Delta RMC151 - Manual (Page 42)

RMC70/150 and RMCTools User Manual
Note:
If the Actual Position filter is applied, the RMC filters the Actual Position after calculating it with
the above formula.
Manually Calculating the Scale and Offset
The Scale/Offset Wizards provide the easiest method of scaling your axis. If you prefer to
do it manually, read this section.
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 first measured position P
0
, and its corresponding
Counts C
0
. Call the second 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
)
Method 2: Using the Encoder Resolution
1. Obtain the encoder resolution (usually from the transducer data sheet), for example
4000 counts/turn.
2. Determine the position units you would like for your Actual Position, for example
inches, meter, mm, etc.
3. Determine how many position units correspond to one count.
For example, consider an encoder with 4000 counts per revolution mounted to a motor
that drives a linear axis through a 4:1 gearbox and then a rack and pinion. The rack
moves 5 inches for every turn of the pinion. The desired position-units of the linear
rack motion are inches.
To find the resolution:
a. Determine the number of counts generated in one revolution of the encoder:
From the datasheet, this is 4000 counts.
b. Calculate the linear travel (position units) in one revolution of the encoder:
One revolution of the encoder equals 1/4 revolution of the pinion.
One revolution of the pinion equals a travel distance of 5 inches.
Therefore, the linear travel in one turn of the encoder is 5/4 = 1.2 in.
c. To calculate Scale, divide the calculated linear travel (position-units) by the
number of counts:
1.2/4000 = 0.0003 position-units/count
4. If you wish to reverse the direction of the feedback, make the Scale negative.
5. Download the Scale to the RMC.
2.2.10. Scaling Rates on a Position Axis
Revolutions per Minute and Feet per Minute
If your system is using a position feedback device (such as an encoder), but your system
is primarily intended for velocity control, and the system must be scaled so that the
velocity of the system is given in feet per minute or revolutions per minute, then this
topic is for you.
22 Delta Computer Systems, Inc.