GB1561632A - Mixers and control systems therefor - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO MIXERS AND CONTROL SYSTEMS THEREFOR (71) We, SPILLERS LIMITED, a British Company, of Old Change House, Cannon Street London EC4M 6XB, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to mixers and in particular to dough mixers used in the manufacture of bread, cakes, biscuits and the like.

In modern bread manufacture, the dough mixer is usually of the high energy type which runs, not for a pre-set period but for the time required for its beater motor to consume a pre-set quantity of energy. This energy setting has been empirically derived for a given mixer batch size to obtain optimum quality of finished bread.

This development has led to a generation of bread dough mixers fitted with energy controllers utilizing a disc type watt hour meter arranged to operate a pre-set counter.

The latter operation has been achieved by using perforations in the disc which are scanned by a photo cell/lamp arrangement driving a relay. The contacts of this relay in turn send low voltage impulses representing energy into a conventional electromagnetic ratchet counter fitted with a pre-set. When the pre-set quantity of energy is consumed the mixer is automatically stopped and the counter reset.

A double wound transformer connected to the current element of the watt hour meter via an adjustable resistor in anti phase provides a "backing off" control. This resistor is set so that the no load energy of the mixer motor is offset, whereby the energy consumed during the mixing is that truely required to blend ingredients and develop the dough to the desired level required to produce a finished loaf of acceptable quality.

This electro mechanical method of measuring energy although well proven in the domestic electricity meter (where the disc drives an indicator via a gear train) has certain short comings as follows.

Firstly, the duty cycle of the photocell relay is high- 220 operations per mixing or 132,000 operations per week, (more than 6 million operations per annum), in an average bakery. Secondly, the ratchet counter used for the first digit also has a similar duty cycle.

Thirdly, flour dust penetrates the counting system affecting performances. Fourthly, noload adjustment is difficult to set up since indication of setting can only be made by checking the movement of the disc and it is difficult to decide when this is stationary.

Fifthly, if the no-load is set too high the disc reverses when the mixer stops causing extra counts which can reduce the total energy consumed on following mixings. Finally, the operation of the unit depends entirely on a photocell exciter lamp which is likely to fail eventually.

In accordance with the present invention, there is provided a control system for a mixer having an a.c. drive motor, the system comprising means for forming a first a.c. voltage signal proportional to the mixer motor current, means for forming a second a.c.

voltage signal proportional to the mixer motor voltage, a device for operating on said first and second a.c. signal voltages to produce a third signal which is instantaneously proportional to the power being consumed by the motor, a backing off amplifier which receives said third signal and which enables a signal level representative of the power consumed by the motor under no-load conditions to be offset from said third signal, an integrator for integrating the output of the backing off amplifier to form a fourth signal proportional to the energy expended by the motor during mixing, and a comparator which is arranged to trigger the actuation of a stop circuit for the mixer motor when the fourth signal reaches a predetermined value.

Preferably, the backing off amplifier is in the form of a summing amplifier, the output of said device for operating on the first and second a.c. signal voltages being connected to the summing amplifier together with a preset voltage connected in opposition thereto such that the summing amplifier output is zero when the motor is running under no-load conditions, i.e. when the mixer is running empty.

Advantageously, the slope of the integrator is adjustable for enabling preselection of the quantity of energy that has to be expanded before said predetermined value of the fourth signal is reached.

Said fourth signal proportional to energy can be continuously displayed on a suitable meter to provide an indication of the proportion of the required energy which has been expended in the mixing operation up to a given time.

The invention is described further hereinafter, by way of example, with reference to the drawing accompanying the Provisional Specification which is a circuit diagram of one embodiment of a mixer control system in accordance with the present invention.

The illustrated system is designed to control the energy expended in mixing a batch of dough by measuring and integrating the instantaneous power being supplied to the mixer motor.

The system includes two transformers 10, 12 which are arranged to monitor the voltage applied and the current taken by a mixer motor 14, respectively. The transformer 12 is a conventional double-wound step down transformer which provides a reference voltage proportional to the mainsfphase voltage being supplied to the mixer motor 14 and may be conveniently arranged as an extra winding on the mains transformer used ot power the complete system. The low voltage output of the transformer 12 is accurately trimmed using two resistors Rl, R2 to give a predetermined voltage, for example 5 volts, across R2 for a mains phase voltage of 240 volts. Any fluctuation in mains voltage is compensated for in the power measurement by variation in the reference voltage.

The current transformer 10 has a small resistance R3 (for example 50 ohms) connected across its secondary which enables a voltage to be taken from the secondary which is proportional to the primary current in the phase being used for providing the aforementioned reference voltage but which is small enough not to disturb the ampereturns balance of the primary and secondary windings of the transformer 10. In this embodiment the current transformer 10 is wound such that 100 amps in its primary winding produces 5 volts across the 50 ohm resistor R8. Thus the voltage across the resistor R3 is always proportional to the line current in the selected motor phase.

The two a.c. voltages from the transformers 10, 12 are respectively fed into x and y inputs of a multiplier 16 which in the present embodiment is a solid state device which performs the function xy/l0.

