Temperature is a widely used parameter in the control of various industrial processes, and one of the most common temperature sensors is the thermocouple. In some ways, a thermocouple is an extremely simple sensing element; it consists of two dissimilar metals joined together at a single point and, due to what’s known as the Seebeck effect, the junction of these two metals produces a voltage that varies with the temperature of the junction We’ll examine thermocouples in detail in a later chapter, but for now the important concept to understand is that the voltage it produces is very small, on the order of millivolts, and is frequently measured in the presence of significant levels of electronic noise, which may be on the order of hundreds of volts. Complicating matters is the fact that the temperature response of thermocouples is nonlinear, so a linearization operation usually must be performed before the temperature reading can be used. There are other serious challenges in using thermocouples, but these two are sufficient to illustrate how intelligent sensors can overcome these issues to provide
accurate readings in an extreme environment: an injection-molding machine.
For those readers unfamiliar with injection molding, it is a manufacturing process in which solid plastic pellets are heated to between 300°F and 900°F to melt them. The melted plastic is then injected into a mold under high pressure (on the order of 10,000–30,000 psi), and the plastic is then allowed to cool back to a solid in the shape of the mold. This process is repeated rapidly so that the manufacturer can make parts as quickly as possible. The key to running a successful injection-molding operation is to keep cycle times (the time it takes to open and close the mold once) short and scrap rates low. So long as a molder can produce good quality parts at a
profit per part, he essentially has the ability to “print money” based on the speed at which he can run his cycle. An important aspect is the proper regulation of the temperature of the plastic at various points throughout the molding machine, which requires the distribution of temperature sensors (thermocouples) at key points in the process.
Unfortunately, one of the drawbacks to using thermocouples is that the wire used to create them is expensive. Molding machines in general are not particularly small, and the machines employ multiple zones of temperature monitoring and control (250 zones or more in the larger systems). The thermocouple wires thus must be run long distances to their associated temperature controllers, resulting in the worst of all possible worlds: multiple strands of expensive wire that have to be run long distances. One pioneering company in the temperature-control field realized that they could save their customers a tremendous amount of money by digitizing the
temperature readings at the mold itself and then shipping the digitized readings to the controller via standard (and inexpensive) copper cables. Furthermore, they could do this for many channels of thermocouple readings and, since the thermocouple readings changed relatively slowly, the digitized readings could be time-multiplexed when sent to the controller. In the end, up to 96 channels of thermocouple data could be reported for each device, thus turning a costly, noise-prone system of long thermocouple wires into an easily managed single pair of copper wires.
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