Dead-Band Crossing; Valve Types Overview; Valve Transfer Function - Danfoss pved-cl Operation Manual

Operation Manual
PVED-CL Controller for Electro-Hydraulic Steering, Version 1.38
Installation
Steering valve tolerance characteristics
Valve type
EH, dynamic spool
EH, static spool
OSPEH
EHPS
Xsp values are valid for programmed linear spool characteristic.
24 | © Danfoss | May 2016
For closed-loop applications such as auto-steering, the hydraulic dead-band has an undesirable effect.
Small position control errors are corrected by proportional spool activations to output a correcting flow.
However, if the spool movement is inside the hydraulic-dead-band, then a steady-state error is present
which will cause the vehicle to drift from the desired course. This is one of most likely reason for
degraded auto-steering performance and shall be avoided.
The performance objective in closed-loop mode is to always output a flow when a steered wheel position
error is present while ensuring that the valve operation is not to crude, leading to jerky vehicle course
corrections.

Dead-band crossing

In open-loop operation mode, the spool operation in the hydraulic dead-band can be configured by
parameters. The spool can either be controlled through the hydraulic dead-band manually by following
the input device set-point. This allows the user to control the level of pressure built-up of the steered
wheels. This is controlled with the valve-specific parameters, as well as the steering device specific
parameters (See the input device specific chapters).
Alternatively the spool operation can be configured to 'jump' between over the dead-band. This
resembles servo valve operation and gives a fast steering response.
In PVED-CL closed-loop operation mode, the spool will always jump over the hydraulic dead-band.

Valve types overview

The below table shows the hydraulic dead-band characteristics and tolerances for the available steering
valves. Due to mechanical tolerances on the valve parts, the exact hydraulic dead-band position cannot
be predicted for any single valve. Similarly, it may differ from valve to valve.
Nominal dead-band
[mm] [Xsp]
0.5 94
0.8 136
0.8 136
1.3 204

Valve transfer function

The valve transfer function matches the PVED-CL to the valve and thus also the hydraulic dead-band. The
PVED-CL converts the calculated requested port flow to a spool set-point, Xsp. Xsp is a scaled
representation of the physical spool displacement where 0 is neutral position and ±1000 is the maximum
possible spool stroke.
Dead-band tolerance
[mm] [Xsp]
0.36 – 0.62 77 – 108
0.6 – 1.1 106 – 177
0.6 – 1.1 106 – 177
1.05 – 1.55 170 - 238
Maximum spool displacement
[mm] [Xsp]
3 430
3 430
4 560
7 915
11025583 | AQ00000216en-US0302