GB2202971A - Control unit for regulation of washing machine motors - Google Patents

Abstract

A control unit for regulating the speed of a washing-machine motor 1 over a wide range of speeds (e.g. in the wash, rinse and spin modes) comprises a tachogenerator 2 coupled to a divider 6 for dividing the output pulses of the tachogenerator according to the operating mode of the machine. The pulses, after being differentiated at 9 are fed to a voltage/time converter 11 which compares the voltage on a charging capacitor with a reference to produce a measure of the pulse interval. The output of the converter 11 is integrated at 15 and compared at 16 with a saw-tooth waveform to control a triac 19 through which the motor 1 is supplied. A memory 13 may be used to trip the supply to the motor if an excessive speed is sensed. <IMAGE>

Description

CONTROL UNIT FOR REGULATION OF WASHING MACHINE MOTORS This invention relates to a control unit for regulating the motor of a washing machine to achieve the required control of the rotation speed of the machine drum in its various operating stages, namely washing, rinsing and spinning.

The speed of the drum of a washing machine is critical, particularly in the washing phase; it has been established in experiments that the most suitable speed is a linear speed at the periphery of the drum of about 1.2 m/sec. The more precise this speed, the better the results which are obtained in washing, with the same amount of detergent. For this reason all washing machine manufacturers seek to get as close as possible to the critical or ideal speed since a failure to do so, whether in the positive or in the negative direction, has a detrimental effect on the results obtained. Washing time and water temperature are believed to be secondary factors; the drum speed in rinsing and spinning is not believed to be critical. Nor is the control of the inlet and drainage valves considered to be a problem.Other features such as soal < ing in anticreasing semi-flotation are not considered important they are significant only from the point of view of the range of services provided for the user's comfort, and they do not affect the quality of the wash.

The dryness of the clothes when they are removed from the washing-machine depends upon the spinning speed, which is generally fixed at between 1000 and 1500 r.p.m. which is equivalent to from 20 - 30 times the drum speed in the washing routine. Thus it can be said that the drum of a washing machine rotates through a range of speeds running from 50 to 1000 or 1500 r.p.m., i.e. a ratio of 1 : 20 or 1 i 30, so that the motor must provide a useful torque at any speed within that range.

It is not possible to fit a washing-machine with a motor for each of the washing, rinsing, spinning, etc. programs which it must carry out - because of the cost and volume; therefore, a motor must be made available which can have its speed adjusted as precise as possible.

Clearly, the cost of a motor giving enough torque at minimum speeds is high, so that the power must be optimised so as to avoid unnecessary expense. However, the risk in using a motor with a highly adjusted power rating is serious since if, for any reason, it should drop below the minimum torque, the motor overheats rapidly. This results in a loss of power which increases the overheating. This progressively damages the insulation of the winding until, in a relatively short period of time, the repeated overheating causes the winding insulation to breakdown, reducing the number of coils in the winding. The motor may burn out and is rendered useless. This is an expensive and common breakdown, usually arising because the drum of a washing machine has been overfilled.

For this reason, manufacturers are obliged to use expensive motors, and generally resort to added luxury services in order to cover the cost of the motor.

A basic cause of this problem is that the motor adjustment devices which have been used up until now do not allow a sufficient degree of control. This creates an error which is at its maximum at slow speeds, which is precisely when the error is of greatest effect and when it is most detrimental to the machine's operation.

In more specific terms, control of the rotational speed of the motor is conventionally achieved by means of a signal generator made up of a tachometric generator mechanically coupled to the motor of the washing machine.

A frequency to voltage converter functions in conjunction with the tachometric generator to provide a voltage which is proportional to the frequency, if the converter is linear. This signal is delivered to an analog comparator which supplies an error signal which is suitably amplified, integrated and processed by analog means, to converted it into a control signal for a power element. The power element, or regulator, acts on a triac governing the motor and is controlled by a synchro circuit and a trip circuit according to the mains frequency.

