This invention relates to a control system for a step motor, and more particularly to a closed-loop control for controlling a step motor in response to rotation of the step motor.
A closed-loop control for a step motor is already known wherein a rotational angular position of a step motor is detected and a field winding of a next phase is energized at an optimum point of time, that is, at an optimum rotational angular position of the step motor to drive the step motor in order to prevent step-out of the step motor. In determining such an optimum point of time, also a rotational speed of the step motor is commonly taken into consideration.
Where a load is relatively heavy or is to be driven at a high speed, the closed-loop control technique is preferably employed. One of application of the closed-loop control is, for example, a daisy wheel typewriter or printer in which a rotary type wheel commonly called a daisy wheel is rotated to position a type of a selected character to a print position to allow printing of the character. In the typewriter or printer of this type, the rotor of the step motor is rotated from one to another rotational angular position in one or the other direction in response to a character selecting instruction. As the time required for rotation from one to another position reduces, apparently the overall printing speed of the typewriter or printer will rise. Accordingly, the closed-loop control is commonly used in high speed typewriters or printers.
A concept of an angle of lead may be involved in order to attain a high speed rotation of a step motor under a closed-loop control. It is known that where the lead angle of a step motor is between 1 step and 2 steps, that is, electrically between 90 and 180 degrees, the step motor will rotate continuously like a brushless motor. In this instance, theoretically an angle of lead of 1.5 steps or electrically 135 degrees will yield a maximum rotational speed whereas the rotational speed will be in the minimum when the lead angle is 1 step or 2 steps, that is, electrically 90 or 180 degrees, and the speed will vary continuously between 1 step and 2 steps, that is, between 90 and 180 degrees.
Conventionally, a rotary encoder such as an optical encoder is used to detect a rotational position and speed of a step motor. Step motor controls including a rotary encoder are normally very expensive compared with controls without a rotary encoder because such a rotary encoder is expensive.
Means is also known which detects an electromotive force (EMF) or back EMF induced in a field winding of a step motor to detect a rotational angular position of the rotor of the motor. This technique is disclosed, for example, in U.S. Pat. Nos. 4,282,471 and 4,480,218. In a step motor control disclosed in U.S. Pat. No. 4,282,471, a back EMF which is induced in a non-excited phase field winding by a mutual inductance with an excited phase field winding is detected to detect a rotational position of the rotor of the motor. On the other hand, in a step motor control disclosed in the former patent, EMF which is induced in a field winding by a permanent magnet of the rotor of a step motor is detected directly by a specifically provided sensing means to detect a rotational position of the rotor.
Feedback signals obtained from a rotary encoder or from back EMF are used in most cases to determine whether or not a step motor is actually at a proper rotational angular position or whether or not a step motor is in a step-out condition so that, when the step motor is in a step-out condition, either a positional error may be compensated for or the motor may be restored to its home position. Otherwise, feedback signals are used to determine when a next phase field winding is to be energized. In the latter case, a timing of next energization may be selectively determined in response to the feedback signal and clock pulse signals of the system. Accordingly, infinite velocity control cannot be attained.