Each of the heater circuits in the oven has its power output level controlled by a Proportional-Integral-Derivative servo algorithm. The temperature derived from a group of assigned "neighboring" thermocouples is compared to the desired "scheduled" temperature and power is applied accordingly.
The oven heater power control system provides for computer control of the temperature via 9 power panels, each panel controlling 30 heaters of 7800 watts. The heaters from each power panel are evenly distributed throughout the oven's base, walls and lid. The power is switched with Crydom solid-state relays (SSR) rated at 480 volts, 50 amps which are mounted on forced air cooled heat sinks.
Each microprocessor controlled power panel communicates with a main on-board VME computer by ASCII serial data through fiber optic cables. The VME computer contains the PID algorithm and tells the control panels which heaters to turn on or off. It then accepts the sense information if voltage is on to the heaters and if current is supplied both at the panel and at the heater. The command status is updated four times per second for each of the 30 solid state relay circuits. Current and voltage status are readback after each command. Each microcontroller is powered by the power source which supplies the heaters being controlled.
The current minute's power level for each heater element is broken into quarter second periods of full power or no power. The on periods are distributed throughout the minute so as to even the total load on each circuit of ten heater elements. Voltage and current sensors are used on each heater circuit to verify that the heater elements have indeed switched on or off at the moment commanded by the computer. Each of the power panel micros can turn any particular heater on or off every quarter second. Thus, the heaters have a dynamic range of 240 within a minute. To avoid surges of current use followed by the current being off for part of each minute, the power panel micro smooths the load so that a constant number of heaters (modulo 1) is on for each phase during the minute. When several heaters are demanding power for only a fraction of a minute, these are interlaced or alternated according to the parameter "Fase Epsilson" which gives the typical timescale that a heater circuit should be on during the minute.
Once a minute, a power level for each heater element is calculated from a weighted average of its neighboring thermocouple temperatures, and the temperature called for at the current point in the schedule of temperature versus time. A proportional, integral, derivative (PID) servo algorithm is used calculate the desired power level. A one quadrant servo is used since losses through the insulation of the furnace provide a significant cooling loss. There are separate temperature versus time schedules for each of several zones of heater elements.
The P-error (etmp - htmp) is multiplied by the P-coefficient to give the P-product in heater power units. This power request is "proportional" to the present error in temperature compared to the schedule. The same is done to get the I-product by integrating over a specified number of lookback minutes, and the D-product by seeing how the error has changed since the previous minute. If the sum of these three products is less than the Power Limit for that heater, this is the hpwr applied to the heater. If the sum is greater, then the request is clipped to be no greater than the Power Limit. If all the 10 heaters on a single phase request more heat than can be accomodated by Fase Max Load, then the hpwr request is clipped even further to keep the load within the limit for than phase. After all these calculations, the Requested Power (hpwr) reported is what actually got sent to the heater.