Thus, until now, the process has always been analog, employing a reference voltage supplied by the program system of the washing machine, and comparing it with a control signal obtained from the frequency to voltage converter to control motor speed as in any analog continuously variable regulator.

This type of regulator has the serious disadvantage that the linearity of - the converter does not, in practice, correspond to the theoretical value and therefore the lower the frequency, the less precise the control.

Non-linearity is more critical at reduced speeds because an equal absolute error represents a relative error which, percentage-wise, is much greater; thus, for example, the accuracy obtained in absolute value is similar in the washing and spinning cycles, while much greater in relative value. For example, an error of 30 revolutions in a rotation speed of 1000 or 1500 r.p.m. is not very important; in a speed of 50 r.p.m. it is of the very greatest importance and will mean that the motor will shortcircuit within a relatively short period of time.

Clearly therefore, the speed adjustment systems used in current technology have disadvantages.

According to one aspect of this invention, there is provided a control unit for regulating washing machine motors of the type where a signal is obtained from a tachometric generator coupled to the washingmachine motor, wherein, means are incorporated for the comparing a reference signal with the signal measured by the said tachometric generator, following the transformation into time of the said reference signal or control voltage, by means of a voltage/time convertor, and in digital form, said means for comparing comprising a logic gate which compares the duration of the impulses according to their coincidence or non-coincidence, and generates a logic error signal, with provision in the said convertor for the participation of a condenser with exponential charge through a resistor, so that, from a known charge and from the control voltage, the condensor voltage is compared so as to detect the moment at which the said coincidence takes place, with the assistance of an analog comparator.

The applicants have designed an embodiment of control circuit for washing machine motors which, although receiving a signal from a tachometric generator, does not need to rely wholly on analog techniques to secure particular accuracy in the control of low speeds, for example in the washing cycles. The embodiment of control circuit designed by the applicants may hold the optimal linear drum speed within very strict limits, irrespective of the load. To do this, because it is a frequency which must be controlled, and a frequency always corresponds to a period, a digital circuit is used as the basis, incorporating other analog and digital circuits, until finally reaching logic circuits.

Instead of converting the tachometric generator signal into an analog signal the tachometric generator signal is converted into a time signal and the control voltage representing the required velocity is converted, into a reference time, by means of a voltage/time converter.

In the embodiment illustrated hereinafter, the tachometric generator signal is measured and compared digitally using a logic gate, which compares the pulse duration of the reference pulse with that of a measured pulse, so as to give a logic error signal, which will indicate "1" when the values are different, and' "0" when they are the same.

Basically the control voltage is converted into a reference time by means of charging a capacitor, specifically a capacitor of reverse exponential type, which is fully defined. Charging of the capacitor continues until the stored voltage matches that of the reference voltage, at which point the capacitor is discharged. Thus the period between the time when the capacitor starts charging and the time it discharges will depend on the magnitude of the reference voltage.

In the illustrated arrangement, a simple digital comparator compares the voltage stored on the capacitor and the reference voltage so as to detect the moment at which coincidence takes place.

From this, it turns out that the longer the period the more exact the measurement; in other words, the greatest accuracy is obtained at relatively slow rotational speeds of the motor.

Preferably the signal generated by the tachometric generator drives a shaper circuit which gives a square wave of the same period and frequency as the sinusoidal signal generated by the tachometer, but with a standardised amplitude. Because the tachometric generator has a greater resolution at high speeds than is necessary for adequate speed control, it is preferred for the square wave to be supplied to a divider circuit which divides the signal frequency by 2, 4 or 8. The divider may be a simple type which is cheap and effective, and may be designed in two or more stages.

There is, in addition, provision for a digital multiplexer which acts as a signal switch, controlled by a logic signal that selects the origin of the signal from the input or output of the divider.

Thus, at speeds other than that for washing, the machine may operate with the signal divided, so that, for all normal operating speeds, the periods to be regulated are of the same order, and even very similar.

Obviously, digital division does not introduce errors and the ratio of 1:20 or 1 : 30 when the shaper circuit is used is reduced to 1 : 5, so that the operating errors of the absolute type are of the same order, being divided by four or five, and produce a parallel effect in terms of relative errors.