The multiplier 16 operates instantaneously in the following manner: Let the voltage representing mains voltage supplied to input x = Va and the voltage representing current supplied to input y= Vb then VA = v = Vm Sin wt Vb = i = Im Sin (wt- O) where v and i represent instantaneous values.

Vm= Maximum voltage Im = Maximum current w = 2sF Radians per second t = time in seconds = = Phase angle between current and voltage.

The multiplier operates on Va and Vb as Va x Vb = vi = Im Vm Sin wt (sin wt cos 0 - cos wt sin ) = Im Vm (sin2 wt cos 0 - sin wt cos wt sin ) = Im Vm i cos 2 wt cos 0 -- 9 sin 2 wt sin ) =Im Vm (i cos 0 - + cos (2 wt - =+ImVmcos - +ImVmcos(2 wt - Converting from maximum to RMS values.

= VI cos - VI cos (2 wt - n) The first term VI cos 0 represents the steady state power measurement, whereas the second term - VI cos (2 wt -- O) is removed during integration since it is time dependent and oscillatory.

The multiplier thus instantaneously multiplies the signals representative of the motor voltage and current to obtain an output which is proportional to the instantaneous power being supplied to the mixer motor.

A resistor and diode network 18 connected to the multiplier inputs x, y is provided to protect the multiplier 16 against mains spikes and surges, the voltage drop occurring across the input resistors of this network 18 being compensated by the gain of the backing off amplifier described below.

The output of the multiplier 16 representing power is fed to one input of a differential amplifier 20 which is arranged to subtract from the latter signal the voltage on the wiper of a potentiometer 22. As stated above, the present circuit is intended to control the energy being expended in mixing a batch of dough. Obviously, a certain amount of power is supplied to the mixer motor to drive the mixer impeller simply to overcome friction.

This is equally true when the mixer is running empty. Vnder such no-load conditions, a particular voltage value is obtained at the output of the multiplier 16. The latter voltage is "backed-off" using the potentiometer 22 until the output of the amplifier 20 is zero.

Thus, the present arrangement enables the "no load" power of the mixer to be "backedoff" by adjustment of the potentiometer 22 during setting up of the system so that the output of the amplifier 20 is zero when the mixer is running empty. The output of the amplifier 20 thus represents only that part of the total power expended which is required to blend and develop the dough in the mixer.

A second function of the amplifier 20 is to enable the overall gain of the system to be adjusted for calibration purposes, this being achieved by an adjustable resistance 24 in a feedback circuit of the amplifier 20.

The output of the amplifier 20 is applied to a further amplifier 26 which is set up as an integrator by the provision of a variable input resistance R4 and a feedback capacitor C which is operable when a switch contact K2 (1) in parallel therewith is open. The contact K2 (1) is part of a relay K2 and is opened when the mixer is started, as is described further below. This contact K2 (1) is arranged to be closed for resetting purposes. In a preferred embodiment, the input resistance R4 is in the form of a ten turn helical potentiometer fitted with a three decade digital indicator directly scaled in watt hours or watt hours x 10.

By varying the input resistance, for example from 0 to 500K ohms, the time constant of the integrator may be varied between predetermined time limits, for example 0 to 20 seconds. This means that for a time constant of 20 seconds and 1 volt at the input, the output of the integrator will reach 1 volt in 20 seconds or 10 volts in 200 seconds. The voltage at the input of the integrator is proportional to the instantaneous power to the mixer motor so that, by varying the time constant, the amount of energy required for the integrator output to reach a predetermined level, for example 10 volts, is also varied.

Thus, the rising output of the integrator 26 represents energy consumed by the mixer and is calibrated in watt hours. A meter 28 continuously displays the output of the integrator and is calibrated 0-100%, reaching full scale deflection when the integrator output is 10 volts. Thus, the reading on the meter 28 at any time indicates the percentage of the setting of the variable resistance R4 setting in watt hours that has been expended in the dough and also approximately the fraction of the mixing time reached.

The rising output from the integrator 26 is supplied to one input of a comparator 30 and is compared with a further reference voltage at that input, in this case a - 10 volt level. When the integrator output voltage is less than +10 volts, the comparator output voltage is arranged to be high, for example +14 volts. However, at the point where the integrator output just exceeds + 10v the comparator output goes low, in this case to - 14 volts. As described below, the attainment of the - 14 volt level at the output of the comparator is arranged to cause a relay circuit to disconnect the mixer from the power supply and hence terminate mixing.

The relay circuit includes a drive device in the form of a high-gain Darlington transistor pair 32 which is coupled to the output of the comparator 30 and which energises a control relay K3 when conductive. The switching contact K8 (1) of relay K3 is normally closed but is opened upon energisation of the relay K3 to open a mixer stop circuit 34 which causes the mixer motor to stop. The relay circuit further includes a switch contact 36 which is arranged to close at the start of a dough mixing cycle whereupon it connects the - 24 volt supply to the coil of a further relay K1.Operation of the relay K1 causes closure of a contact K1 (1) located between the amplifier 20 and integrator 26, thus switching the signal representing power onto the input of the integrator 26, and also causes closure of a contact K1 (2) which connects the - 24 volt supply to the coil of the relay K2. The operation of relay K2 causes contact K2(1) to open, thus allowing the integrator output to rise, and closure of a contact K2(2) which holds relay K2 energised until a reset or tipping switch 38 is opened.

