New Technology Controls the Heat
Aline Medical Packaging, a division of Aline Heat Seal Corp. (Cerritos, CA), has introduced an impulse sealer that uses new technology to control temperature. Precise-Seal is a tabletop impulse sealer that can also be mounted on a movable stand.
The sealer is the result of a joint effort with Aromat, a subsidiary of Matsushita, and Donald Barcan, a medical device package engineering consultant and founder of Donbar Industries Inc. The equipment now incorporates a touch screen and PLC, which maintains precise temperature control of the heating element to ±1ºF without overshooting. The touch screen and controller combine all the functions of the normal impulse sealing process (e.g., temperature, heat time, cooling temperature, pressure, and vacuum, where installed). Unique programming of the PLC is necessary to provide this precise control.
Current methods of controlling temperature on impulse sealers are straight time, thermocouple on the heating
element, and control using heating element resistance. Straight time has been abandoned for the medical device packaging community because it does not offer precise process control of temperature, Barcan says. This system does not control temperature, but rather the time power is applied across the heating element. The result is in-consistency of the heating element’s surface temperature as the resistance of the heating element changes over time.
|The Precise-Seal uses a touch screen and PLC to help control temperature.|
A thermocouple on the heating element provides for accurate control of the heating element temperature but is limited by the controller’s ability to sense and respond to temperature fluctuations. There are two primary types of controllers: on-off and proportioning types. Though they control temperature during the process, cycle duration is a combination of temperature control and an external timer. This process, though adequate for many applications, is limited by the thermocouple and controller response time.
Finally, heating element resistance is an alternative to temperature control. It is a process of monitoring and controlling temperature by monitoring the resistance and current of the heating element. Though accurate and reproducible, the process requires a special calibrated heating element that is uniform from a resistance standpoint. As the temperature of the heating element increases, the resistance increases. The process controls temperature by monitoring resistance and not actually reading the temperature of the heating element.
Precise-Seal combines the advantages of the proportioning thermocouple controller with the functions of heat dwell, cooling dwell, bar pressure, and vacuum for snorkel or chamber machines (when equipped). The key to the system is an extremely fast temperature control based on I-PD (integral, proportional, and derivative) implemented on the PLC, using the following three technologies.
Phase control. The PLC output signal to turn on and off power to the heating element is constantly in phase with the ac power. Phase control repeatedly gives a consistent level to the output power applied to the heating element. If phase control is not used, the controller randomly chops the power at any part of the ac cycle. Sometimes, it may turn on when the level is close to zero cross point, getting low power; or when it is near peak, getting too much power.
Phase control means that the controller is capable of always knowing ahead of time what stage or level the incoming ac power will be at, on each half cycle (8.333 milliseconds) if it is turned on. It gives a pulse signal of about 400 milliseconds to the SSR to turn it on, and when the ac power becomes zero (zero crossing) the SSR turns off by itself and the process is repeated on every half cycle.
When the cycle time is long enough, like 1000 or 2000 milliseconds, the error is small. But when trying to do I-PD every 20 milliseconds, the error is significantly high. Using phase control, it is possible to always control the amount of power applied to the heater on every half cycle of ac signal exactly at the same position, maintaining it in a steady state. This provides consistent energy and is fundamental to control fast-reacting temperature accurately.
Without the ability to control the energy level consistently, it is impossible to obtain and maintain the temperature set point accurately.
Floating point calculated PID algorithm with built-in auto tuning capability. The PLC from Aromat/Matsushita used in this machine has proven PID (proportional, integral, derivative) and I-PD algorithm. The autotuning runs and the gain is adjusted to increase process speed-up. If the controller does not have good PID algorithm, or if the PLC does not have floating point calculated PID (for example on an integer PID, 1÷3 = 0, not 0.3333), it is difficult to get good results.
Fast PID algorithm. Even with a well-designed algorithm and floating point calculated PID, it is necessary to process PID in floating point quickly. The Aromat/Matsushita PLC calculates accurate floating point PID in 32 milliseconds. The PLC has the ability to update the PID output every 0.5 milliseconds. The fast reacting thermocouple and thermocouple reading device gives 17-millisecond updates. With this combination, it is possible to update the PID output every 20 milli-seconds. If the temperature, for example, usually gets from the ambient temperature of 70º F to the high target of 350º F within a matter of less than 2 seconds (2000 milliseconds), it means that the PID algorithm updates the output 100 times. This gives enough number of cycles for PLC to correct the process temperature.