SSI 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. RMCTools provides Scale/Offset wizards to guide you through the scaling process. This topic describes how to manually calculate these parameters for an SSI transducer.

To scale rotary SSI axes, see the Rotary Scaling topic.

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

Scaling Counts to Position Units

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.

 

To correctly scale the axis for your particular applications, use the methods below to find the Position Scale and Position Offset parameters.

Determining the Scale and Offset

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

ClosedMethod 1: P0/P1 Calculation

The accuracy of this method depends on how accurately you can measure two positions of the axis.

  1. Physically measure the axis position at two points and record the value of the counts register at each point. Call the smaller measured position P0, and its corresponding Counts C0. Call the greater measured position P1, and its corresponding Counts C1.

  2. Calculate the Position Scale with the following equation:

    Position Scale = (P0-P1)/(C0-C1)

  3. Calculate the Position Offset with the following equation:

Position Offset = P0 - Position Scale x C0

 

ClosedMethod 2: Using the Transducer Resolution

Linear Transducers:

  1. Obtain the transducer resolution (usually from the transducer data sheet), for example 0.005 mm. The resolution of the transducer indicates the smallest increment of the transducer. This increment is one count in the RMC.

  2. Determine the position units you would like for your Actual Position, for example inches, meter, mm, etc.

  3. The Position Scale is equal to the number of position units per count, for example, using inches and a resolution of 0.005 mm:

  4. If you wish to reverse the direction of the feedback, make the Scale negative.

  5. Download the Scale to the RMC.

  6. Calculate the Position Offset:

    • Look at the Actual Position register and the Counts 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.

  1. Download the Offset

 

 

See Also

Scaling Overview | Scale/Offset Wizard | Position Scale | Position Offset | Rotary Scaling

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