of pattern averaging, noise that may appear due to an accidental variation of the rotation speed of the main shaft 4 can be removed because rotation speed variations of the main shaft 4 occur periodically, Therefore, the rotation speed of the main shaft 4 can be controlled with high reliability. As the means of statistical processing for producing the synthesized current pattern shown in the step 172, a method may be employed in which rotation speed variations detected in past plural revolutions (m revolutions) for each of the N-divided angular positions of rotation of the main shaft 4 are simply averaged, and such averaged values are used to form the current pattern. In such a case, smoothing of the current pattern signal by a low-pass filter is effective for removing high-frequency noise components. In the step 170 shown in Fig. 5, the current pattern corresponding to one revolution of the main shaft 4 is formed by detecting the speed error Δv x in each of the N sections. However, such a manner of current pattern formation may pick up excessively large, temporary variations attributable to external factors. Effective means for preventing pickup of such variations is shown in Fig. 7. In Fig. 7, a threshold I x * is provided, and variations exceeding this threshold I x * are ignored in the step of averaging so as to minimize an adverse effect due to disturbance. Referring to Fig. 7, the step 170 of forming the current pattern at time k is followed by a step 175 in which judgment is made as to whether I x ≧ I x * or not. When the result of judgment in the step 170 is YES, I x is regarded to be I x = I x *, and the current pattern is corrected by ignoring variations exceeding I x + I x *. Then, the controller 34 is actuated to drive the base driver 35 on the basis of the corrected current pattern. When, on the other hand, the result of the judgment proves that I x ≦ I x *, the current pattern produced in the step 170 is not corrected, and the controller 34 is actuated to drive the base driver 35 on the basis of this noncorrected current pattern. The threshold I x * is determined in two ways. In one way, the threshold I x * is previously experimentally determined. In the other way, the threshold I x * is computed on the basis of I x-1 and I x+1 each time these values are detected. As another method of statistical processing made in the step 172 shown in Fig. 6, a running average model method as shown in Fig. 8 is also effectively used. According to this method, weighted running averages of rotation speed variation data (input data) applied in a time serial fashion are computed as shown by the following expression (3): where x k is a k-th time-serial input data (applied at a time interval T), y k is an output data (a statistic), and a ℓ is a coefficient parameter. Such a method of statistical processing is carried out by the use of a linear, nonrecursive type digital filter as shown in Fig. 9. The digital filter shown in Fig. 9 is composed of multipliers 47, unit-time delay elements 48 and an adder 49 to generate the output y k in response to the application of the input x k . In the modification shown in Fig. 8, the step 170 of forming the current pattern is followed by a step 175 in which current information i i acquired immediately before the present stage among those acquired in the past is weighted by a weight a i to correct the current pattern. Then, such corrected current patterns are synthesized in a step 176, and the controller 34 is actuated to drive the base driver 35 on the basis of the synthesized current pattern. It is a notable fact that there are no effective methods of detecting torque variations in the rotating side, except the method of acquiring the information of rotation speed variations of the main shaft 4. In the aforementioned embodiment, the gap sensor 23 is used as means for detecting rotation speed variations of the main shaft 4 and generates rotation speed pulses to be referenced. However, a sensor-less arrangement which does not require provision of such a sensor in the compressor may be used for acquiring the information of rotation speed variations. That is, it is known from the Fleming's rule that, when current flows through each of conductors of a DC motor, a counter-electromotive force is induced in a direction opposite to the flowing direction of the current , and its magnitude is proportional to the rotation speed of the motor. Thus, the information of rotation speed variations of the rotating system can be obtained by detecting changes in the current due to changes in the counter-electromotive force caused by the rotation speed variations. Fig. 10 shows the general structure of another embodiment of the torque control apparatus in which a brushless DC motor is used as the motor element. Referring to Fig. 10, an AC power source 50 is connected through a rectifier circuit 51 and a smoothing capacitor 52 to an inverter 62 to supply a DC voltage E d to the inverter 62. This inverter 62 is of a 120° conduction type composed of transistors TR₁ to TR₆ and feedback diodes D₁ to D₆. In the inverter 62, the 120° conduction period of the transistors TR₁ to TR₃ connected to the positive potential side of the capacitor 52, across which the DC voltage E d appears, is pulse-width modulated, so that these transistors operate as a chopper to control the AC output voltage of the inverter 62. A resistor R₁ having a low resistance is connected between common emitter terminals of the transistors TR₄ to TR₆ and common anode terminals of the diodes D₄ to D₆. A compressor part designated generally by the reference numeral 70 includes a synchronous motor 70-1 having a two-pole permanent magnet of a brushless DC motor in its field system and a compression element 70-2 which is the load of the synchronous motor 70-1. The armature current I L flowing through the armature windings of the synchronous motor 70-1 flows also through the resistor R₁, and, by detecting the voltage drop accross the resistor R₁, the armature current I L can be detected. A control circuit controlling the output torque of the synchronous motor 70-1 includes a microcomputer 53, a pole-position detecting circuit 54 detecting the