Product quality and productivity always have been the top goals for production systems. Because infrared heat processing systems can provide high energy transfer efficiencies, the technology is a viable choice for drying, curing, baking and other process heating applications. For consistent, repetitive results, proper control of the process is crucial.
Two types of control schemes generally are used:
- Open-loop control, where the energy set for the system power is done manually, without any feedback from the heaters or process.
- Closed-loop control, where a measurement is taken from the process to provide a reference point for the process to remain stable.
Examples of open-loop control include:
- A heater connected directly to a power source.
- Heaters connected to a manual, adjustable power supply such as a silicon-controlled rectifier (SCR).
- Any device that requires the operator to manually set up the system based on the operator’s judgment without the aid of process information.
The advantage of open-loop control is simplicity. The disadvantage is that the plant voltage supply to the system often changes during the course of the day, week, month or season. For this reason, the power levels in the process also change unless the power source has voltage stabilization built in. In addition, plant ambient temperatures will affect the process, and changes in the entry temperature of the product may also change the final result. And, of course, operator error may be a concern.
Closed-loop control systems rely on a sensor to check the actual operation of the process. Sensors can be used to monitor variables such as:
- The entry temperature of the product.
- The ambient temperature in the plant.
- The temperature of the heat source.
- The voltage supply.
- The moisture content level of the product.
- Exit or interim temperature of the product.
For a stable process, closed-loop control of the heaters is accomplished using a thermocouple that measures the source temperature. Such a control scheme will provide a stable output day in and day out. As product moves through the system, the thermocouple senses the heater temperature dropping due to the energy absorbed by the parts. Then, the system compensates for the energy usage by increasing the energy to the heater, thereby maintaining the set temperature. Thermocouple placement is critical to maintain close coupling between the heater and process. The thermocouple should responsively track the actual heater temperature.
A refinement of the thermocouple closed-loop control is to use a device to measure the actual product temperature at the oven exit as well as intermediate positions within the process, if critical. Some products absorb more infrared energy at specific wavelengths. In these situations, the initial zones can be set to the heater temperature that most closely couples the transfer of energy to the product as set by a thermocouple in the heater. In the final zone, the control modulates the power to achieve desired end-product temperature. Similar schemes can be used when measuring other process variables such as product moisture, gloss or color match.
Zone Temperature Control with Infrared Heating
Zone control comes into play for two reasons. For many products, the process involves raising the product to a desired temperature and then holding it at temperature for a prescribed period of time. Other processes may require multiple interim steps to achieve the desired results. Both of these desired temperature profiles can be accomplished with zone heating. Zone control provides the ability to change the power/temperature of zones in the direction the product is moving or, if in a fixed position, changing temperature during the cycle to help achieve the profile.
With continuous-web applications, edge effects may cause the center of the web material to be hotter than the edges. In these situations, closed-loop control can be used to set the edge zone temperatures higher and the center zones lower to improve the uniformity of the end product.
Zoning also is used to correct an imbalance across a web that may have been introduced into the product from a prior process. For example, often the moisture level is not uniform across a roll of coated or coated paper. Selectively controlling the heater zones across the product helps increase uniformity, thus leveling the moisture content.
Sensors for measuring heater temperatures used in closed-loop control include:
- Thermocouples, which generate small voltages that change with temperature.
- RTDs, which change in resistance with temperature.
- Noncontact temperature devices such as optical pyrometers or thermopiles, which measure the product temperature directly and optically. Infrared scanners can continuously and automatically scan multiple points across a continuous web of material, creating a temperature map.
In addition, moisture-measuring systems can detect high and low moisture content in the target and send information back to the heater control system. The control system uses this moisture-level information to provide energy where it is needed.
Power controls provide the electrical energy to the heaters. These can be simple switches or contactors to turn power on or off via a mechanical means. They can utilize contacts that can arc and wear out, solid-state relays/contactors, which can switch the power electronically, or SCR power controls. Solid-state relays/contactors are available so that they change from no power to full power when the sine wave crosses the zero point. Alternating current provides electricity so that the power alternates (changes) from plus to minus 60 times a minute (in North America). If power is switched during the middle of a cycle (other than a zero point), it may cause heat, possibly creating noise on the line which may influence other devices and reduce life of the components. By switching the power at the point where the sine wave crosses the zero point (see illustration), no power is switched and little noise is generated on the line.
