GB2289551A - Method of electrical heating control - Google Patents

Abstract

A method of electrical heating control for A.C. powered devices involves resolving power to that delivered in one half-cycle of the sinusoidal supply. This provides reliable, readily interfacible control to any desired accuracy and power, without risk of generating Electromagnetic disturbances, emitted or conducted. The use of parallel or cascading configurations of heating element combination ensures no minimum limit on power resolution or maximum limit on power handling capacity under electronic control.

Description

PATENT APPIICAIION Title: The Control ofElectrical Heating Devices Powered Bv an Alternating Current Supply This claim pertains to the method of control used for elements or devices designed to provide heat by the flow of an alternating electrical current. The voltage and frequency of the supplied current is typically that ofthe national norm, for example; 240 volts ( soon to become 230 volts ) at an oscillation frequency of 50 Hz for the U.K This claim is not to preclude any voltage or frequency that may be deemed suitable for any specific purpose.

The power to the heating device is controlled using a solid state semiconductor device known as a 'Triac'. The use of such a device is common and need not be explained further for the purposes ofthis document. The originality ofthe control means is based on the combination of the following attributes: i) The control ofthe Triac is synchronised to the frequency ofthe alternating current supply, in real time.

ii) The power control is therefore resolved to that of one half-cycle ofthe sinusoidal current supply.

iii) By driving the Triac using a readily available 'Zero Crossing' device the control ofthe heating system, regardless of the power being switched, does not produce or incur any electromagnetic emissions. Currently an area of concern in electrical appliances, as compulsory regulations are due to be enforced from 1996.

iv) Provision oftemperature measurement feedback to the control system, which then applies a simple, but unique temperature controlling Algorithm, completes the control loop.

The resolution ofthe system control outlined above can be determined as may be required for the specific purpose. The choice of measurement transducer is wide and selection can be made on any number of characteristics or requirements. Typically thermistors, thermocouples, platinum resistance devices or proprietary integrated parts may be used to provide a variation in voltage proportional to the temperature. This can then be measured or 'compared' as a feedback input to the control system. See Fig 1.

The drive method devised is to produce a drive window equal to a multiple ofA.C.

half-cycles, 'n', during which the control system can provide power, from no half-cycles in 'n' up to 'n' in 'n' or 100%, incremented in half-cycle steps. The power resolution then becomes lJn x 1000/o. See Fig 2.

It is aclalowledged that there are two established drive methods which may utilise a Triac device, namely; a) 'Burst Fire' and b) 'Phase Controltor 'Proportional Control' The claimed method differs as detailed: a) 'Burst Fire' control is not intentionally synchronised to the mains frequency, other than by the inherent action ofthe Triac component itself The Tire' or 'On' period is not controlled as a wholly repeatable quantum of power as defined by a resolution to one half cycle of the supply fi-equenc-. The arrival of the Triac gate firing pulse is typically 'event' driven If timed, the timing is likely to be of an asynchronous nature in respect of the AC. half cycle.

b) 'Phase' o? Proportional' Control involves the control ofpower within each and every AC.

half-cycle . If the Triac 'gate' or 'firing' pulse occurs at the beginning ofthe half cycle, frill power or at least 94% offlill power is delivered. As the firing pulse is delayed from the start of the half cycle, after each zero crossing point > the power delivered is reduced in proportion to the area under the sinusoidal curve, to aminimum of zero at an approximate delay of 94% of the half-cycle period.

The major drawback of this control method is the generation ofboth radiating and conducted emission caused by the instantaneous rising edge occurring when the Triac is switched on during a half-cycle. The amount of emission and the difficulties of controlling the effects increase with the power switched. The undesirable electromagnetic effects require filtering and suppression in order to comply with regulations and such measures become increasingly difficult and expensive as the power switched increases.

Control Detail The control electronics can be configured either as discrete or integrated logic, or, as a programmable system using a microprocessor or microcontroller. The use ofprocessing systems obviously allows for detailed flexibility ofresponse, a degree of customisation and complex interfacing of heating control as part of a larger programme. All variations would employ the basic temperature control Algorithm, detailed as follows.

Having established a 'drive window' comprising a number ofA.C. half-cycles the power is then delivered in the form of a synchronous 'mark-space' within that window.

At a regular interval, not less than one drive window, the control system will act to increment or decrement the power input dependent on the measured difference between the actual temperature and the desired or demanded temperature. If the actual temperature is below that required, the power input is incremented by aminimum of one half-cycle. If the actual temperature is above that required, the power input is decremented by aminimum of one half-cycle. The interval of action, or 'Action Interval' is typically a multiple of drive window lengths derived by empirical means. The exact period is likely to be dependent on the response ofthe temperature measurement device(s) employed. The delav in response created by the 'action interval' provides the necessary damping to the system.

