48
CMA USER MANUAL
E-VLMB-TT005-E April 2019 www.eaton.com
CMA User Manual
10.3 Passive and overrunning margins
In the CMA system, it is important to understand how
the valve determines if the load is passive or overrunning.
A description of how the valve calculates the current load
condition is given in the following.
If the previous load condition was passive, then the load is
considered overrunning only when the differential pressure
(including area ratio) is higher than the overrunning margin.
Refer to the figure below for the following discussion.
Figure 17 .
P
1
A
1
P
2
A
2
Assuming P1 is the upstream port and the Area Ratio is
defined as A1/A2
If P2/Aratio – P1 > Overrunning Margin then
Differential Load = overrunning
else
Differential Load = passive
If the previous load was overrunning and P1 is the
upstream port (area ratio is still A1/A2)
If P2/Aratio – P1 < Passive Margin then
Differential Load = passive
else
Differential Load = overrunning
We can further describe this scenario with a load versus
time plot, shown below
Figure 18 .
Di
ffe
re
nt
ia
l l
oa
d
Time
Passive margin
Overruning margin
Point at which load is
considered Passive
Point at which load is
considered Overruning
Starting at time 0, the load is considered truly passive.
As we move through time the P1 port pressure is
reducing and the P2 port pressure is increasing. When
that load differential is greater than the overrunning margin,
then we will consider the load to be truly overrunning.
As we continue through time, we can see that the P1
pressure is starting to increase and the P2 pressure is
starting to decrease. When the load differential drops
below the passive margin, then the load is passive.
To summarize, if you want to force the valve to transition
to passive quicker as the load differential drops, increase
the passive margin. If you want the valve to stay in a
overrunning state longer, than decrease the passive
margin. For the overrunning margin, if you lower it the
valve will transition to the overrunning state at lower load
differentials. If you increase the overrunning margin, the
valve will remain in the passive state until a higher load
differential is achieved.
10.4 Digital filter bode plot
As mentioned above, the valve has the capability to
digitally filter the raw pressure signals for each work port.
The filter is a first order lag filter and its response can be
characterized with a Bode plot. The plot below is for a filter
coefficient of about 29,820 or about 10 Hz.
Figure 19 .
-25
-20
-15
-10
-5
0
M
ag
ni
tu
de
(d
B)
10
0
10
1
10
2
10
3
-90
-45
0
Ph
as
e
(d
eg
)
Bode diagram
Frequency (rad/sec)
