The present invention relates generally to motors, and more particularly to a direct current (dc) motor that generates a time-varying signal for closed-loop feedback control.
Motor control systems may be used in a variety of applications including those such as computers, printing mechanisms, video cassette recorders (VCRs), automobiles, and stereos. These applications include VCR head motors, spindle motors for computer flexible disks, spindle motors for compact disks, tape drive capstans for tape drives, and automobile seat positioning motors. Motor control systems may be used in printing mechanisms for such things as moving the print carriage, moving the print media, and moving elements of the inkjet printhead service station.
Both stepper motors and direct current (dc) motors may be used in such motor control systems. Stepper motors are typically used in an open-loop configuration in which the stepper motor receives a command signal which causes a shaft of the stepper motor to rotate in a predetermined direction for a predetermined number of degrees. The command signal may be in the form of one or more pulses from a microprocessor which is programmed to generate the one or more pulses to rotate the shaft of the stepper motor in the predetermined direction for the predetermined number of degrees to perform a specified function (e.g., actuating a service station to cap an inkjet printhead). The polarity and/or phasing of multiple signals may be used to control the direction of stepper motor shaft rotation. The open-loop system is also capable of tracking the position of the stepper motor shaft by, for example, noting the initial shaft position and counting the number of pulses already transmitted. The open-loop stepper motor control system provides accurate motor control and positioning of controlled devices.
DC motors may be used in either an open-loop or a closed-loop configuration. In an open-loop configuration, a shaft of a dc motor rotates as long as a dc voltage control signal is applied and for a period after the signal is removed until the inertia of the rotating shaft is dampened. The direction of rotation of the shaft of the dc motor is controlled by the polarity of the control signal. Accurate motor control and positioning of controlled devices is difficult with such open-loop dc motor control systems because the shaft does not rotate a predetermined amount each time a control signal is applied, as with stepper motors. An advantage, however, to control systems that utilize dc motors is that a dc motor based control system utilizes power more efficiently, and is thus less expensive to operate, than an open-loop stepper motor because power isn't disrupted to turn the dc motor shaft as in a stepper motor. DC motors are also currently less expensive than stepper motors.
Various solutions have been offered to deal with this positioning problem, including the use of stops and the use of a closed-loop dc motor control system. Stops are fixed structures that are placed in the path of a controlled device that prevent further movement of the device in a particular direction. Closed-loop dc motor control systems utilize a feedback device, such as an encoder, a switch, or tachometer, to track the actual position of a shaft of the direct current motor or of the controlled device. The actual position is fed back and compared with the desired position. Any difference between the two is corrected by sending a further signal to the motor to rotate the shaft until it properly positions the controlled device. This design provides accurate motor control and positioning of controlled devices, comparing favorably with open-loop stepper motor control systems. A drawback to such closed-loop systems is that these feedback devices are expensive. This added expense may result in the cost of this closed-loop dc motor control system approaching or exceeding that of an open-loop stepper motor control system.