Load unbalance detection washing machine

Abstract: This invention relates to a control circuit for a machine for treating clothes. The circuit includes means (2) for providing a desired speed signal for the machine drum, means (14) for comparing the desired speed signal with an actual speed signal to provide an error signal (E) for controlling a switch (8) which switch is arranged to supply electrical power to an electric motor (10) for driving the machine drum and means (14, 24) for detecting a tachogenerator (12) signal representing speed of the motor and providing said actual speed signal. The detection means includes strobe means (44-48) responsive to the output of the tachogenerator for generating a strobe signal at a frequency related to the tachogenerator speed frequency for sampling the actual speed signal or error signal, the strobe means being responsive to zero crossing points of the tachogenerator signal in order to generate a strobe signal at or near to a zero crossing point and means (72) for comparing the sampled error signal or actual speed signal with a reference value (V L ) to ascertain the existence of load unbalance.

Description: Field of the Invention This invention relates to the detection of load unbalance in a rotable drum of a washing machine, dryer or like device (herein referred to collectively as "a machine for treating clothes"). Background Art European Patent Specification No. EP-A-0 071 308 discloses a control circuit for a machine for treating clothes, the circuit including: control means for providing a desired speed signal for the machine drum; comparing means for comparing the desired speed signal with an actual speed signal to provide an error signal for controlling a switch which is arranged to supply electrical power to an electric motor for driving the machine drum; a tachogenerator for producing a tachogenerator signal representing the speed of the motor at an output thereof; and detection means for detecting the tachogenerator signal representing the speed of the motor and providing said actual speed signal. Fig. 1 shows a known universal motor speed regulation loop for a washing machine with a ramp generator 2 for defining a speed characteristic connected to a first error amplifier 4 input, controlling a pulse generator 6 which switches a triac 8. This triac 8 supplies a universal motor 10 having a tacho generator 12 whose output frequency is proportional to the motor speed and is fed to a pulse integrating frequency to voltage converter 14, whose output, actual speed voltage, is filtered in RC filter 16 and is looped to a second input of error amplifier 4. Load unbalance phenomenon of the load in a washing machine drum appears if such a circuit as an oscillation of actual speed around the desired speed as set by the ramp voltage provided by generator 2. In order to detect such unbalance it is necessary to compare actual with desired speed as set by the ramp voltage in a comparator 18 of an unbalance detector 20, the result of the comparison (E) being compared with a threshold voltage level (V L ) in a comparator 22 in order to reset the ramp voltage, if necessary. However in such applications, the actual speed voltage is superimposed by an undesirable ripple, whose amplitude is 100mV or more. This arises since with the technique used in pulse integrating FV converter 14, a fixed current pulse is delivered to RC network 16 once per input signal period and this leads to a ripple on actual speed signal. A higher capacitor in the RC network for better filtering is not possible because of loop stability constraints. This ripple peak to peak amplitude may mask or amplify oscillation of the speed signal due to load unbalance. Summary of the Invention It is an object of the invention to provide a control circuit for a machine for treating clothes which overcomes or reduces the above noted problem. The present invention provides a control circuit for a machine for treating clothes, the circuit including: control means for providing a desired speed signal for the machine drum; comparing means for comparing the desired speed signal with an actual speed signal to provide an error signal (E) for controlling a switch which switch is arranged to supply electrical power to an electric motor for driving the machine drum; a tachogenerator for producing a tachogenerator signal representing the speed of the motor at an output thereof; and detection means for detecting the tachogenerator signal representing the speed of the motor and providing said actual speed signal; wherein the detection means includes strobe means responsive to the output of the tachogenerator for generating a strobe signal at a frequency related to the tachogenerator signal frequency, the strobe means being responsive to zero crossing points of the tachogenerator signal in order to generate a strobe signal at or near to a zero crossing point; means for sampling the actual speed signal or the error signal at the frequency of the strobe signal to produce a sampled signal; and means for comparing the sampled signal with a reference value (V L ) to ascertain the existence of load unbalance in the machine drum. The present invention stems from the realization that since the frequency of the load unbalance signal, being the same as the drum frequency, is many times slower (say between 10-100 times slower) than the frequency of the electric motor driving the drum, by sampling the speed signal at or near zero crossing point of the a.c. signal from the tachogenerator (which, having n pairs of poles, is in itself a multiple (n) of the motor frequency), undesirable ripple in the actual speed signal is eliminated and any remaining difference between the tachogenerator signal and desired speed signal will be due to load unbalance. Brief Description of the Drawings A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which:- Fig. 1 is a block diagram of a known control circuit for a washing machine drum motor; Fig. 2 is a diagram illustrating the phenomenon of load unbalance in a washing machine, causing variations in drum speed; Fig. 3 is a series of timing diagrams showing the way in which strobe signals are generated in accordance with the invention; Fig. 4 is a circuit diagram for a preferred embodiment of the present invention. Description of the Preferred Embodiment Referring to Fig. 2, this shows the phenomenon of load unbalance in which a ramp signal which is provided to the motor control for regulating the speed of the motor is denoted V(ramp). In the case of load unbalance, the actual speed of the motor is denoted by the line V(actual speed). It may be seen there is a maximum differential of A and in the preferred embodiment it is desired to determine when this differential exceeds a threshold value. Referring to Fig. 3, Fig. 3a shows a tachogenerator voltage signal as would be generated by the tachogenerator 12 of Fig. 1. It may be seen that the wave form is sinusoid, with positive going zero crossing points B and negative going zero crossing points C. Fig. 3b shows the actual speed signal which is developed at the output of converter 14 and filter 16 as a ramp wave form again with similar zero crossing points B, C but a maximum error signal E. It may be noted that the characteristics of converter 14 are such that the actual speed signal has a step jump at points B and is quasi-linear between consecutive points B but that at points C the actual speed signal is close to, and slowly varying relative to, the reference signal V(ramp). In accordance with the invention strobe signals are generated at points C as shown in Fig. 3c in order that the absolute magnitude of the motor speed may be measured at points at which the ripple voltage is not causing any error. It will be appreciated, referring back to Fig. 2 that in the case of load unbalance, the ripple of Fig. 3b will be superimposed on the signal V(actual speed) of Fig. 2. By effecting a comparison at point C, this ripple can be eliminated when comparing the actual speed with the ramp voltage. Referring now to Fig. 4, a frequency to voltage converter 24 is shown. It will be understood that this converter 24 is to be incorporated into the circuit of Fig. 1 and to replace converter 14 of Fig. 1, other parts of the circuit remaining the same. An input port 40 receives an input signal from tachogenerator 12 and this is amplified in an amplifier 42 and applied to the bases of PNP transistors 44, 46. The collector of transistor 44 is coupled to a further PNP transistor 48 which has a resistor R2 in its collector path. The base of transistor 48 and the collector of transistor 46 are coupled to a nodal point 50 to which capacitor C(pump) is coupled. A switch indicated schematically at 52 is coupled via a resistor R1 to nodal point 50. Switch 52 is also coupled to a NPN transistor 54 which is coupled in current mirror configuration to a further NPN transistor 56 which is arranged to supply an output port 60 to which filter 16 is coupled. In operation of the circuit of Fig. 4, as the tachogenerator signal voltage reaches positive going zero crossing points B transistors 44, 46 are switched off and, switch 52 being in the closed position, capacitor C(pump) is charged to voltage V CC . The current pulse thus provided is mirrored through transistor 56 to output port 60 to provide the "actual speed signal", as a series of current pulses which are integrated to a voltage value in RC network 16. As the tacho signal proceeds and reaches point C, switch 52 is switched off whereas transistors 44 and 46 are switched on. Switching on transistor 46 creates a current I1 which discharges C(pump). Transistor 48 is switched on enabling a current I₂ to flow through resistor R2 for as long as the voltage at node 50 V(pump) is below V cc . When V(pump) reaches ground, current I₂ is terminated by reason of transistor 48 being switched off. The signal voltage developed across resistor R2 is employed as an enabling strobe. It has a short duration (40 microseconds). The enabling strobe is directed along line 70 as shown in Fig. 4 to control a threshold detector 72 which receives as input signals a voltage V L which indicates the minimum acceptable load unbalance voltage, and the output E from error amplifier 18. When signal E exceeds threshold V L , comparator 72 when enabled by the strobe signal generates a signal U which is employed to switch or re-start the washing machine drum motor. The biggest problem in washing machines is drum unbalance which creates heavy mechanical stresses. The tendency is to electronically control the unbalance level to save weight and consequently cost, and to increase the overall reliability. The invention described provides a big improvement in this feature while insuring a total integration without any extra outside components. The impact will be a better competitiveness on this market.

