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Conversion Factor on the NI 9505

Hardware: Motion Control>>Power Drives>>cRIO-9500

Problem:

The NI 9505 is a Servo Drive module that can be used for current loop control algorithms. These algorithms require the user to specify the Current Setpoint and Current Limit (Amps) as floating point [DBL] numbers. Because the FPGA on the NI 9505 does not currently interpret double precision number, all the floating point numbers must be converted to integers. Why is 0.0062 used as the conversion factor for this module?

Solution:

Although it is recommended not to exceed +/- 12 Amps when specifying the current set point for a current loop control algorithm on the Real-Time controller, the actual current range of the module is +/- 12.7 Amps.

The ADC on the 9505 has 12 bits of resolution, thus the values range between -2048 and 2047 which needs to map to a current range of +/- 12.7 Amps (See Table 1).

Table 1: Mapping

Therefore the conversion factor is

Amps/integer value.

This explains why most of the examples in the LabVIEW Example Finder use 6.2 mA/integer value as the conversion factor between floating point numbers on the Real-Time controller and integers on the FPGA.

Related Links:

NI 9505 Product Manual

Attachments:


- Mapping.JPG - Equation.JPG

Report Date: 03/13/2007
Last Updated: 03/13/2007
Document ID: 47CK5JUI

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