The multiplexed and digitised signal is controlled by an outside signal and supplied to a differentiator which provides a train of very short impulses. The impulses pass to the voltage-time converter and then to an integrator which gives a mean voltage of this signal output of the converter, which is referred to as the control voltage, which is supplied to an analog comparator. At the analog comparator the control voltage is compared with the voltage from a saw-tooth generator, the maximum levels of which will determine the mean voltage levels and, therefore, the periods required in the tachometric generator in order to provide the different degrees of actuation of the triac which controls the motor.

The invention will now be described by way of example, reference being made to the accompanying drawings in which: Figure 1 is a voltage/frequency diagram for a standard regulator, where a broken line shows the loss of linearity of the converter as against the theoretical value; Figure 2 is a schematic diagram of a conventional speed regulator for a washing machine motor; Figure 3 is a schematic diagram of a control unit for the regulation of a washing machine motor designed in accordance with an embodiment of the present invention: Figure 4 shows different voltages and signals for the circuit in the Figure 3, and which will be used in the description thereof, and Figure 5 is a diagram illustrating the voltage time converter used in the embodiment of Figure 3.

Figure 2 illustrates a standard speed controller, which includes a motor 1, a tachometric generator 2, a frequencylvoltage converter 3, an analog comparator 5, supplied with a reference voltage 4, a regulator 6, a synchronising circuit 7 and a triac 9 each supplied with mains voltage 8.

This circuit gives a lack of linearity in the voltage/time ratio (V/t), as represented by the broken line of Figure 1.

Referring now to the embodiment of the invention illustrated in Figure 3 shows how the motor 1 is mechanically linked to a tachometric generator 2, in the usual way; the sinusoidal signal 3 output by the generator is supplied to a wave shaper 4, which delivers a square wave signal 5 of only two logic levels, "high" and "low", each of constant amplitude having a period exactly the same as that of the signal delivered by the said tachometric generator 2. The square wave signal passes to a frequency divider 6 comprising a binary counter where the signal is divided by a multiple of 2, e.g. 2, 4, B, 16 etc.

The output from the binary counter 6, and a parallel output from the wave shaper 4, are applied to a multiplex digital selector 7 which is controlled by an external pre-divider signal 8. In this way the signal from the multiplexer 7 may have the same period as that from the tachometer 2, or it may be a binary submultiple of this period.

As has already been pointed out, the digital multiplexer 7 acts as a signal switch controlled by a logic signal which controls whether the signal is to come directly from the shaper, as happens with slow speeds, or, as in the case of high speeds, it is to be divided so that the periods to be regulated are of the same order as those of the undivided slow speed signals.

The signal from the digital multiplexer 7 enters a differentiator 9, which is sensitive to the rising edge 10 of the square wave 5; the differentiator 9 delivers a very short pulse, synchronised with the edge 10. The synchronism may be with the rising or falling edge; it is important however that it should be sensitive to only one edge, so that only one pulse is obtained for each complete period of the square wave 5.

The pulse from the differentiator 9 is introduced into a voltage/time converter 11 and, with a set of reference components, such as the resistor 12, associated with a capacitor and the control voltage 14, it delivers a longer pulse, synchronised with the input pulse from the differentiator, and which constitutes a reference period which is a fraction (i.e. 80-900h) of the required period required.

Figure 5 shows an example of voltage time converter 11 used in the control unit of Figure 3. A R/S flip-flop A is driven by the input pulses (ISIS Figure 4). Logic output Q goes to logic level 1, whereas logic output Q goes to logic level 0 (zero), thus cutting transistor B and giving rise to an exponential in time charging of capacitor C through resistor D.

An analogue voltage comparator E receives the voltage of capacitor C and compares it always with the control voltage 14 (Figure 3).

When the voltage in capacitor C reaches the control value, the output of comparator E goes to logic level 1 and disables flip-flop A.

In this situation transistor B conducts and discharges immediately the capacitor, which remains uncharged until a new pulse comes.