Claims: A control circuit for a machine for treating clothes, the circuit including: control means (2) for providing a desired speed signal for the machine drum; comparing means (14) for comparing the desired speed signal with an actual speed signal to provide an error signal (E) for controlling a switch (8) which switch is arranged to supply electrical power to an electric motor (10) for driving the machine drum; a tachogenerator (12) for producing a tachogenerator signal representing the speed of the motor (10) at an output thereof; and detection means (14, 24) for detecting the tachogenerator signal representing the speed of the motor and providing said actual speed signal; characterised in that the detection means includes strobe means (44-48) responsive to the output of the tachogenerator for generating a strobe signal at a frequency related to the frequency of the tachogenerator signal, the strobe means being responsive to zero crossing points of the tachogenerator signal in order to generate a strobe signal at or near to a zero crossing point; means (72) for sampling either the actual speed signal or the error signal at the frequency of the strobe signal to produce a sampled signal; and means (72) for comparing the sampled signal with a reference value (V L ) to ascertain the existence of load unbalance in the machine drum. A control circuit as claimed in claim 1 wherein said control means (2) is arranged to provide a ramp waveform. A control circuit as claimed in either claim 1 or claim 2, wherein the detection means (24) includes means (50-60) for charging and subsequently discharging a capacitor (Cpump) for every cycle of the tachogenerator signal to provide a series of current pulses to an R-C filter means (16) to consitutute the actual speed signal. A control circuit as claimed in claim 3 wherein the charging means includes a switch means (52) responsive to the tachogenerator signal for providing a charging path between a supply voltage and ground. A control circuit as claimed in claim 4 including current mirror means (54, 56) for mirroring the charging current in the charging path to an output port (60). A control circuit as claimed in either claim 4 or claim 5, including transistor switch means (46) which is arranged to be switched on for providing a discharge current path for the capacitor (Cpump). A control circuit as claimed in claim 6 wherein the strobe means (44, 48, R2) is responsive to the switching on of the transistor switch means (46) for providing a current pulse for as long as the voltage on the capacitor (Cpump) remains above a predetermined level (GND). A control circuit as claimed in claim 7 wherein the strobe means comprises a first transistor (44) having a common base with the transistor switch means (46), and a second transistor (48) having a common main current path with the first transistor and having its base coupled to the capacitor (Cpump).

Inventors: L HERMITE FRANCOISE, OLLITRAULT STEPHAN, L'HERMITE, FRANCOISE, OLLITRAULT, STEPHAN
Applicants: MOTOROLA SEMICONDUCTEURS, MOTOROLA SEMICONDUCTEURS S.A.
Application year: 1992
Main classification: D06F37
IPC codes: D06F37