WO1997046928A1 - Control system for electrical heating equipment - Google Patents

Abstract

A control system for electrical heating equipment has a thermal parameter sensor, such as a temperature sensor (7), a control processor (12), and a power output control circuit (6) which uses triac devices (11). The system can control heating of water or food or other fluid or solid materials.

Description

CONTROL SYSTEM FOR ELECTRICAI HFAT1NG FnniP_MFNT

This invention relates to a control system for electrical heating

equipment. The invention can be used for the temperature control of fluids

and more particularly the temperature control of liquids especially water.

However, the invention may also be used in cooking to control heating of

solids such as food or for other purposes, e.g. for industrial materials.

The temperature control of fluids has been accomplished in many

ways. Usually a temperature sensor is provided which controls a heater so

that the heater is switched off when the fluid temperature reaches, or

exceeds, a predetermined level and/or switched on when the fluid

temperature drops to, or below, a predetermined temperature. If the

temperature of a fluid flow is to be controlled as is the case with a shower

it is frequently necessary to monitor the rate of fluid flow as well as the

temperature.

One particular form of temperature control used in kettles and the like

for boiling water comprises means for sensing the presence of steam which

in turn controls the heating means.

According to one aspect of the invention there is provided a control

system for electrical heating equipment for heating a medium, comprising

an electrical power supply switching circuit, a sensor for monitoring a thermal parameter of the medium, and a control device for controlling the

switching circuit in relation to said thermal parameter as sensed with the

sensor, wherein the control device comprises programmable processing

means, and the switching circuit has power output control properties.

According to a second aspect of the invention there is provided an

apparatus for controlling the temperature of a fluid comprising an electric

heating element disposed on a stainless steel or ceramic substrate and a

temperature sensor printed on said substrate, means responsive to the

sensor for monitoring the change of temperature of the fluid with respect

to time and means controlled by the monitoring means for controlling the

operation of the heating element.

With the second aspect of the invention, monitoring the rate of

change of temperature can be used to indicate that a liquid is boiling since

in that condition there is no temperature change. Other parameters can also

be controlled by monitoring the rate of change of temperature. For example

if the rate of change is very fast it is an indication that there is no liquid or

fluid present to be heated. This condition can be used to cut off power to

the heater element.

The invention also enables a heater to be disabled when the

temperature of a liquid heated by the heater element reaches a

predetermined temperature below boiling. For example in the preparation of coffee using so called "instant coffee" it is not necessary to use boiling

water and water at 80 °C for example is satisfactory, particularly for

vending machines and the like.

The control of cooking of food items is often required for items which

change colour upon heating such as the toasting of bread.

The control of toasting of bread in toasting devices is conventionally

by time. However, this is a somewhat hit and miss method. New bread

takes longer to toast to a particular shade of brown than does rather older

bread, for example. It is now quite common practice to store bread in a

frozen condition and then to make toast using frozen slices of bread. A

toaster set to toast by time only will often only unfreeze a frozen slice of

bread rather than produce good looking toast.

According to a third aspect of the invention there is provided a device

for controlling the cooking of food items which change colour upon heating,

comprising means for directing radiation at the food item while it is being

cooked and receiving means for detecting the level of radiation reflected

from the food item, said receiving means being adapted to stop the cooking

process when the reflected radiation reaches a predetermined level.

In a preferred embodiment of the third aspect of the invention the

radiation is infra red radiation and the receiving means is an infra red

receiver. The receiver can be set to disable the power supply to the cooker when the radiation reflected from the food item reaches a preset level.

Preferably a plurality of receivers is provided so that the reflected radiation

from a plurality of sites over the food item being cooked can be monitored.

The third aspect of the invention is particularly applicable to making

toast from bread. The invention enables toast of a desired level of browning to be obtained irrespective of the condition of the bread.

The third aspect of the invention can be used in connection with

toasters or other forms of cooker for institutional and domestic use.

Preferably the heaters for the toaster comprise heating elements disposed

on stainless steel substrates. The heaters may contain means for sensing

the temperature thereof which can be used for further control. In addition

means may be provided for keeping the toasted product warm at least for

a period of time after toasting has been completed. The mode of the

device, i.e. on, off, toasting, and so on can be indicated by audio and/or

visual means such as an LED and/or audio alarm.

The invention will now be described further by way of example only

and with reference to the accompanying drawing which is a circuit diagram

of one form of a control system according to the invention.

The control system comprises a microprocessor 12 connected to

input and output circuits.

