Galvanometer Recorders: Types and Operation
Galvanometer Recorders
Galvanometer recorders are based on the same principle as the moving coil meter mechanism. The main difference is that the opposing force to the one generated by the current through the coil is produced by the torsional suspension coil springs. A pointer can be attached to the suspension, with a stylus at the end. This then produces an ink line graph. If R is the length of the pointer and θ the deflection, the pen’s displacement will be y = Rsinθ. Since the angle θ is proportional to the current passing through the coil, the relationship between current and displacement is nonlinear. If angular deflections are restricted to less than ±10º, the error due to this nonlinearity is less than 0.5%.
The biggest problem is the fact that the pen moves in an arc instead of a straight line. Therefore, curvilinear paper is used for graphics rendering, presenting some difficulties for interpolating points between the curved lines.
Knife-Edge Recorder
An alternative type of graphic pen recorder is the knife-edge recorder. This uses heat-sensitive paper moving along the edge of a razor and a hot needle instead of an ink pen. The paper is impregnated with a chemical that shows a marked color change when heated in contact with the needle, or the needle burns the outer layers sensitive to temperature, covering the paper. The use of a curved knife edge prevents arcing, but still gives a nonlinear relationship between θ and the displacement y. The length of the line on paper produced by a deflection θ is: y = Rtanθ.
The nonlinearity error is slightly larger than that obtained for a pen recorder. If deflections are limited to less than ±10º, the error will be less than 1%. With this recorder, there isn’t any electronic amplification of the input signal.
This leads to sensitivities that are usually about one centimeter of pen displacement per millivolt (mV), with input resistances of about 10kΩ, with a bandwidth ranging from DC to 50Hz, and accuracies of the order of ±2% of full-scale deflection.
Ultraviolet Recorder
There are several ways by which the movement of the coil can be transformed into a line on a chart. An alternative to the pen type may be the ultraviolet recorder. This consists of a small mirror attached to the suspension. An ultraviolet light beam is directed at the mirror, and so when the coil rotates, the reflected beam sweeps across the graph. The chart uses photosensitive paper that reveals a trace when exposed. Using an optical pointer instead of a physical pen allows for longer pointer lengths and, therefore, greater sensitivity.
With a bandwidth ranging from DC to 50Hz, the sensitivity is usually close to 5cm/mV, with the coil having a resistance (r) of 80Ω. Other instruments have higher bandwidths but lower sensitivities; for example, a bandwidth covering up to 5KHz with a sensitivity close to 0.0015 cm/mV and a coil resistance (r) close to 40Ω. The frequency limit for this type of instrument is close to 13KHz. The typical accuracy is about ±2% of full-scale deflection. Since optical pointers can cross without interference, it is very common to have 6, 12, or 25 galvanometer mechanisms together on the same magnet block and to perform simultaneous recordings of many variables.