digital control systems for synchronizing hydraulic servo cylinders

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15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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411
Scientific Bulletin of the
Politehnica University of Timisoara
Transactions on Mechanics Special issue

The 6
th
International Conference on
Hydraulic Machinery and Hydrodynamics
Timisoara, Romania, October 21 - 22, 2004
DIGITAL CONTROL SYSTEMS FOR SYNCHRONIZING HYDRAULIC SERVO
CYLINDERS

Nicolae VASILIU, Professor*
Department of Hydraulic Machinery
“Politehnica” University of Bucharest
Constantin CALINOIU, Associated Professor
Department of Hydraulic Machinery
“Politehnica” University of Bucharest
Daniela VASILIU, Professor
Department of Hydraulic Machinery
“Politehnica” University of Bucharest
Dragos OFRIM, Senior Researcher
Department of Hydraulic Machinery
“Politehnica” University of Bucharest
*Corresponding author: Spl. Independentei, nr.313, sector6, 060042, Bucharest, Romania
Tel.: (+40) 021 4105642, Fax: (+40) 021 4106480, E-mail: vasiliu@fluid.fluid-power.pub.ro


ABSTRACT
This paper presents the theoretical and experimental
researches performed by the authors in order to design
a digital control system for electrohydraulic servo-
motors. The system contains a digital control module
and two position servomechanisms. A mathematical
approach was developed for obtaining a mathematical
model describing the whole control system. The
dynamic performance of this system was studied by
Matlab – Simulink software. For validating the mathe-
matical model, the authors designed and built an ex-
perimental testing bench, containing a constant pres-
sure

supply

source,

two

hydraulic

cylinders,

two

electro-
hydraulic flow control valves (BOSCH) and an indus-
trial process computer (ADwin – Pro). The program-
ming language was ADBasic, a high performance
software, specialized for DSP modules, made by
KEITHLEY. The experiments created a realistic im-
age of the efficiency of the different control algorithms.

KEYWORDS
Digital electrohydraulic control systems, proportional
valves, control algorithms
1. INTRODUCTION
The synchronization of the hydraulic cylinders rods
is an important practical problem, which can be solved
by different methods: mechanical, electrical, hydraulic
and

electrohydraulic

ones.

The

modern

electrohydraulic
servo systems, which contain flow servo valves or
proportional flow control valves (direct drive valves)
can be controlled by two algorithms:
a) the analog servocontrolers of the two servo-systems
receive the same reference signal from the control
unit; the position error between the two rods is
used for generating two aditional control signals
for the two servocontrolers; this method seems to be
a natural one, especially for the systems containing
two cylinders only, as for water gates;
b) one of the servo system is designed as a „leader”,
and the other have to follow the movement of the
first; this method is suited for controlling a set of
cylinders used for the same purpose, as in case of
the Pelton turbine with 2...6 nozzles.

This

paper

contains

an

evaluation

of

the

two

methods.

The experimental researches were carried out on a
full test bench designed and built in the Fluid Power
Laboratory

from

the

Hydraulic

and

Hydraulic

Machines
Department. The test bench diagram is presented in
figure 1; some partial views are presented in figures
2,3, and 4. The digital compensators included in the
overall loops are proportional on PI ones. The input
signal of the compensators is the error between the
hydraulic cylinders strokes. The paper contains the
experimentalresults for PI compensators only. The
proportional compensator use offers a good dynamics,
but the steadystate error strongly depends on the cyl-
inder load difference. This fact was systematic checked
by numerical simulation and extensive experiments.
A very simple control system cannot solve the problem
of the accuracy, even the real requirements seem to be
low. The main characteristics of the test bench are the
following:

industrial

process

computer

type

ADwin-Pro
(KEITHLEY); servo solenoid valves with integrated
amplifier NG 10 (BOSCH); analog speed/position
servo controller AVPC (BOSCH); hydraulic cylinder
∆e =180 mm/ ∆i = 100 mm /500 mm; inductive po-
sition transducers (PENNY & GILLES), industrial
412
computer IPC connected by an Ethernet interface
with the industrial process computer. The pressure
supply

is

a

constant

one:

40

bar.