The processed output OP is sent to circuit 15 (Figure 3).

In addition, there is a memory 13, which is a memory of the so-called D constant/D non-constant type, which memorises the state of the output impulse from the voltage/time converter 11 at the moment of the arrival of a trip impulse from the differentiator 9 and checks that there is at least some "LOW" level in each period of signal V. The memory has control means which detects a speed which is 10% greater than normal, which would mean that two pulses from the voltage/time converter 11 have overlapped, and the system will no longer operate correctly. If the memory detects an excessive speed, it effects direct actuation of the trip circuit to interrupt power supply to the motor.

The impulses from the voltage/time converter 11 are integrated by a circuit 15 which may be analog, using a simple RP circuit, giving a voltage which is the same as the average voltage of the impulses from the voltage/time converter 11.

This signal, which is the control voltage, is supplied to an analog comparator 16, where it is compared with the saw tooth voltage waveform from a saw tooth generator 17, which must he of an amplitude whose maximum and minimum levels determine the average voltage levels and, therefore, the periods required in the tachometric generator 2 in order to obtain the different degrees of actuation of the triac 19 which commands the motor through the trip circuit 18.

So that the level from the integrator 15 is within the saw tooth, it is necessary to ensure that the output of the voltage time converter contains pulses of a given length between 1 and 0. in other words, if the capacitor associated with the voltage/time converter 11 is always charging, the control voltage output obtained would be "0" and, therefore, less than the saw tooth.

In order to ensure actuation, the control voltage is set within the saw tooth amplitude, so that there is always a difference between the period measured by the tachometric generator 2 and the one generated by the voltage/time converter 11. This ensures that there will be a discharge time.

Figure 4 shows graphically the voltage of the tachometric generator (I), the signal shaped between 0 and 5 V (II), with the period corresponding exactly to the zero crossing of the alternating signal from the tachometric generator 2. The differentiated signal output by differentiator 9 is shown at (III). This signal rIII) controls the initiation of charging of the capacitor 20 of the voltage/time converter 11; it can be seen how, as the period of the tachometric generator signal increases, the length of the period during which the discharge is activated increases, while the time at which it is being charged does not change, so that the mean voltage of the curve (III) increases.

Also in Figure 4, the capacitor charge is marked (IV) and it can be seen how it begins in synchronisation with the appearance of the rising edge of the signal (II). Between signals (II) and (V), a period difference signal can be seen between the tachometric generator square wave signal and the reference period which is controlled by the reference voltage 14 (see Figure 3).

Also in this Figure, the average voltage obtained by the simple integration of the signal (V) is marked (VI). As the discharge impulses increase in length, the average voltage rises, with charge predominating over discharge in the capacitor of the integrator 15.

The saw tooth, which is not synchronised with the mains supply is marked (VI) in Figure 4. In this case, the saw tooth generator gives a frequency which is higher than the mains frequency.

Depending on the particular type of motor, such as induction models, the tachometer frequency at washing frequencies may be lower than main frequencies.

Finally, graph (VIII) in Figure 4 shows the control voltage entering the comparator 16, forming an increasing sloping line, in which three areas can be clearly distinguished, A, B and C. In the first zone, A, because the control voltage is less than the saw tooth voltage, the triac is not activated; in zone B, the triac is activated for pulse length of increasing length and, in zone C, the control voltage, is clearly greater than that of the saw tooth, so that a minimum activation is produced.

With a pulse amplitude of 5V, zone A is obtained from voltages of less than 0.5\', which do not operate the triac, because the impulses in the wave form (V) from the voltage/time converter 11 have a percentage at the output of the integrator 16 of less than 10%, corresponding to not more than lOP'0 longer than the reference period. In practice, operation takes place in the proportional regulation zone between 10% and 20%, giving voltages of between 0.5 V and 1 V, which are voltages for the B zone of the graph (VII) in Figure 4.

