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Differential Current Sensing System for Diagnostic Purposes

IP.com Disclosure Number: IPCOM000051357D
Original Publication Date: 1981-Jan-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 43K

Publishing Venue

IBM

Related People

Paniccia, AE: AUTHOR [+2]

Abstract

Current sensing for self-diagnostic purposes is known. For example, in trouble shooting printer systems with the aid of a microprocessor, current sensing is used to differentiate between a shorted driver circuit condition and an open circuit condition between the driver circuit and the driven circuit. The driver circuits drive the hammer circuits, stepper motor circuits and relay circuits of the printer system. In case of a driver failure, all stepper motor drivers, which are normally on, would be degated and, of course, all hammer drivers would be turned off in response to the microprocessor. An error code is displayed in the microprocessor's status indicator display, which indicates either a driver turn-on failure condition, a driver turn-off failure condition, or the aforementioned open circuit condition.

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Differential Current Sensing System for Diagnostic Purposes

Current sensing for self-diagnostic purposes is known. For example, in trouble shooting printer systems with the aid of a microprocessor, current sensing is used to differentiate between a shorted driver circuit condition and an open circuit condition between the driver circuit and the driven circuit. The driver circuits drive the hammer circuits, stepper motor circuits and relay circuits of the printer system. In case of a driver failure, all stepper motor drivers, which are normally on, would be degated and, of course, all hammer drivers would be turned off in response to the microprocessor. An error code is displayed in the microprocessor's status indicator display, which indicates either a driver turn-on failure condition, a driver turn-off failure condition, or the aforementioned open circuit condition. Discrimination between the last two conditions cannot be made without current sensing if the sense voltages are sensed at the collector of the driver transistor, as both conditions result in a DOWN level at the collector. It can be shown that the inability to discriminate reduces the fault location isolation capability and hence adds to the repair time.

Certain prior-art printer systems require an extra sensing component, e.g., a resistor, diode or coil, in the associated power supply circuit for current sensor. This has certain disadvantages that add to the cost and complexity of the system. The component dissipates additional heat and thus increases the heat sink requirements. Moreover, the component causes an undesirable voltage drop in the power supply and, if the power supply voltage is, in addition, load dependable, it causes further poor regulation.

The system of the figure does not have these disadvantages. The voltage drop directly across the power supply rectifier diodes CR1 and CR2 is sensed by differential amplifier 1 of the current sensor through the coupling Schottky diodes D1 and D2. Diodes D1 and D2 have a lower voltage drop than diodes CR1 and CR2. Filter capacitor C1 smoothens the AC ripple and together with resistor R1 determines the filter time constant. Voltage comparator 2 in response produces a logical signal at output 3 which is fed into the printer status multiplexer (not shown) which is decoded by the microprocessor (not shown).

The power supply has a basic load consisting of a bleeder current through resistor R2 and the leakage current through the schematically shown output LOAD of drivers, only one of which is shown in detail. Each driver includes a switching transistor 4, a driven circuit such as, for example, the hammer coil 5, and a voltage...