Low-Cost 4-20 mA Loop Calibrator

Abstract

Instrument calibration is vital to a successful control system because signal inputs to the system controllers come from such instruments. This paper presents a method for actualizing a standard low-cost loop calibrator for the famous 4-20 mA electrical signaling scheme. The loop calibrator generates a linear current signal from 4 to 20 mA over a 250 Ω typical process instrument load for calibration. The realization of the loop calibrator relies on a voltage-to-current converter to build a constant current source. The voltage controlled constant current source is built from discrete components and an op-amp to keep the cost low. Results from simulations and the prototype demonstrate the performance of the 4-20 mA loop calibrator which utilizes a greatly reduced number of components. The cost of these components is approximately 34% of the least expensive calibrator sampled, though other production costs are not included. This conclusion reinforces the fact that loop calibrators can be cheaper.

Further Analysis of Machine Tool Dimensional Accuracy and Thermal Stability under Varying Floor Temperature

“…Data is obtained and analyzed to determine the most suitable combination of these factors and what works best for achieving high dimensional accuracy. Measurement accuracy is crucial to instrument calibrators [18], and dimensional accuracy is vital to machine tools.…”