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# Print Hammer Flight Time Temperature Compensation Circuit

IP.com Disclosure Number: IPCOM000051555D
Original Publication Date: 1981-Feb-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 47K

IBM

Ganim, M: AUTHOR

## Abstract

A method to control the flight time of print hammers is to control the time constant of the current in the hammer coil. Since temperature affects the resistance in the hammer coil, the approach to compensate for resistance changes is to maintain a constant time constant. This is explained in the following expression: T(c) = L over R also T(c) = L over V over I = LI over V. where L = inductance of a coil, V = coil applied voltage, I = coil current, and R = coil resistance.

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Print Hammer Flight Time Temperature Compensation Circuit

A method to control the flight time of print hammers is to control the time constant of the current in the hammer coil. Since temperature affects the resistance in the hammer coil, the approach to compensate for resistance changes is to maintain a constant time constant. This is explained in the following expression:
T(c) = L over R also T(c) = L over V over I = LI over V. where L = inductance of a coil, V = coil applied voltage, I = coil current, and R = coil resistance.

Because the number of turns in the coil is fixed, L remains constant. R changes due to temperature. A chopper circuit applied to the hammer drive maintains a constant average current. Considering the above formulas it can be seen that the voltage across a coil varies directly with coil resistance. The basic approach is to detect the coil change in resistance and compensate by changing the voltage applied to the coil.

In the circuit of Fig. 1, operational amplifier (Op Amp) 10 senses the voltage of hammer coil 11 and, depending on this voltage difference, drives transistor 12 which is shunting a source resistor 13. The operation here is to by-pass some source current around resistor 13 by driving transistor 12. The amount of current bypassed is determined by the voltage across hammer coil 11. This will have the effect of varying the voltage across the hammer coil, and compensates for temperature effects on the hammer coil resistance.

The operation of the circuit is explained as follows: At time T(o), transistor 14 is turned on; this applies an immediate maximum voltage across hammer coil 11. The Op Amp 10 is turned on fully and drives transistor 12 full on. This now applies...