5-5 Speed Compensation
Leading/Trailing Speed Comp (cont’d)
Figure 27—Example for Calculating Leading and Trailing Edge
RPM
Glue On
Glue Off
Difference
1st Line Speed:
200
73
°
156
°
83
°
2nd Line Speed:
680
49
°
144
°
95
°
Note that the length of the pulse is 83
°
at 200 RPM, and 95
°
at 680 RPM. This means that
the leading and trailing edges require different speed compensation values.
Leading Edge: Difference in Position:
73
°
- 49
°
= 24
°
Difference in Speed:
680 RPM - 200 RPM = 480 RPM
Speed Compensation Value: Divide difference in position by difference in speed:
24
°
/480 RPM = 0.05
°
per 1 RPM
Since a shaft at 1 RPM rotates 0.006
°
/msec (see page 4-2), this shaft would require (0.05/
0.006), or 8.3 msec to rotate 0.05
°
. The speed compensation value is 8.3.
Trailing Edge: Difference in Position:
156
°
- 144
°
= 12
°
Difference in Speed:
680 RPM - 200 RPM = 480 RPM
Speed Compensation Value: Divide difference in position by difference in speed:
12
°
/480 RPM = 0.025
°
/1 RPM
Since a shaft at 1 RPM rotates 0.006
°
/msec (see page 4-2), this shaft would require (0.025/
0.006), or 4.2 msec to rotate 0.05
°
. The speed compensation value is 4.2.
Negative Speed Compensation
Negative Speed Comp
Normal speed compensation advances the setpoints in an output channel to compen-
sate for a fixed response time in the device being controlled. In some applications,
however, negative speed compensation is required to retard the setpoints in an output
channel. Negative speed compensation is usually found in two situations:
“Wrap-Up”
As some machines increase in speed, the drive train at some point between the re-
solver and the product “wraps-up,” or shifts with respect to the resolver. If the wrap-up
is proportional to machine speed, negative speed compensation can be used to retard
an output channel’s setpoints from the true resolver position, thus maintaining output
accuracy.
Sensor Lag
While output channels are usually used to switch devices on and off, another use is to
“gate” a sensor into a PLC or other computer. Figure 28 on the preceeding page illus-
trates a basic sensor gating scheme. In the illustration, the signal from the sensor reaches
the PLC only when the output channel from the PLS is turned on.
Most sensing devices have very fast response times. However, if a sensor’s response
time is slow, its signal will appear later and later in the machine cycle as the machine
speeds up. Eventually, the sensor may lag the resolver so much that its signal fails to
appear during the window programmed into the PS-6244’s output channel.
Negative speed compensation will correct this problem by causing the output channel
to lag its programmed machine position by a specified number of milliseconds. Nega-
tive speed compensation is calculated using the same method as standard speed com-
pensation. See SPEED COMP in Section 3 for details on programming negative speed
comp.