The vital addition to this regular control is that of a 'kick pulse'. As soon as the actual temperature passes tllrough the required 'set point, from above or below the direction or 'sign' of the temperature difference or 'error will change. Wllellever this challge occurs the control will cause an opposing increment or decrement to occur immediately, within the current or subsequent drive window irrespective or asynchronous with respect to the 'action interval'. lii practical terms this would equate to the next controlling edge within the drive window.

See Fig 3.

This deceptively simple Algorithm, or method, is able to control temperatures to a high degree of accuracy in a dynamic and volatile environment seeking to achieve steady state conditions at all times. Iii practical situations the exact confituratiots ofthe vsiffiles within the system is complex and cannot be flirther specified or detailed. Further additions or embelislmiellts will be required to modify the systems behaviour. Safety requirements may necessitate svstem behaviour modification. for example.By definition this will impinge on the specific system response or activity, but such additions or complications should not be considered sufficient to detract from the core configuration and control Algorithm, as detailed.

Ifnecessary, the control can be further enhanced by 'cascading' the particular power input requirement by creating a set of elements in a binary ratio. For example, using simple numbers: A 7Kw heating requirement can be supplied using a set, or group, ofthree independently controlled elements rated at; 1Kw, 2Kw and 4Kw. The control system can then be configured to increment through the array in binary manner. By adopting this drive configuration the power input control resolution is improved by a factor better than six.

i.e.: Assuming a drive window of 15 A.C. half-cycles.

1 x 7Kw element can be resolved to: 466 watts, or 6.66 input power.

1 x 1Kw +1 x 2Kw + 1 x 4Kw can be resolved to: 66.7 watts, or 0.95 ó input power.

This cascading method can therefore be applied to the control of large power requirements to a high level of accuracy or equally well to low power systems giving a phenomenally fine control.

An ideal example of how this system can be implemented to improve existing control methods is that ofthe Instantaneous Electric Shower.

The common control method of an instantaneous electric shower involves a fixed power input to the system, typically a 'low' or 'high' option setting. The water output temperature is then controlled purely by the water flow rate through tile system. Iliis method requires a mechanical regulating device which attempts to maintain a constant flow rate irrespective of input water pressure.In practice the regulating device is often poor at regulation and is unable to cope with anything but very small pressure changes. Most systems also assume that the ambient temperature ofthe input water is constant, during a typical shower period this is not necessarily true. Seasonal variation of input water ambient temperature is quite substantial and adjustments have to be made accordingly.

if the devised system is applied to the instantaneous electric shower situation it will provide temperature regulation regardless of changes in water pressure or ambient temperature. Ifsufficient power input capacity is provided, the water flow rate can be adjusted to suit an individual requirement the control system automatically adapting to the change. With this facility the previous fixed power consumption of 'high' and 'low' is no longer a prerequisite. The flow rate can be adjusted independent ofthe desired temperature allowing the user to effect the quantity ofpower used and therefore the cost. In simple terms the user is allowed to control variables which effect the shower comfort while the technical detail becomes automatic and imrisible.

The ability ofthis 'solid state' system to be so adaptable, easily interfaced with digital or software control, providing ultimate control over temperature and power, without incurring E.M.C. penalties makes the concept very powedill and wide ranging in its usefiilness.

Claims (4)

1. A method of electrical heating control comprising a Triac power switching device

controlled to operate in synchronisation with the AC. supply frequency such the power delivered to a heating device can be repeatedly resolved to a minimum unit equal to the power delivered in one half cycle of the supply. The resulting power control and delivery being intentionally benign in respect ofthe production of significant Electromagnetic Disturbances both conducted or radiated.

2. A method ofelectrical heating control, as claimed in claim 1, wherein the power to the heating device or element can be controlled by electronic means to attempt to achieve a constant desired performance in respect of measurements proportional to the heating effect thereof, i.e. temperature control using feedback measurement.

3. A method of electrical heating control, as claimed in claims 1 and 2, wherein the control method is such that response to parametric change in the controlling environment is continuous and comparatively fast acting, seeking to achieve 'steady state' condition at all times.

4. A method of electrical heating control, as claimed in any preceding claims wherein the control is required and acts only to control the proportion of power delivered to the heating device irrespective of any proportional measurement or feedback variable.

5 A niediod of electrical heating control, as in any preceding claim, wherein the control method may be 'cascaded' over a number of individually controlled heating elements, of differing but related multiples of power, such that higher resolutions ofpower control can he achieved.