Closing The Loop; Feed-Forward; Steady State Error; To Achieve Steady State Accuracy - Danfoss pved-cl Operation Manual

Operation Manual
PVED-CL Controller for Electro-Hydraulic Steering, Version 1.38
Steering by Steering Wheel – Closed Loop
Symbol Index
Lx 1y06
YR 1y07
YL 1y08
Lx in quadrant 1 or 4 is located at: [500;Yr*(20-Lx)/40], Lx in quadrant 2 or 3 is located at: [-500;Yl*(20-Lx)/40]

Closing the Loop

56 | © Danfoss | May 2016
much whether a linear sensor is used to detect cylinder piston position or an angular sensor at the king
pin. Lx is typical set at zero when the cylinder piston position is detected using a linear sensor. When the
king pin rotation is detected with an angular sensor at the king pin, Lx is typical set between 2 and 4.
The default value will not effect the resulting relation.
YR, YL The difference between the set values of both parameters define the freedom of the steering
actuator. Normally, YR is set equal at the mechanical end lock that steers the vehicle into a right direction.
YL is normally set equal to the mechanical end lock that steers the vehicle into a left direction. In case an
opposite steering behavior is required, YR must be set at the negative equivalent. YL must be set at the
positive equivalent.
The default value for YR and YL is set equal to the mechanical locks of the steering actuator and provides
steering in the right direction.
Default
0
1000
-1000
YR and YL may not both be zero nor have same sign.

Feed-forward

This variable is used to feed the drivers steering intends forward to the valve. It minimizes effectively lag
in the steering actuator motion. The feed forward has most effect when the system responds 80 to 90%
of the exact intend. This is accomplished by scaling steering wheel speed using the following parameter:
StrkVol scales the feed forward in order to get the specified number of steering wheel turns within the
end-locks. It represents the stroke volume between the mechanical end-locks in cm3.
Kp must be temporarily set at zero to eliminate the closed loop contribution while tuning. Tuning is
finished when the number of steering wheel turns from lock to lock is at 80 to 90 % of the turns specified.
If 4 turns was specified, the number of turns should be between 4.4 and 4.8.

Steady State Error

Since the steering actuator acts as a free integral and the dead band in the valve is compensated in
software, the loop does not necessarily include a second integrate term to achieve accuracy. The
controller in EHPS software has therefore only a proportional term, which keeps tuning relatively simple.

To achieve steady state accuracy:

•
The difference between the location of spool dead band specified in the spool compensation
function in the EHPS software and the true locations should be as little as possible.
•
The amount of internal leakage at all potential locations between cylinder and valve and its
dependency on steering pressure should be as little as possible.
•
The amount of backlash in the feedback signal. Extreme care must be taken when an actuator
position sensor is installed.
The controller has proofed repeatable steady state accuracy at ± 1% of the full control region.

Proportional Band

In order to acquire open loop steering characteristics like absences of stability problems and lag, the
available proportional band for the controller is variable to steering wheel speed as shown in the figure
below.
Value range
-10 to 10
-1000 to 1000; Yr ≠ 0
-1000 to 1000; YI ≠ 0
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