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Control Systems

Ceramicx provide customised control solutions to suit specific heating applications. These solutions can vary in size from simple single zone open loop systems to large complex multi-zone installations with temperature feedback and closed loop control.

The objective in most process heating applications is to match the installed heating load to the process heat requirement in so far as possible. Usually, the power requirement is over estimated to ensure there is always adequate capacity to allow for process variations and in some cases to provide a fast thermal response. For this reason, control can be critical. The ability to control heat energy output as required allows the heating process to be optimised. With all radiant heat transfer applications there is a limit to the amount of energy which can be absorbed by the target over a specific period of time. Modern manufacturing requires short process times with minimal energy usage. Applying excess heat introduces the risk of surface scorch making it critical that the heater output can be tuned to provide both process speed and high quality results.

Control Options

Heating control solutions can be categorised as follows

• Individual zone controllers
• Centralized PLC based control
• Open/closed loop

Individual Zone Controllers

Individual zone controllers are generally suited to smaller less complex systems with a typical zone count of 15 or less. It involves using individual PID temperature controllers for each temperature control zone required. Each controller is individually configured and used to provide a control signal to a solid state relay or similar power switching device. Solid state relays (commonly known as SSR’s) are solid state switching devices which normally use back to back thyristors/SCR’s or triacs for switching AC loads. Ceramicx normally employ zero cross SSR’s which means the device will only switch on and off at the zero point in the sine wave of the ac supply which minimises the initial surge current and helps prevent electromagnetic interference (EMI). Another advantage of SSR’s over conventional mechanical relays is that they can perform millions of switching operations with excellent reliability when specified and utilised correctly.

Centralised PLC Based Control

One of the disadvantages with individual zone controllers is that each controller has to be individually configured. Although each controller is configured before leaving the factory, parameters such as setpoint (the desired temperature value) or % power (required zone output on a % time basis) will need to be set by the customer. For a low zone count this is not an issue but for larger systems (potentially containing hundreds of zones) this is not a practical solution where process conditions and/or requirements are frequently subject to change. In this case, PLC’s (programmable logic controllers) provide the best solution. Ceramicx use Siemens S7 series controllers as the standard control platform for larger multi-zone projects (AB on request). A single human machine interface (HMI) allows the operator to fully interact with the PLC which can be programmed to read all process values (temperatures), set process values, configure alarm thresholds and operator access levels. The PLC can be used to directly switch SSR’s similar to individual zone controllers and also facilitates the integration of multi-channel power switching hardware. These systems use the same switching technology as SSR’s except installed in cards with multiple outputs which can in turn be mounted in racks. The total package can be networked to the PLC and offers a considerable space saving inside the electrical enclosure, a reduction in wiring time, increased functionality and load diagnostics. Systems incorporating PLC’s can also be programmed to exchange digital IO with existing equipment or network directly to plant SCADA software.

Open/Closed Loop

Individual zone controllers and PLC based systems can be programmed to operate in both open and closed loop mode. Open loop mode is the simplest form of control and involves basic power control of the heating load without temperature feedback. Ceramicx normally uses time proportional control where the heating load is burst fired i.e. turned fully on for a period followed by an off period. For example, a setting of 50% with a 2s cycle time results in 1s on and 1s off. Closed loop control involves temperature control of either the heating system or the target object. Where closed loop control of the heating system is employed, a type K thermocouple is normally incorporated into the heating element. This provides temperature feedback to the controller which will then employ PID (proportional-integrative-derivative) control in order to operate the heater at the set value. Closed loop control of the process is also possible and normally implemented through the use of non-contact infrared thermometers (pyrometers) which can be used to directly measure product temperature.

Ceramicx design and build the control enclosure which includes the required control hardware for the control solution selected. Most low voltage supply networks can be catered for. Both heating and control systems are fully tested before leaving the factory to help ensure seamless integration when installed in the customer’s factory. Where closed loop control from the heater system is employed, Ceramicx ensure the PID functionality is tuned to provide close tolerance temperature control. The provision of “quick connect” plugs/sockets on the heating and control system where possible allow for a short installation time with minimal requirement for skilled electrical personnel.

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