1. Field of the Invention
The pesent invention relates to a supervisory device for wrong operation of varying quantity measuring apparatus, and more particularly to a supervisory device for wrong operation of varying quantity measuring apparatus which measures a physical varying quantity by relative movement between a main scale and an index scale.
2. Description of the Prior Art
There has been a well-known physical varying quantity measuring apparatus which measures a certain physical varying quantity by relative movement between a main scale and an index scale, and is of wide use for the measurement of length and physical varying quantity in length measuring instruments, coordinate measuring machines, etc. or in positioning or the like of machine tools, or for the other purposes.
This varying quantity measuring apparatus includes a main scale and an index scale which are moved relative to each other in accordance with a physical varying quantity and an encoder which detects the relative movement of such two scales and measures the requested physical varying quantity based upon the relative movement of such two scales which can be detected by this encoder.
The encoder mentioned above detects the relative movement of both two scales as an electric signal by optical, electromagnetic or well-known means using contact resistance, and further converts the detected signal into a count pulse signal in accordance with the relative movement per unit time to supply this to a counter provided in the encoder. The counter counts the supplied count pulse signal, and the counted value of the count pulse signal is displayed as a digital value representing a measured value of the physical varying quantity or is used for an other control signal.
Accordingly, this varying quantity measuring apparatus can measure the physical varying quantity with accuracy as a predetermined digital signal by means of relatively moving the main scale and the index scale in accordance with the desired physical varying quantity.
The varying quantity measuring apparatus mentioned hereinabove, however, clears the counted value of the count pulse signal stored in the encoder, when the electric power stops or lowers the voltage to great extent for some reason, and restart counting the count pulse signal from the very beginning, when the electric voltage recovers up to the optimum level. Accordingly, when the electric power stops or lowers its voltage, there arise measuring errors between the relative moving quantities of the main scale and the index scale which are converted into the electric signal by the encoder to be displayed and the actual moving quantities of these two scales. Furthermore, it is a problem that the error occurrence is not known to those at measuring work in most cases.
Especially, in such a case that the electric power suspension or low voltage is instantly caused by the start or the stop of a large machine, the sudden load change, or the like for example, the encoder immediately restart counting the count pulse right after the counted value of the count pulse signal is cleared, which is not known to those at measuring work in most cases, and an effective countermeasure has been requested.