Control and Supervisory Systems for Windscreen Forming

Over the last decade the use of glass panels in cars has increased rapidly along with the complexity of shape and integration of sensors.

With the automotive industry demanding more and more stringent tolerances and introducing more complex forms and shapes, the production process of automotive glass panels has become extremely complex.

Resulting, for example in a Tunnel furnace, having hundreds of sub zones, requiring precise and complex temperature control running multiple recipes to allow maximum production flexibility. Resistive elements as well as medium wave infra red heaters have to be controlled to very tight parameters whilst maximizing energy efficiency.

Eurotherm temperature control, power control and supervisory systems for windscreen forming are in operation around the world and provide:

• Accurate multi-zone temperature control
• Accurate open or closed loop power control
• Advanced single cycling for infra-red elements, eliminating DC components
• Power control set point transfer and monitoring by digital communication, reducing the I/O requirements of the control system
• Feed through control
• Track and Trace capability

Eurotherm has always been present and successful in power glass applications such as:

• Flat glass: roof heating, annealing
• Bottle manufacturing: boosting, feeder heating
• Glass wire manufacturing: boosting, feeder heating, bushing
• Windscreen forming

The principle of windscreen forming is that the flat windscreen is going through a tunnel furnace on a very precise form. There is one type of form per type of windscreen. When at the right temperature, the soft glass will take accurately the form banding by gravity.

The first part of the furnace is the pre-heating zone, temperature controlled. Second part is the forming zone with a precise electrical power control. Heating elements are metallic resistors, medium wave infra-red, with a low temperature coefficient. Heat transfer to the windscreen is a combination of air heating and infra-red emission.

The quality and precision of the windscreen banding is given by the accuracy of the pre-heating zone temperature control and the accuracy of forming zone power control.

For such furnaces, the number of zones can be very high, 100 to 300 zones. Present tendency is to increase the number of zones to improve precision of banding and to manufacture more complicated and various windscreen shapes. Total power of such furnaces ranges from several hundred kW to several MW.

Architecture of installation

The architecture of the installation in terms of electrical control as follows:

• A PLC supervisory system is controlling the furnace: temperature in pre-heating, zone, power informing zone.
• Heating elements power control is made with multi-channel communicating thyristor power units, EPower Series for both pre-heating and forming zones.
• Firing mode: single cycle.
• Control mode: precise power U x I, very important for banding accuracy.

Through communication link (Modbus or Profibus depending on type of PLC). All EPower parameters are available (read and write) on the supervisory system: power set point, load output power, status word/alarms.

Fast set point transfer

On the furnace forming part, during process time each windscreen stays in a heating zone; its next working set point, for the next heating zone it will move into, is loaded in each EPower channel. When all the windscreens reach their next heating zone, a single command through the communication link will transfer all pre-loaded set points for each zone as an active set point, this being done in a few milliseconds.

Advantages of Eurotherm solution

• Thyristor firing mode: single cycle. Perfectly adapted for this application.
• Power control mode for a better process control
• Predictive load management avoids peak power demands.
• Multichannel EPower reduces installation cost: floor space saving and smaller panels, lower cost per channel.
• Very good accuracy of EPower power control U x I: absolutely necessary to get a better process control and windscreen banding accuracy.
• Communication
• Reduces cost of installation (wiring etc).
• Improves process control efficiency and quality.