1
AN IMPROVED PSEUDO-BONDGRAPH MODEL FOR PNEUMATIC SYSTEMS WITH EXPERIMENTAL
VALIDATION
de las Heras, S.*, Quevedo, J.**, Puig, V.**, Escobet, T.**
* Department of Fluid Mechanics - Campus of Terrassa
Universidad Politécnica de Cataluña (UPC)
Colon, 7 - E08222 Terrassa, SPAIN
delasheras@mf.upc.es
** Automatic Control Department (ESAII) - Campus de Terrassa
Universidad Politécnica de Cataluña (UPC)
Rambla Sant Nebridi, 10. E08222 Terrassa (Spain)
joseba@esaii.upc.es
ABSTRACT
The present paper deals with the improvement of pneumatic systems modeling. A pseudo-bondgraph model is described
taking into account the real gases behavior and the thermal constant time for estimating heat transfer. The main loss of
accuracy in predictive models of pneumatic systems is due to the lack of improved models for variable orifices, gas
thermodynamics and piston friction. A careful experimental work has been done in these directions.
Double-acting cylinders are usually operated by a five-port directional control valve to fully extend or retract pneumatic
cylinders. Estimating the cycle time and gas consumption is very important to improve the system behavior and reduce the
designing time. The validity of this method is demonstrated by experiments.
1. INTRODUCTION
The pneumatic technology is widely used throughout
manufacturing industry to provide linear motion, medium
force applications. Low cost pneumatic actuators are
cheaper, fast enough and they avoid pollution in the event
of external fluid leakage. This technology can present a real
advantage for positioning systems but, until now, it has
been essentially used for pick and place applications where
industrial plants have been served primarily by hydraulic
and electric servo drives. The main reason for this is the
difficulty in using compressed air as a control medium.
Firstly, a valve pneumatic controlled drive has very low
stiffness, small natural damping, and a natural resonant
frequency in the order of 10Hz or less. And secondly, the
inherent non-linear behavior of pneumatics makes it very
difficult to predict accurately and to control later.
Pneumatic actuator performance can be determined in first
approximation at steady state basing on the coupling of the
actuator and the regulation valve. In this way a prediction of
the real working conditions cannot be achieved, especially if
the design specifications require reduced motion times.
Although the linear models can only represent
approximately the dynamics of the physical system, they
have been used as an aid to the design of suitable control
strategies for pneumatic servos (Weston et al. 1984; Virvalo
1989; Brun et al. 1999).
Different non-linear control models have been proposed
in the specialized literature for describing the global
behavior of the pneumatic chambers (Brun et al. 1999;
Scavarda and Richard 1994), but often they are after
linearized in the vicinity of an equilibrium position.
Usually, these models produce an improved performance,
even considering the polytropic hypothesis, the gas as
perfect and some other simplifications.
Bondgraph is used for modeling (Karnopp et al. 1999;
Thoma, 1990) and monitoring (Ould et al. 2000) physical
systems and is essentially based on the energy flow through
the systems and between the subsystems contained in them.
Bondgraph provides a unified approach to modeling since it
is applicable to all engineering sciences focused on energy
or power. The causality relationship and the energy and
continuity principles are inherent in bondgraph unlike other
modeling procedures allowing a better understanding of the
physical system.
Traditionally, thermal systems have been presented with
temperature and heat flow as effort and flow power
variables (Karnopp et al. 1999). However, it is well known
that the product of temperature and heat flow is not a power
since heat itself has the dimensions of power. These
bondgraphs are called pseudo-bondgraphs, distinct to the
true bondgraphs, and use heat flow as flow variable because
it is conserved in conduction opposite to the entropy flow.
Pseudo-bondgraphs preserve all the virtues of bondgraph
modeling and system identification, and use the known
partial differential equation between heat flow and
temperature to calculate temperature distributions (Thoma,
1990). Unfortunately, they are not power-conservative,
which is a small price to pay in exchange for the enormous
modeling flexibility the technique affords.
A true bondgraph for pneumatic systems that was able to
predict experimental temperature variations accurately was
presented in (Ikeo et al 1992). But upstream and
downstream the valves both the volume and entropy flow
rates are different because of the compressibility of air and
.....