Delta RMC151 - Manual (Page 39)

2 Starting Up 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.
2.2.7. 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.
Method 1: P
0
/P
1
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 P
0
, and its corresponding
Counts C
0
. Call the greater measured position P
1
, and its corresponding Counts C
1
.
2. Calculate the Position Scale with the following equation:
Position Scale = (P
0
-P
1
)/(C
0
-C
1
)
3. Calculate the Position Offset with the following equation:
Position Offset = P
0
- Position Scale x C
0
Method 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.
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