pole positions of the rotor of the synchronous motor 70-1, a torque controller 80 controlling the output torque of the synchronous motor 70-1, a base driver 55 driving the base of the transistors TR₁ to TR₆, a speed commanding circuit 56 applying to the microcomputer 53 a signal commanding a desired rotation speed, an electromagnetic pickup 72 detecting variations of the rotation speed of the main shaft of the compressor part 70, and a wave shaping circuit 74 shaping the waveform of the output of the pickup 72. As disclosed in JP-A-52-80415, the pole-position detecting circuit 54 is a circuit to which terminal voltages V A , V B and V C of the armature windings of the synchronous motor 70-1 are applied through a filter circuit and which generates a position signal 6S indicative of the detected angular position of the rotor of the synchronous motor 70-1. The microcomputer 53 includes a CPU 53a, a ROM 53b, a RAM 53c and an interface 53d which are interconnected by an address bus, a data bus and a control bus (not shown). The ROM 53b stores a control program required for driving the synchronous motor 70-1 on the basis of data acquired in preceding one revolution of the compressor main shaft or statistically processed data. On the other hand, data of speed errors detected in past one revolution of the compressor main shaft of N sections of the shaft revolution or statistically processed data of speed errors detected in past plural revolutions of the compressor main shaft are written in the RAM 53c. On the basis of the information of rotation speed variations of the compressor main shaft, a current pattern corresponding to one revolution of the compressor main shaft or an averaged current pattern corresponding to plural revolutions of the compressor main shaft is produced, and the torque controller 80 controls the armature current using such a current pattern. In the brushless DC motor, the current flowing through the armature windings is proportional to the output torque of the motor. Therefore, by controlling the armature current of the motor the output torque of the motor can be controlled. Fig. 11 shows the detailed structure of the torque controller 80. As shown in Fig. 11, the torque controller 80 includes a D/A converter 80b acting as a current commanding circuit, an amplifier 80c acting as a current detecting circuit, and a comparator 80d acting as a comparing circuit having a hysteresis characteristic. An 8-bit speed error data S₀ read out from the RAM 53c of the microcomputer 53 is converted by the D/A converter 80b into an analog data which provides a current command value 80a as shown. The armature current I L detected as the voltage drop across the resistor R₁ is amplified by the amplifier 80c to provide a detected current value V IL which is compared with the current command value 80a in the comparator 80d. A chopper signal S₁ appears at the output of the comparator 80d for switching the transistors TR₁ to TR₃ in the inverter 62. The foregoing has referred to the manner of controlling the output torque of an inverter-driven motor element of a closed rotary compressor. However, the present invention is in no way limited to such a specific application and is equally effectively applicable to any one of various rotary machines which are driven by a motor element and in which load (absorption) torque varies periodically. It will be apparent from the foregoing detailed description that, by application of the present invention to a rotary machine which is driven by a motor element and in which load torque varies periodically, variations of the rotation speed of the rotating system can be greatly reduced. In other words, vibrations of the entire housing of the stationary side can be greatly reduced, and, especially, vibration-free low-speed operation of the rotary machine near the resonance point or in a range lower than the resonance point can be achieved. Further, since vibration of the rotary machine can be reduced, the vibration-proof structure can be greatly simplified. Therefore, the power consumption of the rotary machine can be saved by operating the rotary machine in a wide range of variable speeds, and the scale of a system in which the rotary machine is incorporated can be reduced.

Claims: A torque-controlled rotary apparatus comprising a motor element (20), a load element (21) connected to said motor element (20) through a rotating shaft (4) to be driven by said motor element (20) and a torque control apparatus (28,34,35,36) for controlling an electromagnetic torque produced by said motor element (20) so as to eliminate the difference between the electromagnetic torque of said motor element (20) and a load torque of said load element (21), characterised in that: said control apparatus is arranged to divide, by rotational angle, each revolution of said rotating shaft (4) into a plurality of angular sections; and in that said control apparatus further includes: sensor means (23,29) for obtaining a plurality of variation values of the rotating speed of said rotating shaft (4), corresponding to said plurality of angular sections during at least one previous revolutions to said rotating shaft (4); calculating means (31) for calculating a plurality of control values based on the plurality of variation values of the rotating speed of said rotary shaft obtained at said plurality of angular sections so that each control value corresponds to one angular section; and control means (34,35) for applying each of said plurality of control values to said motor element (20) at succeeding revolution of said rotating shaft (4); thereby to cancel the difference between the electromagnetic torque and the load torque at each of said angular sections. A torque-controlled rotary apparatus according to claim 1, wherein the plurality of variation values of the rotating speed of said rotating shaft at the plurality of angular sections are obtained during a plurality of previous revolutions of said rotating shaft.

Inventors: ENDO, TSUNEHIRO, HATA, HIROAKI, MACHIDA, SIGERU, NAKAMURA, MITSURU, NAKAMURA, YOZO
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: G05D19
IPC codes: F04B49, F16F15, H02P29, F04C28, G05D17, H02P6