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DEVELOPING AN AC CURRENT GENERATOR

by Ernesto Gradin & Aubrey Kagan

Start ý Features ý Magnetics ý Primary and Secondary Turns ý Primary and Secondary Winding ý Primary Inductance ý Hardware ý Firmware ý Analog to Digital Conversion ý User Interface ý Sources and PDF

ANALOG TO DIGITAL CONVERSION

Three A/D channels are read. When all three have been read, they are transferred to a register and a flag set. Averaging is only done on Channel 1, which is used for the 4- to 20-mA signal from the true RMS Current Monitor.

Most of the functions and their implementation are self-explanatory. On reset, the parameters stored in the EEPROM are read, with some loaded into RAM. The process (mode) number is one of these. On command, it can be changed from the host PC.

The closed-loop process is based on a PID loop. Provisions are made for suitable integral, differential, and proportional constants. In reality, it was found that the system worked best as a purely proportional loop. This was attributed to the long settling time and filtering affect of the external AC-to-mA converter, coupled with the coarse AC current adjustment of only nine bits.

SERIAL COMMUNICATIONS

The serial communications portion of the software implements the Intercommunications Protocol as detailed in Listing 2.

Parameters are stored in EEPROM. The original driver software for the EEPROM was developed in assembler. It was then ported to applications in C. There are simple translation routines to match the used registers to the calling conventions used in IAR C. Only a small portion of the EEPROM is used for the application. These locations are protected by a checksum to detect whether the unit has been set up. It is possible to access all the available locations in the EEPROM for any purpose by following the communications protocol.

Most constants are only stored in EEPROM and read from EEPROM when needed. A few are read and stored in RAM, so they can be changed "on the fly" by the host PC when the change in operations is desired without having to go through an EEPROM change. These include the process type and the target output for closed- or open-loop operation.

As part of failure protection, an EEPROM read must provide two identical reads for the procedure to continue. Failure of the two subsequent readings to agree in three attempts will lead to a hardware reset.

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