BACKGROUND TO THE INVENTION Heaters, for example, high power heaters are used in industrial printers to dry and cure printed images. These heaters are driven by complex electronics that handle a large amount of power with very high accuracy to obtain the necessary temperature profiles to obtain a image quality. In existing systems, this requirement translates into a very fast adjustment servo control. However, another requirement of such systems is to handle several kilo-watt of power without disturbing the power network and complying with regulatory legislations with respect to flicker and harmonics.

BRIEF DESCRIPTION OF DRAWINGS For a more complete understanding, reference is now made to the following description taken in conjunction with the accompanying drawings in which:

Figure 1 is a flowchart of an example of a method of controlling a heating system; Figure 2 is a flowchart of more detail of the example of a method of controlling a heating system;

Figure 3 is a simplified schematic of an example of a heating system; Figures 4a to 4d is an illustration of operation of an example of a heating system;

Figure 5 is a simplified schematic of an example of a control module of the heating system of Figure 3; Figure 6 is a simplified schematic of an example of a control module of the heating system of Figure 5;

Figures 7a to 7c is an illustration of another example of operation of the heating system of Figure 6; and Figures 8a and 8b is an illustration of yet another example of operation of the heating system of Figure 6.

DETAILED DESCRIPTION

To minimise the effects of flicker in high power heater circuits, the power within a power line cycle may be adjusted, for example, phase control, chopper, sinewave. The AC sinewave of the power source is converted to a controlled sinewave adjusted to the power that the heating elements needs. The slope of the sinewave may be modified, solving the flicker problems that cause the commutation of large heating elements. These kinds of converters have two limitations in that they are very complex and that they have difficulty meeting harmonics power network regulations since they produce lots of distortions to the line. To fix these problems bulky line filters are required. An On/Off system simply switches on and off the power using a triac (switch) element. Although less complex, it is difficult to meet flicker power network regulations as it can only switch the heater element at a very slow rate. This translates into a very inaccurate temperature adjustment, thus resulting in poor cured/dried image, and hence poor printout quality.

With reference to Figure 1 , the heating system 100 comprises a heater 105. The heater 105 comprises a plurality of parallel heating elements 105_1 , 105_2, 105_3, 105_4. Although 4 heater elements 105_1 , 105_2, 105_3, 105_4 are illustrated in Figure 1 , it can be appreciated that the heater 105 may comprises any number of parallel heating elements. The heater 105 is connected to an ac power source 101 via a control module 103. The ac power source 101 is configured to supply ac power to each of the plurality of parallel heating elements 105_1 , 105_2, 105_3, 105_4. Therefore, the heater is divided into a plurality of heating elements which can be spatially distributed over a heating zone, for example, a printzone in a printer. The positions of the heating elements can be finely tuned in order to provide the desired heat distribution over the heating zone of the printer and the activation sequence of the heating elements can be controlled based on real time line power measurements and the heating requirements.

With reference to Figures 2 and 3, the control module 103 is configured to switch, 201 each of the plurality of heating elements on/off at zero crossings, 203, of the ac power source such that the number of the plurality of heating elements that are activated at any one time remains constant. The temperature profile of the heater can be controlled by switching appropriate heating elements having appropriate power and also the sequence of switching can be controlled top produced a desired, predetermined temperature profile across a zone depending on the power of individual elements and their position within the heating zone.

Further a target power P _T for the heater 105 may be provided and the actual temperature P _H of the heater 105 is determined, for example, from the voltage of the ac power source and time that individual heating elements are activated. Further, a target temperature T _T for the heater 105 may be provided and the actual temperature T _H of the heater 105 may be measured. This may be achieved by a temperature sensor provided in the vicinity of or on each heating element providing a measurement of the temperature of each heating element. The target power P _T and the actual power P _H are compared, 205. If it is determined, 205, that the actual power P _H of the heater 105 and the target power P _T is substantially equal, the process then returns to step 203 maintaining activation of the same number of the plurality of heating elements and switching any one of the heater elements 105_1 , 105_2, 105_3, 105_4 off or on at zero crossings whilst maintaining the same number of heater elements on. The actual power P _H may be determined based on the number of heating elements activated and the power output of each heating element. If it is determined, 205, that the actual power P _H of the heater 105 and the target power P _T is not substantially equal and a zero crossing has been reached 209, the number of activated heater is adjusted, 21 1 , until the actual power P _H of the heater 105 is substantially equal to the target power P _T . The switching, 201 , is then carried out to maintain a constant number of heating elements activated at the new dajsute3d number of heating elements.

In addition, the target temperature T _T and the actual temperature T _H of the heater 105 are compared. If it is determined that the actual temperature T _H is different to the target temperature T _T (for example, depending on the requirements of the heating zone, the temperature may be considered different for a variation greeter that 1°C or 5°C), the target power P _T is adjusted (not shown in Figure 3). Figures 4a to 4d is an illustration of operation of an example of a heating system. In this example, the heater 105 comprises 4 heating elements 105_1 , 105_2, 105_3, 105_4. The heating elements 105_1 , 105_2, 105_3, 105_4 are a resistive type of heating element, each comprising respective resistor elements 401_1 , 401_2, 401_3, 401_4. Further each of the heater elements 105_1 , 105_2, 105_3, 105_4 provide substantially the same wattage output.

