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Menu 2
Digitax ST Advanced User Guide 37
Issue Number: 1 www.controltechniques.com
Setting this parameter enables the S ramp function. S ramp is disabled during deceleration when the standard ramp voltage controller is active. When
the motor is accelerated again after decelerating in standard ramp the acceleration ramp used by the S ramp function is reset to zero.
This parameter defines the maximum rate of change of acceleration/deceleration. If the S ramp is disabled (Pr 2.06 = 0) a linear ramp is used and the
time in seconds taken for the ramp output to change by speed (Δ w*) is given by:
T
Ramp =
Δw* x A / 100
Where A is the selected ramp rate in s / 1000rpm
If the S ramp is enabled (Pr 2.06 = 1) then the ramp time is extended as shown in the diagram below.
The time taken in seconds for the ramp output to change by speed (Δw*) is given below. Two cases are given because the total ramp time must be
calculated with a different equation depending on whether the acceleration is able to reach the selected ramp rate (A) or not. If the required change is
small the selected ramp rate is not reached and the ramp does not include the central linear ramp region. If the required change is larger the ramp
does include the central linear region as shown in the diagram above.
Δw*
linear
= 1000 x J / A2
where:
A is the selected ramp rate in s / 1000rpm
J is Pr 2.07, the S ramp acceleration limit in s
2
/ 1000rpm
If the required change is less than Δw*
linear
then T
Ramp1
should be used, but if the speed change is greater or equal to Δw*
linear
T
Ramp2
should be
used.
T
Ramp1
= 2 √ (Δw* x J / 1000)
T
Ramp2
= (Δw* x A / 1000) + (J / A)
The default values for the ramp rate and S ramp acceleration limit have been chosen such that for the default maximum speed, the curved parts of the
S ramp are 25% of the original ramp if S ramp is enabled. Therefore the ramp time is increased by a factor of 1.5.
2.06 S ramp enable
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Default 0
Update rate 4ms read
2.07 S ramp acceleration limit
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
3111
Range
0.000 to 100.000 s
2
/1,000rpm
Default 0.030
Update rate Background read
t
Acceleration
Actual Speed
Programmed
ramp rate
TT
T/2 T/2 T/2T/2
S ramp
acceleration
ramp
Demanded Speed
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