Control Engineering Practice 10 (2002) 645–654
Theoretical and experimental investigation of a pulse-width
modulated digital hydraulic position control system
Omer Keles*, Yucel Ercan
Department of Mechanical Engineering, Faculty of Engineering and Architecture, Gazi University, Muhendislik Mimarlik Fakultesi, Maltepe,
06570 Ankara, Turkey
Received 4 January 2001; accepted 14 December 2001
Abstract
In this study, the open loop and closed loop behaviours of a pulse-width modulated hydraulic system are investigated both
theoretically and experimentally. First a mathematical model is formulated for the system and methods for obtaining system
responses to pulse-width modulated inputs are developed. Using these methods, the responses of the servo valve, the open loop as
well as closed loop hydraulic systems are calculated for pulse-width modulated step and sinusoidal inputs. Effect of pulse frequency
on the sinusoidal response is investigated. An experimental set-up, which consists of a PC as the controller, a pulse-width modulator,
a servo valve driven hydraulic system and feedback elements, was constructed. Experiments show that precise position control can
be achieved by using pulse-width modulated inputs in electro-hydraulic control systems. The results obtained for sinusoidal
reference inputs show that the pulse frequency should be more than 50 times the reference input frequency for a satisfactory
performance of the tested system, if the reference frequency is less than 10 Hz. r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Pulse-width modulation; Electro-hydraulic system; Electro-hydraulic servo valve; Hydraulic position control
1. Introduction
Recent advances in digital hardware technology and
digital signal processing have opened new frontiers for
the hydraulic servo systems. Digital hydraulic servo
systems can implement complicated control strategies
and reference inputs, allow control of more than one
servo valve by the same control unit, adapt themselves
easily to changing system parameters, have easy data
storage and monitoring, and can be constructed
inexpensively by the use of standardized hardware such
as PC’s, data acquisition/control cards, etc. Use of
pulse-width modulation (PWM) in hydraulic systems on
the other hand makes possible the use of inexpensive but
fast switching on/off valves, replaces the dither signal in
the case of slowly moving loads and easily implemented
to digital systems by using hard real-time programming.
Digital hydraulic controls have become important
especially in advanced applications such as industrial
manipulators, manufacturing, welding and sheet metal
cutting machinery, simulators, etc.
Advantages of digital technology and PWM may be
used conveniently in newly designed hydraulic servo
systems. However, there are still vast numbers of
analogue hydraulic servo systems, which have been
installed in the past and still used in the industry. These
systems are usually designed for limited tasks, have fixed
configurations and lack many of the advantages of
digital systems summarized above. Modernization of
existing analogue hydraulic systems usually involves
conversion of their electronic parts to digital systems.
The system still uses the existing servo valve after the
conversion. However, a digital device such as a PC reads
the feedback information, compares it to the reference
value and synthesizes a control input to the system. The
digital system gives out an analogue control signal, if a
D/A convertor card is used. In that case a simple PWM
circuit may be used along with a voltage to current
converter, and the use of more complicated servo
amplifiers with sophisticated control actions and dither
capability may be avoided. However, one can alterna-
tively use a PC to directly generate the pulse-width
modulated control signal, thus removing the D/A
*Corresponding author. Tel.: +90-312-2317400 ext 2413; fax: +90-
312-231940.
E-mail address: omer@mmf.gazi.edu.tr (O. Keles).
0967-0661/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0967-0661(02)00021-7