The

common

reference

synchronizing system diagram is presented in figure 5.
The diagram of the synchronizing system containing
a leader cylinder is indicated in the figure 6.



Figure 1. Test bench diagram: DP1, DP2 – high speed proportional valves; BE – analog servo controller



Figure 2. Partial view of the test bench: electrohydraulic servomotors

413


Figure 3. Partial view of the test bench: analog components




Figure 4. Partial view of the test bench: ADwin and IPC


Figure 5. Control diagram with common reference signal: EC – comparison element, AVPC – analog servo
controller; SM – servomotor, Tr – position transducer; PID – digital controller; Y – servomotor stroke

414

Figure 6. Control diagram with a leader servo system: EC – comparison element; AVPC – analog servo
controllers; SM – servomotor, Tr – position transducer; PID – digital controller; Y – servomotor stroke



2. NUMERICAL CONTROL SYSTEM
FEATURES

The best solution for fast real time applications is
to place a dedicated CPU close to the signal source
and therefore having dedicated resources for the pur-
pose of processing this data. Only this structure gives
the ability of exact response times with predictable
delays.
If the intelligence is not localized and dedicated, but
centered on a host PC platform, all calculations are
under the control of PC’s operating system, Windows,
and its available resources. As a consequence, there
can be no guarantees for response times to either an
external event interrupt or an internal timer interrupt.
Furthermore, processes executing based on timer
feedback will become erratic, at best, due to incon-
sistencies in the system timer. However, Windows
offers comfortable user interfaces, multitasking func-
tionality, and great possibilities for network function-
ality. In order to take advantages of a Windows envi-
ronment and run fast and stable real-time processes,
it is necessary to use the ADwin family of products.
ADwin real-time systems are complete process
controllers with analog and digital I/O, a local CPU,
and local memory. ADwin systems use DSP’s, which
guarantee response times of as little as 1.2 micro-
second to an interrupt, while maintaining complete
software stability, even in a Windows environment.
Since the local processor handles the process control
and/or data acquisition, the PC processor is free to run
a user interface program, for example, a man-machine
interface with data visualization, user input, data
storage, etc., without regard for the effect the user
front-end software has on PC resources.

Since the programs runs on the processor of the
ADwin board, up to 10 processes can run simultane-
ously on one CPU, with priorities assigned where
required. Processes can interact, exchanging parame-
ters and data.
Thus, each process has its own independent timing,
but it is also possible to exchange parameters and data
with other running processes. Finally it is possible to
develop complex applications with processes, which
interact more or less.
The timer has a resolution of 25 ns (depending on
the

processor).

The

sequences

are

compiled

by

ADbasic,
and

therefore,

are

optimized

to

run

quickly

on

the

RISC

CPU. The response to a new measurement result is
immediately and completely executed, so that reliable
process execution times for analog controls below 3µs
(A/D conversion, online calculation, and D/A output)
can be guaranteed.
An important feature of these systems is that an
ADbasic application processes each new measurement
immediately. Data transfers between ADwin and the
PC do not have any affect on the real-time capability.
The data transfer is executed in the background,
so that ADbasic programs can use 100% of the CPU
resources.
The applications for ADwin devices are unlimited.
This list is a sampling of typical applications: usage of
very-fast discrete-time controllers (PID up to 400 kHz,
PI, adaptive, state space, etc.); fast data acquisition,
up to 1250 KHz sampling rate; simultaneously signal
synthesis; generation and measurement of analog
signals performing system monitoring and control
tasks in less than 300 ns.


