Delta Tau GEO PMAC User Manual download pdf (Page 63)

Geo PMAC Drive User Manual

Setting Up Encoders 53

Software Setup

Sinusoidal Encoder Decode Value Description

3 x4 clockwise Decode

I910 for ENC#1 or I920 for ENC#2

7 x4 counterclockwise Decode

Note: This permits the user to set the direction sense by setting the decode variable to 3 or 7. However, if the

variable is changed, the user must save the setting using the SAVE command and reset the card $$$ before the

fractional direction sense matches.

To read the sinusoidal encoder feedback, set up the Encoder Conversion Table.

High-Resolution Encoder Interpolation Entries ($F): The $F entry converts the feedback from

sinusoidal incremental encoders through the Geo PMAC’s high-resolution interpolation circuitry,

producing a result with 4096 states per line of the encoder.

Method/Address Word: The first line of the three-line entry contains $F in the first hex digit and the

base address of the encoder channel to be read in the low 16 bits (the third through sixth hex digits). In

the Geo PMAC, the first encoder channel is at address $C000 and the second encoder channel is at

address $C008, so the first setup line is set to $F0C000 or $F0C008.

A/D-Converter Address Word: The second line of the entry contains $00 in the first two hex digits and

the address of the first of the two A/D converters in the low 16 bits (the last four hex digits). The second

A/D converter will be read at the next higher address. In the Geo PMAC, the first A/D converter for

Channel 1 is at address $FF00, and the first A/D converter for Channel 2 is at address $FF20, so the

second setup line is set to $00FF00 or $00FF20.

Sine/Cosine Bias Word: The third setup line in a high-resolution sinusoidal-encoder conversion entry

contains bias terms for the sine and cosine ADC values. The high twelve bits (the first three hex digits)

contain the bias term for the sine input; the low twelve bits (the last three hex digits) contain the bias term

for the cosine input. Each 12-bit section should be treated as a signed 12-bit value (so if the most

significant of the 12 bits is a 1, the bias value is negative).

Each 12-bit bias term should contain the value that the high 12 bits of the matching A/D converter report

when they should ideally report zero. In action, the bias term will be subtracted from the high 12 bits of

the corresponding ADC reading before subsequent calculations are done.

For example, if the bias word were set to $004FFA, the sine bias would be +4 LSBs of a 12-bit ADC, and

the cosine bias would be -6 LSBs ($FFA = -6) of a 12-bit ADC. In use, 4 12-bit LSBs would be

subtracted from the sine reading, and 6 12-bit LSBs would be added to the cosine reading each cycle

before further processing.

Result Word: The output value of the high-resolution sinusoidal-encoder conversion in the Geo PMAC

is placed in the 24-bit X-register of the third line of the conversion table entry. Bit 0 of the result contains

the LSB of the conversion, representing 1/4096 of a line of the encoder. Since PMAC software considers

the contents of Bit 5 to be a count for scaling purposes when used for servo feedback or master data, bit 0

will be considered 1/32 of a count. This means that PMAC software will scale the data as 128 software

counts per line of the encoder.

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Delta Tau GEO PMAC User Manual Page 63