CONTROL SYSTEM FOR INDUCTIVELY
CONTROLLED MULTI-PHASE MOTOR
This invention relates generally to the control of an 5 inductively controlled multi-phase motor. More specifically, it relates to the control of a stepper motor whose rotational motion is translated into linear motion.
While the invention is useful in the control of stepper motors for a wide variety of applications, in the pre- 10 ferred embodiment it is used to control a stepper motor for positioning the printhead of a thermal printing mechanism. Printing is accomplished by applying heat to the printhead in a prescribed fashion, the printhead contacting thermally sensitive paper to produce a de- 15 sired alpha-numeric character. The printing is done when the printhead is moved from left to right. At the end of the printline, the printhead is returned to the left rapidly, in a "carriage return" (SLEW) mode. The printhead can also be moved to the left in a "backspace" 20 mode.
A typical prior art system is that disclosed in U.S. Pat. No. 3,953,778, and entitled "Motion Control System for an Inductively Controlled Multi-Phase Motor". The system disclosed therein is comprised of a plurality 25 of TTL integrated circuits, interconnected to provide the required control functions. One problem with such a combination of elements is the reduction of its size and weight to permit the use of the control system in a small lightweight portable computer terminal. The system 30 disclosed in the above referenced U.S. patent also requires a limit switch to indicate when the printhead is in the left hand limit position. This limit switch further adds to the size and weight requirements of the prior art system. Another disadvantage stemming from the TTL 35 integrated circuit implementations of the prior art system is the relatively large expense involved in modifying the system once it has been built.
It is an object, therefore, of the present invention to provide a small light-weight motion controller for an 40 inductively controlled multi-phase motor.
It is another object of the invention to provide a motion controller using a microprocessor device.
It is a further object of the invention to provide a motion controller which employs a single position sen- 45 sor.
It is yet another object of the invention to provide a motion controller for a thermal printer having a constant velocity during a substantial portion of the carriage return. 50
It is still a further object of the invention to provide a motion controller for a thermal printhead wherein the aforementioned single position sensor is utilized to locate the left-hand limit for the printhead travel.
It is another object of the invention to provide a 55 motion controller for a thermal printhead wherein the aforementioned single position sensor is further utilized to detect a paper out condition.
It is a further object of the invention to provide a motion controller wherein the mode of operation of the 60 controller can be modified by changing the contents of a ROM.
Briefly, in the preferred embodiment of the invention, a microprocessor operates under the control of a readonly-memory (ROM) to generate the requisite control 65 signals. A random access memory (RAM) is used to store variables such as various types of pointers, flags, counts, and the microprocessor stack. The microproces
sor communicates with various other hardware devices of the printer through an input/output (I/O) integrated circuit which is formed on a single semiconductor chip. The stepper motor drive circuit which provides current to the stepper motor phases is an improved version of the circuit disclosed and claimed in U.S. patent application Ser. No. 452,320, filed Mar. 18, 1974, now abandoned, and entitled "Switching Regulator Control for Inductively Controlled Electro-Mechanical Device".
Operationally, power is selectively applied to the various mesas of the thermal printhead so as to print a character on the thermally sensitive paper, this print operation consuming approximately 10 milliseconds. During the next 18.5 millisecond period, the printhead is stepped to the next position for a subsequent print cycle. To initiate this step from the last detent position to the next detent position, the current in the last phase of the stepper motor is terminated while a high level signal is generated to initiate a rapid build up of current in the next motor phase. This continues until a position sensor located on the stepper motor provides a signal indicating that the motor has traversed as predetermined portion of the angular distance between adjacent detent positions. This sensor signal, after a short delay, is used to turn off the current in the next motor phase, and generate a high level logic signal which controls the values of motor current between one of two levels to cause a rapid build up of current in the lagging motor phase. This build up of current in the lagging motor phase provides a braking torque, and continues for a period of two milliseconds. At the end of this two millisecond period, the current is removed from the lagging phase, and the high level logic signal again induces a build up of current in the leading motor phase. The build up in the leading phase is allowed to continue for four milliseconds so as to pull the motor into the next detent position. At the end of four milliseconds, the logic signal is reduced in amplitude so as to cause the current in the leading motor phase to decay to a level just sufficient to hold the motor in the detent position. If the aforementioned position sensor signal is not received within 7.2 milliseconds after the initiation of the step operation, the controller asserts that a paper out condition has occurred.
Backspace operation functions in a similar manner, but in this case the 10 millisecond print period is eliminated.
During the carriage return operation the logic signal which controls the current in the various motor phases is maintained at a high level. This current is switched consecutively from phase to phase of the stepper motor in an order such as to cause the motor to drive the printhead toward the left-hand limit stop. To initiate the carriage return from a detent position, the current is switched to the adjacent leading motor phase to begin rotation of the motor in the carriage return direction. At the same time, a 1.5 millisecond timer is initiated. This continues until the aforementioned sensor provides a signal to indicate that the motor has rotated through a predetermined fraction of the angle between the start detent position and the adjacent leading detent position. At this point in time, the current is switched to the next leading motor phase and the 1.5 millisecond timer reinitiated. This current switching between motor phases and reinitiation of the 1.5 millisecond timer continues iteratively until the motor reaches an angular velocity such that the signal from the position sensor on some iteration occurs prior to the termination of the 1.5 milli