A potentiometer is an circuit element comprising a resistor which has a movable tap for adjusting the ratio of the resistances from the tap to the respective ends of the resistor. A frequent application of a potentiometer is to obtain a control signal which is intermediate between the electrical potentials present at its opposite ends. Such use of potentiometers is common in circuits for controlling amplifiers, power supplies and other electrical and electronic devices. The desired control signal is obtained from the tap by adjustment of its location.
Sometimes it is desirable to control a device with an electrical signal from a separate circuit. One such application is robotic welding where a welding power supply is coupled to a programmable controller for automatically controlling the welding process. In such coupling, it is necessary to adapt the output of the controller to the control circuitry of the power source. Further, it is frequently essential to provide direct current isolation between the programmable controller and the welding power source. One possible solution is to use the controller output and a servomotor to drive the tap of a potentiometer in the power source to the desired position.
One welding power source which has been used for robotic welding is the PULSE ARC 350 welding power source available from the Airco Welding Products Division of The BOC Group, Inc. This power supply adjusts the welding current in response to a control signal in the range 0 to 10 volts. In the manual control mode this signal is obtained by adjustment of a potentiometer. The welding current control has been automated by coupling a programmable controller to the welding current control circuit by means of a conventional dc-dc isolation device. This device linearly converted a 0 to 10 volt input level from the controller into a 0 to 10 volt output for power source control while providing no dc current path from the controller to the output power source.
A further feature of the above mentioned welding power source enables the welding arc voltage to be adjusted within a range about a nominal value. Again, the arc voltage can be manually trimmed by setting a control potentiometer. In this situation, however, the potentiometer is in the arc voltage feedback loop and the voltage trim control signal must have the desired proportion intermediate between the high and low potentials across the potentiometer. Since these high and low potentials are floating and vary with the feedback signal, it is not possible to use a simple dc-dc isolation device as described above in connection with the welding current control.
The electronic potentiometer simulator of the present invention is suitable for substitution for each of the control potentiometers in the welding power system just described to enable a programmable controller to control both the welding current and the arc voltage. The potentiometer simulator is also suitable for use in many other situations where control circuitry is coupled to electrical devices.