At the end of the dough mixing cycle, the contact 36 (delta auxiliary start contact) opens de-energising K1. As mentioned above, the relay K2 remains energised until reset.

This has the effect of holding the final voltage on the integrator until reset by the switch 38.

Thus a measurement of the energy being expended begins when the auxiliary delta contact 36 is closed and the mixer starts.

Closure of contact 36 energises relays K1 and K2 so that K1(l), K1(2) and K2(2) are closed and K2(l) is open. The Darlington pair 32 is initially non-conductive, the relay K3 thus being non-energized and its contact K3(1) being closed. The voltage signal proportional to instantaneous power is connected to the input of the integrator via contact K1(l) and the output of the integrator begins to rise.

When the integrator output reaches 10 volts, so that the required watt hour input has been reached, the comparator changes state and causes the Darlington pair to become conductive, the relay K2 being thereby actuated to open the mixer stop circuit 34 by way of its contact K3(1). The mixer is thus stopped.

This de-energises the relay K1. When the mixer tips to discharge the mixed dough, the tipping switch 38 is actuated to release relay K2 and hence to reset the integrator 26.

It will be noted that it is not strictly necess ary to use a multiplier to produce the signal proportional to power and other devices, such as a modulator or thermo electric bridge, can equally well be employed.

WHAT WE CLAIM IS: 1. A control system for a mixer having an a.c. drive motor, the system comprising means for forming a first a.c. voltage signal proportional to the mixer motor current, means for forming a second a.c. voltage signal proportional to the mixer motor voltage, a device for operating on said first and second a.c. signal voltages to produce a third signal which is instantaneously proportional to the power being consumed by the motor, a backing off amplifier which receives said third signal and which enables a signal level representative of the power consumed by the motor under no load conditions to be offset from said third signal, an integrator for integrating the output of the backing off amplifier to form a fourth signal proportional to the energy expended by the motor during mixing, and a comparator which is arranged to trigger the actuation of a stop circuit for the mixer motor when the fourth signal reaches a predetermined value.

2. A control system as claimed in Claim 1 in which the backing off amplifier is in the form of a summing amplifier, and the output of said device for operating on the first and second a.c. signal voltages is connected to the summing amplifier together with a preset voltage connected in opposition thereto such that the summing amplifier output is zero when the motor is running under no-load conditions.

3. A control system as claimed in Claim 1 or 2 in which the slope of the integrator is adjustable for enclosing preselection of the quantity of energy that has to be expended before said predetermined value of the fourth signal is reached.

4. A control system as claimed in Claim 1, 2 or 3 including a meter for continuously displaying said fourth signal proportional to energy whereby to provide an indication of the proportion of the required energy which has been expended in the mixing operation up to a given time.

5. A control system as claimed in any of Claims 1 to 4, in which said device is in the form of a multiplier adapted to multiply said first and second a.c. signal voltages to produce the third signal.

6. A control system constructed and arranged to operate substantially as hereinbefore particularly described with reference to and as illustrated in the drawing accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. ary to use a multiplier to produce the signal proportional to power and other devices, such as a modulator or thermo electric bridge, can equally well be employed. WHAT WE CLAIM IS:

1. A control system for a mixer having an a.c. drive motor, the system comprising means for forming a first a.c. voltage signal proportional to the mixer motor current, means for forming a second a.c. voltage signal proportional to the mixer motor voltage, a device for operating on said first and second a.c. signal voltages to produce a third signal which is instantaneously proportional to the power being consumed by the motor, a backing off amplifier which receives said third signal and which enables a signal level representative of the power consumed by the motor under no load conditions to be offset from said third signal, an integrator for integrating the output of the backing off amplifier to form a fourth signal proportional to the energy expended by the motor during mixing, and a comparator which is arranged to trigger the actuation of a stop circuit for the mixer motor when the fourth signal reaches a predetermined value.

2. A control system as claimed in Claim 1 in which the backing off amplifier is in the form of a summing amplifier, and the output of said device for operating on the first and second a.c. signal voltages is connected to the summing amplifier together with a preset voltage connected in opposition thereto such that the summing amplifier output is zero when the motor is running under no-load conditions.

3. A control system as claimed in Claim 1 or 2 in which the slope of the integrator is adjustable for enclosing preselection of the quantity of energy that has to be expended before said predetermined value of the fourth signal is reached.

4. A control system as claimed in Claim 1, 2 or 3 including a meter for continuously displaying said fourth signal proportional to energy whereby to provide an indication of the proportion of the required energy which has been expended in the mixing operation up to a given time.

5. A control system as claimed in any of Claims 1 to 4, in which said device is in the form of a multiplier adapted to multiply said first and second a.c. signal voltages to produce the third signal.

6. A control system constructed and arranged to operate substantially as hereinbefore particularly described with reference to and as illustrated in the drawing accompanying the Provisional Specification.