1. Field of the Invention
This invention relates to stepping motors, and more particularly to a method and apparatus for determining the speed and position values of such motors.
2. Description of the Prior Art
The stepping motor is increasingly used for applications calling for the motor to be operated quasi-continuously, because of its relatively simple digital control. The open loop mode and the feed-back mode are two operating modes for applications where the stepping motor is operated quasi-continuously. The open loop mode is highly susceptible to oscillations and the feed-back mode necessitates a feed-back signal indicating the respective operational stage of the motor shaft. Optically coded disc, attached to the motor shaft, provide this feed-back signal, by emitting one pulse for each motor step. To ensure that this pulse is accurate within a few percent of the step width, the discs pattern and the adjustment of the axle must meet very stringent requirements. An increase in the degree of accuracy of the disc is possible by using larger discs, but that would mean an increase in the rotational mass of the motor. The protection needed against contamination, the light source, and the light detector required render this method very expensive.
With each pulse a coded disc of the kind described provides information on the position of the motor shaft. Information on the speed can be obtained only as a function of the time difference between two feed-back pulses. Whenever this method is employed the speed during a step just completed can only be determined afterwards. This is a serious disadvantage when the motor moves very slowly shortly before it reaches its stand-still position. In other words, instantaneous speed data is not available.
Where a stepping motor is used to control the operational sequence the greatest problem is to accurately control the motor into its end position.