Power Controls for Infrared Heating
SCR power controls are available as zero-voltage or phase-angle firing units. Zero-voltage controls proportionally turn on and off each full cycle: The heater is full on or full off. Phase-angle firing units “chop” of the top of the sine wave, producing an even level of power to the heaters. (Phase-angle firing is similar to the way a wall dimmer works on your lights at home.) Unless properly filtered, this type of control also can introduce line noise that may influence other equipment.
In addition, smart SCR power controllers are available to improve communications with the brains of the control system such as a PLC.
Variables that influence the choice of the power control type include:
- The product being processed.
- The speed of the product through the heated zones.
- The type of heater used.
For instance, a low mass product will be sensitive to heater temperature changes. Thus, such a product needs a power supply that is stable. This can achieved with a phase-angle firing SCR or a zero-voltage firing device switching on a very fast cycle. A low mass, fast-responding heater also may benefit from this type of control.
Larger mass products or heaters that have a slower response are less affected by variations that may be provided from the power control. The brains of the system — whether it is a standalone temperature controller, PLC or computer system — will either be in an on/off mode, time-proportioning mode, direct power or a similar scheme.
The on/off mode provides the least process control. With this control scheme, if the heater or process temperature drops below the setpoint, the heater comes on. When the process temperature exceeds the setpoint, the heater goes off. Often, there is a small bandwidth or temperature range that allows time for the heater to stay on instead of just quickly cycling. This allows some equalization of the process. A home thermostat works similar to the on/off control scheme.
A time-proportioning control scheme will set a bandwidth dependent upon the responsiveness of the heater. The scheme is designed so that when the process temperature is below the lower limit, the heater is 100 percent on. Likewise, when the process temperature is above the upper limit, the heater is 100 percent off. Within the bandwidth range, however, the device is pulsed on and off — possibly very rapidly — to hold the required process temperature. Both solid-state relays (SSRs) and zero-voltage firing SCRs work in this fashion. Phase-angle SCRs will constantly modulate the power to the heaters without an on/off cycle.
Heaters with low masses and fast response times include tungsten halogen lamps, carbon-fiber quartz-tube heaters and foil and ribbon heaters. Standard quartz-tube heaters also are relatively fast but not as responsive as the heaters above. Flat-panel heaters generally have higher mass, resulting in slower response times both up and down. They can use a simpler control scheme to maintain a uniform output. Each heater is different, and your supplier can assist you in selecting the proper sensors and power controls.
The Role of the Operator
Because your machine operator makes or breaks the process, making his job easier helps you. Manual controls that just set a power level or turn heaters on/off rely heavily upon the operator to steadily monitor the process and manually make the adjustments as needed. Moving to closed-loop control and standalone temperature controllers that display the setpoint temperature and the actual temperature work well.
If there are multiple zones in the system and many different products run on the line, however, the operator must correctly input information into each temperature controller. Programmable logic controllers (PLCs) are cost effective, and they allow for individual recipes to be set up for each product. These recipes can include the temperature settings as well as other variables for process like line speed and airflow. A PLC can accept multiple sensors, a feature that can be used to create a scheme for each product. Error information can be displayed and recorded. The operator only has to input a recipe number to set up the entire line. Graphic displays can be provided on the PLC for the process. Process records can be stored and uploaded for quality control history, plus PLCs can be set up to communicate with each other over a network with data made available to management in their offices. Supervisory control and data acquisition (SCADA) systems can interface with PLCs for overall plant process control.
Thus, a properly controlled electric infrared system can yield a more efficient operation. Benefits include saving energy, enjoying precise and repetitive process control, improved product quality and consistency, and easy operator setup. The control system can incorporate alarms to alert the operator if part of the process is not within specification and keep a record of the process for quality purposes. Overall, with these improvements, cost of operation will be reduced, rejects will be reduced and customers will be satisfied with your product quality and consistency.