The input circuits include: a temperature sensor circuit 1 , a temperature setting circuit 2, and reset and

interrupt circuits 3, 4.

The output circuits include:

multiple lamp signalling circuits 5, and a power drive circuit 6.

Referring now in more detail to the input circuits, the temperature

sensor circuit 1 includes a temperature sensitive element 7, in this case a

thermistor.

The temperature setting circuit 2 includes a potentiometer control 8.

The reset and interrupt circuits 3, 4 have press buttons 9, 10.

Referring now to the output circuits, the lamp signalling circuits 5

comprise LEDs.

The power drive circuit 6 has live power supply input and output

connections with triac circuitry 1 1 therebetween.

In use, the circuitry described can be connected to a kettle or

domestic shower or some other equipment which requires temperature

control. In this case, the thermistor 7 is exposed to the water in the kettle

or other medium for which the temperature is to be monitored, and a

heating element is connected to the output of the power drive circuitry 6.

The system is switched on and a desired temperature can be selected

using the potentiometer 8.

The power drive circuit 6 is switched on and electric current flows to, and operates the heating element. When the desired temperature is

reached, as selected with the potentiometer 8 and as detected with the sensor 7, the current is switched off.

The microprocessor is pre-programmed in correspondence with

requirements whereby power output from the power drive circuit 6 can vary.

Thus, in the case where fast boiling of water is required, the power

drive circuit may produce maximum power output. In the case where

slower heating of water to a temperature lower than boiling is required a

lower level of power output may be appropriate. Also, where continued

heating for a period after attainment of a temperature is required, e.g. to

maintain water at boiling level, or at a constant temperature less than

boiling, this may be achieved with a reduced level of power output.

Power output is selected by utilisation of 'firing angle' of the triac

devices 1 1 in conventional manner.

There may be provision for preselection of a desired program with

regard to power output levels, using input controls in conjunction with an

LCD display or otherwise (not shown). Alternatively or additionally, this

may be achieved automatically in correspondence with selection of a

predetermined temperature. Other programs may also be selectable, relating

to temperature, timing or other parameters or combinations. The LEDs 5 indicate operational status of the system.

With this embodiment, it will be appreciated that precise control of

temperature and heating is possible having regard to the use of

microprocessor and triac circuitry. Thus, for example, it is possible to use

the system with a kettle to select a temperature less than boiling point (e.g.

86 °-88 ° C) for coffee, or boiling point for tea.

The use of triac switching circuitry ensures careful control without

generating undue interference problems. Careful control of switching and

power is possible whereby this is particularly suitable for controlling heating

of fluid flow as is the case with a domestic shower. It can be ensured that

the fluid is maintained at a constant or desired temperature in relation to the

rate of flow. Triac switching also assists in maintaining a prolonged

temperature e.g. in cooking pans or the like.

The use of microprocessor circuitry also permits incorporation of

other parameters into the control system either derived from external

sensors or from internal programming. Thus, for example, there may be

additional sensors which respond to level of water or steam temperature,

so as to switch off power if the level of water in a kettle falls below a

predetermined level.

The system can be used in cooking in combination with an input from

a sensor which receives reflected radiation e g infra red radiation from toast in a toaster so that the power can be switched off when the toast reaches

a predetermined colouration.

The system can receive flow level input, so that power output is

varied with variation in flow.

The system may also monitor passage of time to determine, for

example, whether heating is proceeding at a desired rate so that the power

output can be adjusted accordingly to attain a desired rate.

Due to the design of the circuit described the heat can be controlled

by proportional control at zero crossing thus eliminating interference on all

other circuits. This facility enables the user to apply the heat at only half

of the time the element is switched on or one third or one quarter or

whatever proportion of the time that you choose to achieve the function

required. This is a good facility to use in the heating of water or liquids in

water heaters, showers, electric kettles and other liquid or food heaters

when a different range of temperatures are required to complete the

necessary cycle. This can be useful in toasters and sandwich makers,

skillets, rice pans, deep fat fryers, cooker tops and other cooking utensils

where heat control is essential for to get the correct cooking temperature.

The system provides for water temperature sensing and verification

in the case of hot and cold supplies. The controls built into the control

circuits would include this facility. This would be particularly useful in the case of a water heater or shower to enable the heater to take in either the

hot or cold water depending on the time of year or demands etc.

The circuits can sense the water or liquid level in any vessel to be

heated and will not allow any power to be switched on if there is no

water/liquid in the vessel.