In this way, an optimal wash is obtained for the clothes, with improved yield from the detergent used, the water consumed, the washing time and the energy employed. In parallel, optimisation is secured in respect of the motor power as far as the adequate mimimum torque is concerned. which amounts to reduced cost for the motor and better sales offer. Finally, increased electro-mechanical reliability is achieved, with greater duration and fewer maintenance costs as a result of breakdowns.

This described embodiment provides a control unit for the regulation of washing-machine motors, which is designed to ensure perfect control thereof in any of its various working regimes - wash, rinse and spin.

On the basis of a tachometric generator which is coupled to the motor of the washing machine. the central feature of the embodiment is the conversion of the signal generated by the said generator into a reference time, using a voltage/time converter. The signal obtained is compared with a reference signal so as to give a logic error signal detected by an analog comparator.

Furthermore, the information from the tachometric generator is, once it has been shaped, divided optionally by 2, 4 or 8, using a shaper circuit so that, when the motor is operating at high speed, a sub-multiple of the signal generated by the tachometer, is used, one or other of the signals being chosen, i.e. either the tachometer signal directly, or the divided signal, with the assistance of digital multiplexor which acts as a switching unit.

It is not thought necessary to further extend this description in order for any expert in the subject to appreciate the scope of the invention and the advantages which arise from it.

The materials, form, size and layout of the elements will be susceptible of variation.

Claims (9)

1. A control unit for regulating washing-machine motors of the type where a signal is obtained from a tachometric generator coupled to the washing-machine motor, wherein means are incorporated for comparing a reference signal with the signal measured by the said tachometric generator, following the transformation into time of the said reference signal or control voltage, by means of a voltage/time convertor, and in digital form, said means for comparing comprising a logic gate which compares the duration of the impulses according to their coincidence or non-coincidence, and generates a logic error signal, with provision in the said convertor for the participation of a condenser with exponential charge through a resistor, so that, from a lZnown charge and from the control voltage, the condensor voltage is compared so as to detect the moment at which the said coincidence takes place, with the assistance of an analog comparator.

2. A control unit according to claim 1, wherein the signal produced by the tachometric generator is supplied to a the shaper circuit which creates a square signal of the same period and frequency as the sine wave generator by the tachometer, but with a standardised amplitude.

3. A control unit according to claim 1 or claim 2 which includes a divider circuit for providing a signal having a period which is a binary submultiple of the signal from the tachometric generator. and further incorporating a digital multiplexor which acts as a signal switch, controlled by an external logic signal and operable to select a signal, having the same period as the tachometric generator, or the signal from the divider circuit.

4. A control unit according to claim 3 wherein the signals generated by the tachometer are sinusoidal and are supplied to a wave shaper which creates a square wave signal, in turn, is supplied to a digital multiplexor selector, either directly or through a binary counter, according to an external pre-divider signal supplied to said multiplexor, the output from the multiplexor being supplied to a differentiator sensitive to one of the edges of the square wave, the output from the differentiator being supplied to a voltage/time convertor which, is also supplied with a reference voltage to supply an output to an integrator, the output from the integrator being supplied to an analog comparator, together with a signal from a saw tooth generator, the output from said analog comparator, being used to control the motor governor triac, wherein a memory co-operates with the convertor to memorise the state of the impulse at the output from the voltage/time convertor at the moment of arrival of an impulse from the derivator.

5. A control unit for a washing machine motor, comprising a tachometric generator for outputting a tachometer signal representative of the speed of said motor, control means responsive to the length of the period of said tachometric generator and to the length of the period of a signal representing the required rotational speed to output an error signal for effecting speed control of said motor.

6. A control unit according to claim 5, wherein said control means is responsive to said tachometer signal to output a periodic square wave signal having a period substantially equal to the period of said tachometer signal and including a portion at one logic level of length representing the period of the required rotational speed.

7. A washing machine incorporating a control unit according to any one of the preceding claims.

8. A control unit for a washing machine motor, substantially as hereinbefore described with reference to and as illustrated in, Figure 3 or Figure 4 of the accompanying drawings.

9. Any and all novel features and combinations and subcombinations thereof disclosed herein.