3. ADbasic SOFTWARE FEATURES


Because the ADwin products are based on an em-
bedded RISC CPU, application-specific programs can
be downloaded to it. However, even with all the power
of the localized CPU, onboard memory, and optimized
hardware of the ADwin series, an easy-to-use devel-
opment environment that runs under Windows is a
necessity for achieving real-time systems in a quick
and cost effective manner. Without an easy-to-use,
yet robust, development environment, fast controls,
415
analysis, visualization, or archiving of data can and
does become difficult, if not impossible.
To facilitate this ease of development, ADbasic
was created. Programs developed in ADbasic are
written and compiled in the PC, and transferred to the
ADwin Series boards. For developing user interfaces
to set and read parameters of the processes, transfer
data, visualize and store data, the following packages
and programming languages are supported: Test Point,
Visual

Basic,

Visual

C++,

Borland

C++,

Excel,

Access,
Word, Matlab, LabVIEW, Lab Windows/CVI Delphi,
Turbo Pascal.
ADbasic is an integrated development environment
with powerful debugging functions to create fast
realtime measurement and control processes for the
ADwin Series CPUs. ADbasic is a real compiler, but
has the familiarity and ease of use of BASIC. Special
commands like adc, dac, digin, etc. allow direct access
to all inputs and outputs.
Additional functions allow automatic transfer of
data between ADwin and the PC. ADbasic generates
fast binary code for all ADwin CPUs. This code can
be downloaded either by the supplied utility program
at startup of the PC, or with the available drivers
from Test Point, LabVIEW, Matlab, programming
languages, etc. With ADbasic, run-time programs,
which can be loaded and started simultaneously with
an evaluation program, can be generated. That is
why ADbasic is needed only for the development of
these programs, but not for their run too. In the same
time, the programs compiled with ADbasic can be
run only on an ADwin system.


4. EXPERIMENTAL RESULTS


The experiments performing need a set of programs
for data aquisition and control for each computer of
the test bench. The programs have to point out the
electrohydraulic synchronizing system static and
dynamic performances. The ADbasic 3.1 language
was used for programming the Adwin - PRO indus-
trial process computer. The interface and monitoring
computer, running under WINWOS 2000, was loaded
with a dedicated program written under Test Point 5.1.

The figures 7 and 8 contains the experimental
results generated by a common reference step signal.
A wide range of step input was used: 0.1V, 0.5V, 1V
and 2V. The dynamic performance of the same servo
systems for a linear input signal can be identificated
on the figures 9 and 10. The overal ramp imput ampli-
tude was 0.1V, 0.5V, 1V and 2V, the overal ramp
period being shorter than the overal stroke time. The
sampling period of the real time control was 10 ms,
good enough for a speed of about 20 mm/s. The tuning
parameters of the digital error amplifier were corre-
lated

with

the

tuning

parameters

of

the

analog

amplifiers

of the two servo systems.



Figure 7. Synchronizing error evolution: common
reference signal, 2V step input signal



Figure 8. Synchronizing error evolution: leader
servosystem, 2V step input signal


Figure 9. Synchronizing error evolution: common
reference signal, linear input signal with 2V final
value

416

Figure 10. Synchronizing error evolution: leader
servosystem, linear input signal with 2 V final value

From the point of view of the cylinder load, the
studied case was a very difficult one: the start
pressures of the cylinders were very different: from
1 bar to 5 bar. Some technological errors lead to this
difference in the first part of the stroke. The precision
is recovered slowly inside this part of the stroke.

5. CONCLUSION


In all the investigated cases, the best dynamic
precision is obtained with common reference ramp
input signal. The temporary operation in a saturation
state of one system component leads to a bigger
synchronizing error and a longer stabilizing period
of the error.
The PI compensators offer a null steady-state
error, without connection with the difference between
the static friction and load of the cylinders. The syn-
chronizing system controlled by proportional
compensators cannot avoid the steady-state errors
generated by the difference between the cylinders
loads and frictions.
The numerical control system developed by the
authors has wide capabilities of connection with other
realtime systems in a distributed and hierarchical
complex control system. The programming facilities
of the entire system essentially the applications de-
velopment by medium level IT staff.

Finally, we can say that the use of the high-speed
proportional valves in conjunction with ADwin hard-
ware and ADbasic software leads to high-performance
control systems both for civil and military appli-
cations.


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Data

Acquisition

Low

Cost

Automation,

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