MDT Scaling

To have any useful meaning, the counts from the transducer must be scaled to position units. The Position Scale and Position Offset parameters define the position units as a function of transducer counts. This topic describes how to correctly calculate these parameters for an MDT transducer.

Delta Recommends using the Scale/Offset Wizard for scaling the position. If you need to do it manually, read this topic.

The RMC calculates the Actual Position every control-loop time using the following formula:

Actual Position [pu] = (Counts [cnt] x Position Scale [pu/cnt]) + Position Offset [pu]

Note:
If the Actual Position filter is applied, the RMC filters the Actual Position after calculating it with the above formula.

The Scale/Offset Wizards provide the easiest method of scaling your axis. If you prefer to do it manually, read this section.

There are two methods of finding the correct Position Scale and Position Offset.

Obtain the following information (usually from the transducer label or data sheet):
- Calibration Number (calibration constant) in μs/in or μs/μm (typically about 9 μs/in)
- Number of Recirculations (if it has no recircs, use a value of 1)
Calculate the Position Scale with the following formula:

RMC75 and RMC200:

RMC150:

where

"pu" is the position units, e.g. in, mm, m, etc. If you want position units other than what is given by the Calibration Number (e.g. in,mm, etc.), you must convert the calibration number to the units you want.
The 240MHz and 120 MHz values comes from the RMC's internal counter. If you change the units from the above equation, make sure the units for the equation work out properly. The units for MHz is [106/sec].

If you wish to reverse the direction of the feedback, make the Scale negative.
Download the Scale to the RMC.
Calculate the Position Offset:

Make sure the Position Offset is zero (0) and downloaded to the controller before continuing.
In the Axis Tools window, look at the Actual Position register.
Move the axis to a point where you know what you want the position to be (such as 0). Called this the Desired Position. Look at the Actual Position register. Call this position P0.
Calculate the Position Offset using the following equation: Position Offset = Desired Position - P0.

Download the Offset to the RMC.

Example

Connected to an RMC75 is an MDT transducer with a calibration number of 9.1615 μs/in and no recirculations (this means it has just 1 recirculation). The system needs to be scaled to inches.
- To calculate the Scale:

- Download the Scale to the RMC.
- To calculate the Offset: Move the axis to where it should be 0. The Actual Position shows 2.4528. The Position Offset should be: 0 - 2.4528 = -2.4528.
- Download the Offset to the RMC.

Connected to an RMC75 is an MDT transducer with a calibration number of 9.012 μs/in and 2 recirculations. The system needs to be scaled to inches.
- To calculate the Scale:

- Download the Scale to the RMC.
- To calculate the Offset: Move the axis to where it should be 10.125. The Actual Position shows 11.241. The Position Offset should be: 10.125 - 11.241 = -1.116.
- Download the Offset to the RMC.

Connected to an RMC150 is an MDT transducer with a calibration number of 9.1313 μs/in and no recirculations (this means it has just 1 recirculation). The system needs to be scaled to millimeters.
- We must first calculate the scale, but in order to do that we need to know how many position units there are in an inch: 25.4 mm/in.
- To calculate the Scale:

- Download the Scale to the RMC.
- To calculate the Offset: Move the axis to where it should be 0 mm. The Actual Position shows 24.8. The Position Offset should be: 0 - 24.8 = -24.8.
- Download the Offset to the RMC.