A range of temperatures can be selected to enable the user to

preselect a desired temperature. This facility can be useful in the case of

showers where you may require the temperature to be at 40° or say 42° .

In a kettle or coffee maker one could select 100° or boiling point to make

tea or 86/88 ° to make coffee as required. In more sophisticated heating

devices for industrial use one could preselect the temperature required

which would be shown on an LCD display. A flashing indicating LED would

display or an alarm sounder would indicate as soon as the preset required

temperature was reached.

The facility is included in the circuitry to enable the user to control

the water supply rate to the shower or water heater as required. This is a

useful facility in the case of a water shower heater where the rate of heat

being applied to the water passing through the shower is constant and one

wants to vary the temperature of the water by speeding up or slowing down

the rate of flow of the water. The quicker the water flow the cooler the

water and vice versa the slower the water flow the more the water heats up.

The control circuits and sensors enables the user to sense and control

the temperature of water to +/- 1 °C. This is a very fine tolerance that has

many applications in domestic appliances and water heaters/showers etc.

where fine temperature control has not been available up to this point in

time.

The control circuitry will enable the user to display on LCD the

temperature of the water/liquid being heated at the moment it is switched

on until it has completed its cycle. The LCD can be preset to any given

temperature if required to be.

Alarm indicators can be included in the controls as indicators or

warning devices to tell the user at what state the process is at. These can

take the form of LED display light as visual display or an audio sounder to

indicate the process is complete. This facility can be used on water/liquid

heaters, kettles and coffee makers to indicate that the process is complete

or the kettle has boiled.

Auto diagnostics of sensing probes is a standard feature which

enables the circuits to decide whether or not to switch on the power to the

elements. This facility is essential in water/liquid heaters as it is a very

good safeguard which prevents possible fire and or fire or smoke damage

in houses and business premises when water/liquid heaters are left switched on without any water/liquid in the vessel to be heated.

The circuitry will facilitate zero crossing control of all on/off switching

operations. This has the benefit of eliminating interference on all other

circuits in the house/premises and facilitates the long lasting of the controls

in all electrical products as the contacts will be under less demand when

switched closed or open circuit.

A remote operation status of control unit could be used using an RF

data link.

FEATURES WHICH CAN BE MADE AVAILABLE ON THE ABOVE CONTROL

CIRCUITS ARE AS FOLLOWS:

SHOWER HEAT SELECTION SWITCH. HIGH MEDIUM OR LOW.

WATER FLOW CONTROL RATE IN SHOWER THROUGH CONTROL OF PUMP

SPEED.

KETTLE SELECTION SWITCH FOR TEA 100° C OR COFFEE 86 °C.

AT TEMPERATURE VISUAL OR AUDIBLE WARNING FACILITY.

WATER LEVEL PROBE SENSING TO ENSURE NO HEAT IS APPLIED IF NO

WATER PRESENT.

THREE TRIAC DRIVE CIRCUITS.

PRESET TEMPERATURE CONTROLS ON THE SOFTWARE FOR TEA/COFFEE

MAKER.

POTENTIOMETER CONTROLLER FOR SHOWER TEMPERATURE. SOFTWARE DIAGNOSTIC ROUTINE TO MONITOR SENSORS AND

INDICATORS.

It is of course to be understood that the invention is not intended to

be restricted to the details of the above embodiment which are described by way of example only.

Claims

1 . A control system for electrical heating equipment for heating a

medium, comprising an electrical power supply switching circuit, a sensor

for monitoring a thermal parameter of the medium, and a control device for

controlling the switching circuit in relation to said thermal parameter as

sensed with the sensor, wherein the control device comprises programmable

processing means, and the switching circuit has power output control

properties.

2. A system according to claim 1 wherein the sensor comprises a

temperature sensor.

3. A system according to claim 1 or 2 including an input device

connected to the processor for preselection of a desired temperature to be

attained with the heating equipment.

4. A system according to any one of claims 1 to 3 wherein the

switching circuit comprises a triac circuit.

5. A system according to any one of claims 1 to 4 further including a

fluid flow sensor.

6. A system according to any one of claims 1 to 5 further including a

reflected radiation sensor.

7. A system according to any one of claims 1 to 6 wherein the control -14- device also monitors time elapse.

8. A system according to any one of claims 1 to 7 when used with

water heating equipment.

9, A system according to any one of claims 1 to 7 when used with

cooking equipment.