| 摘要 |
A thermal printer for printing on a continuous print medium by ink transfer from a thermal print ribbon has a print head which is pivotally mounted and which is driven in an oscillatory nodding motion by means of a stepper motor so as repeatedly to bring a linear array of energisable printing elements to bear against a platen roller. Both the element array and the platen extend transversely to respective paths of travel of the print medium and the ribbon. The print medium is fed through the printer from an inlet region, between the platen and print head, and thence to an outlet region. The instantaneous rate of travel of the print medium past the print head is substantially the same as the rate of feed of print medium to the printer. Typically this rate is of the order of 250 to 400 mm per second. The ribbon also travels between the print head and the platen, overlying the print medium and is driven in such a manner that it travels at the same rate as the print medium during each printing operation. This is achieved by driving the ribbon with a motor the speed of which is controlled according to the sensed speed of travel of the print medium, e.g. by coupling the motor to processing circuitry which takes an input signal from a shaft encoder associated with the platen roller. Alternatively, the ribbon may be driven by frictional contact with the print medium. These methods of driving the ribbon have the advantage that the ribbon speed can be varied to take account of different print medium speeds, e.g. due to the requirements of differing packaging processes, and also in response to transient variations in print medium speed during each printing operation.
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