Closed loop thermal printer for maintaining constant printing energy

Abstract

A system and method are disclosed for automatically detecting any change in average printhead resistance due to continued usage of the printhead and for automatically correcting for such resistance change in order to maintain constant printing energy. In a preferred embodiment of the invention a voltage regulator is turned off during a test mode of operation to test or measure each of the thermal elements in a thermal printhead. When the voltage regulator is turned off a constant current is sequentially allowed to flow through each of the thermal elements. The flow of constant current through an element develops a sense voltage which has an amplitude proportional to the resistance of the element being measured. The sense voltages for the elements are sequentially converted into digital signals by an analog-to-digital converter, summed together and averaged in order to develop an average printhead resistance. Each subsequent average printhead resistance after an initial average printhead resistance is compared against the initial average printhead resistance to determine whether a change in average printhead resistance has occurred. In response to a change in average printhead resistance, a processor maintains constant printing energy during a printing mode of operation by changing the pulse width of the printing pulse and/or by developing a voltage which is used to fine tune the voltage regulator to change the head voltage accordingly.