As illustrated in Figure 4b, to achieve an output of 25% of the total wattage output of the heater of Figure 4a. For an ac power source having a period of t _ac , each restive element 401_1 to 401_4 of each heating element is switch on for half a period of each alternate cycle. For example, the first resistive element 401_1 is switched on

(activated) for the positive half wave of every other cycle of the ac power source, the second resistive element 401_2 is switched on (activated) for the negative half wave of every other cycle of the ac power source, the third resistive element 401_3 is switched on (activated for the positive half wave of every other cycle of the ac power source, alternate to the cycle in which the first and second resistive elements are activated, and the fourth resistive element 401_4 is switched on (activated) for the negative half wave of the cycle of the ac power source in which the third resistive element is activated. Therefore the number of heating elements that is switched on at any one time is constant at 1 heating element. As illustrated in Figure 4c, to achieve an output of 50% of the total wattage output of the heating system module of Figure 4a. Each restive element 401_1 to 401_4 of each heating element is switch on for a time interval corresponding to the period t _ac of the ac power source for each alternate cycle. For example, the first resistive element 401_1 and the second resistive element 401_2 are switched on (activated) for the first cycle of the ac power source and each alternate cycle thereafter and the third and fourth resistive elements 401_3, 401_4 are switched on (activated) for the second cycle of the ac power source (whilst the first and second restive elements are switch off) and each alternate cycle thereafter. Therefore the number of heating elements that is switched on at any one time is constant at 2 heating element to give 50% of the total wattage output.

As illustrated in Figure 4d, to achieve an output of 75% of the total wattage output of the heating system module of Figure 4a. . Each resistive element 401_1 to 401_4 of each heating element is switched on for a time interval corresponding to 1 .5 periods t _ac of the ac power source. For example, the first resistive element 401_1 is switched on (activated) for the positive half wave of a first cycle of the ac power source and is then activated for the complete period of the second cycle, the second resistive element 401_2 is switched on (activated) for the first cycle of the ac power source and the negative half wave of the second cycle, the third resistive elements 401_3 is switched on (activated) for the first cycle of the ac power source and the positive wave of the second cycle and the fourth restive element 401_4 is switched on for the negative wave of the first cycle of the ac power source and the complete period of the second cycle. Therefore the number of heating elements that is switched on at any one time is constant at 3 heating element to give 75% of the total wattage output. In these example, each heating element is switch on or off at a zero crossing of the ac power source to reduce flicker and control the temperature of the heater 105 with less ripple whilst enabling commuting at a maximum speed. The heating elements 105_1 to 105_4 can be commuted to maintain flicker and harmonics levels within permitted ranges. Further, since each heating element is switch on or off at zero crossings of the ac power source, such that flicker is reduced, triacs can be utilised to control switching of the elements.

The control module 103 is shown in more detail in Figure 5. The control module 103 comprises a temperature input module 501 connected to a first summer 503 and a second summer 505 in series. The output of the second summer 505 is connected to the input of a first controller 507. A second controller 51 1 of a first feedback loop is provided to the first summer 503. A third controller 509 of a second feedback loop is provided to the second summer 505. The second controller 51 1 provides actual temperature of the heater 105 to the first summer 503. This is compared to the target temperature at the first summer 503. The first controller 507 controls the switching of the heating elements at each zero crossing of the ac power source such that the number of the plurality of heating elements that are activated at any one time remains constant for a predetermined time interval which may be substantially greater than the period of the ac power source. In order to achieve the target power and the distribution of heating across the zone defined by the heating elements.

The third controller 509 provides the actual power of the heater to the second summer 505 which compares the actual power and the target power defined by the temperature differences of the first summer 503. The output of the second summer 505 is provided to the first controller 507 for adjusting the number of heating elements being sw2itched and controlled by the first controller 507. In more detail, with reference to Figure 6, a target temperature input 601 provides the target temperature T _T to the input module 501 from the settings of a printer control (not shown here). An AC input 603, AC frequency input 605, AC zero crossings input 607 and a soft start input 609 provide the voltage of the AC power source, the frequency of the AC power source, the location of the zero crossings of the AC power source and input of any changes to the printer control settings to the input module 501. The AC zero crossing input 607 provides the switching points for the heating elements 105_1 to 105_8. The first to third controllers 507, 509, 51 1 are connected to a storage device 61 1 which stores the flicker curves, number of heating elements and power requirements, for example, the power needed to be dissipated in each heating element so that the N heating elements may be controlled separately; having a more versatile control that allows modification of the temperature profile. In the example shown in Figures 7a to 7c, a zero crossing period (the time between successive zero crossings of the ac power source is 10ms for a 50 Hz power source and 806ms for a 60Hz power source. As illustrated in Figure 7a, 8 resistive heating elements R1 to R8 are switched at zero crossings such that 5 heaters are activated at any one time providing an actual power that is 62% of the total power. However, let's assume that the target power is actually 70% of the total power. This is achieved, for example, as shown in Figure 7b by adjusting the number of activated heating elements between 5 and 6 for a longer period greater than the interval between successive zero crossings. In this example, the interval at which a constant 5 heating elements are activated at any one time is 38% of the total period t _T and a constant 6 heaters are activated for 62% of the total period t _T .

Increased flexibility in the printzone profile can be achieved particularly for large format page wide array printers to dry and cure the Ink from the media. This is because only the printzone required is heated, as illustrated in the example of Figures 8a to 8c. In this example of 8 resistive heating elements R1 to R8, only R1 to R5 are activated at any one time such that a required temperature profile across the printzone shown in Figure 8b is achieved. While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.