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A laboratory system for multiple point, closed-loop, surface temperature control has been developed to test control algorithms for manufacturing applications such as rapid prototyping laminate manufacture and rapid thermal processing of semiconductor wafers. Accurate surface temperature measurements with high spatial resolution are required to provide a signal for feedback control. Image acquisition and conversion to temperature must be fast enough to operate in conjunction with the control method used. Optical methods are a natural fit, since many spatially distributed measurements can be obtained rapidly with a minimum of complexity. The operation of thermochromic liquid crystals in a real time control loop is described. Every 200 msec, the temperature at each of 196 zones must be obtained. Two frames and multiple pixels per zone are averaged to reduce statistical uncertainty in the reading. An in situ calibrator was developed and used to study the errors inherent in this system, including errors due to hysteresis and uneven surface lighting. The statistical uncertainty in temperature measured due to the calibrator uncertainty varied with temperature, but was less than 0.03 degree(s)C over most of the range. The uncertainty in the in situ temperature reading was larger because fewer pixels could be averaged. This uncertainty also depended upon temperature, and was below 0.2 degree(s)C over an 7.5 degree(s)C range. Additional errors were investigated. Hysteresis, or path dependent effects in the temperature response of the liquid crystals, was seen to depend upon the maximum temperature reached, even when the maximum temperature was within the usable range of the liquid crystals. Taking the crystals to temperatures above the usable range led to larger hysteresis errors, as much as 0.2 degree(s)C, while errors below 0.1 degree(s)C were seen for a smaller temperature excursion. Uneven lighting had a large effect on the saturation and intensity measured, and a significantly smaller effect on the hue. However, the small effect on the hue caused an error around 0.1 degree(s)C, so uneven lighting should be avoided if possible. Also, thresholding (determining if crystals are reading valid temperatures) is complicated by variations in lighting intensity over the surface to be measured.
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