Software for Fluid Power Technology

From Editor

The purpose of the Software Review section of the Journal is to present information to the reader about engineering software, including simulation programs, to highlight their specific features and their "fitness to purpose" in the unique field of fluid power and motion control. It is, of course, impossible to establish evaluation criteria matching the needs of all readers, therefore readers should not look for absolute ratings but more or less "fuzzy" opinions of the reviewer. A software program is like a wrench, just a tool to solve problems. It is good to solve some problems and not so good for others and this depends on both the nature of the problem and the users' attitude - and generally when we review software we do not know either. A software tool can be highly specialised and great for a some applications but not so well suited for others, on the other hand another software tool can be more flexible and generally applicable but without outstanding features. It is impossible, and even misleading, to say which one is better. What we hope to accomplish is to give the reader information necessary to take his/her own decision.

Modelling & Simulation of Fluid Power Systems with
HYDRO ANALYST

Abstract

The development of the simulation and modelling package ‘Hydro Analyst’ was undertaken to bridge the gap between the engineer conversant with Fluid Power and Control Theory and the majority of practising engineers with only a rudimentary knowledge of the subjects. The major contributory factor is the very limited time allocated to Fluid Power and Control Theory by most universities.
The main objective of developing the software package has been the challenge of turning a simulation into a practical engineering tool aimed at a wide spec-trum of engineers faced with the task of designing and analysing an electro-hydraulic control system.
The computer programs are based on the mathematical models described in the first part of the recently published textbook ‘Hydraulic and Electro-Hydraulic Control Systems’. The second part of the book is written in the form of an Operating Manual for Hydro Analyst and therefore does not contain any equations or algorithms or require any specific analytical aptitude. Full details for both Hydro Analyst and the textbook can be obtained from the website: http://www.flotron.co.uk (includes provision for downloading a free evaluation copy of Hydro Analyst in either metric or US units)

Description

Most electro-hydraulic control systems are represented by a high order transfer function, of at least a 5th and, in many cases, considerably higher order. This is due to two major factors:

The non-linear characteristics of proportional control valves,
The compressibility of the fluid.

The complexity of analysing systems described by high order differential equations precludes the Application Engineers with limited analytical capability to attempt such a task.
The failure to carry out a full system analysis, in-cluding dynamic performance, can result in either a system which does not meet the specification or an over-designed system. The former can, and not infrequently does, result in costly litigation, whereas the latter will make the proposal less cost-effective and hence less competitive.
Hydro Analyst Package comprises three functional elements:

Part 1 of the 2nd edition of the textbook Hydraulic & Electro-Hydraulic Control Systems
Part 2 of the textbook
The simulation software Hydro Analyst

The aim of the package is to bridge the gap between an analytically competent Engineer and the majority of Application Engineers. Element1 is an in-depth study of electro-hydraulic component and System design, whereas element2 explains the structure and is a ‘hands on’ step-by-step system modelling exercise for Hydro Analyst.
The three elements are complementary in that the system simulation programs presented in elements 2 and 3 could not have been written without the mathematical models and algorithms formulated in Part 1. On the other hand the subject matter described in Part 1 could not form the basis of a practical feasibility study without a computerised analysis package, such as element 3, the simulation software ‘Hydro Analyst’.
Part 2 is presented in the form of an Operating Manual and therefore does not contain any equations or algorithms or require any knowledge of control theory or any specific analytical aptitude.
The main features of the package can be summarised as:

A tutor for the application of the System Simulation software HYDRO ANALYST, providing a ‘hands on’ operating procedure.
Analytical optimisation procedure allows system optimisation compatible with any given stability criteria.
High order system transfer functions can be accommodated; at present up to 15th order, but easily extendable to higher orders if required.
9 feedback options.
Transient Response to any given input func-tion, including composite duty cycle.
A built-in continuous function generator pro-viding many wave forms.
User friendly programs including component and system data bases.
Automatic component selection feature.
Comprehensive graphics display and print-out facilities exportable to documents.
Continuous automatic system integrity check with remedial action messages.
Automatic Looping under optimised or non-optimised operating conditions.
Interactive HELP file covering all modules.
Tutorial with pre-set access to modules.
The programs have been used extensively over a number of years for system modelling, with close correlation between predicted and actual performance being recorded.
Fully interchangeable versions in both metric and US units.

Attached are typical screen print-outs of the Hydraulic Transmission, Components, Frequency and Time modules and multiple graph print-outs of the Frequency and Time modules. Typical Power Efficiency and Power Dissipation plots (Fig. 14 to 17) are also included.
Automatic Looping, i.e. the provision for determining performance trends over any chosen operating range, is a special feature of Hydro Analyst. Automatic Looping is applicable to the Time and Frequency domains as well as to the Power domain.
Transient performance can be established as a function of any of 34 independent variables. An example of A.L. in the Frequency domain is the graph ‘Stability margins’ where Phase and Gain margins are plotted as a function of Loop Gain (Fig. 7). The plots of Power Dissipation and Power Efficiency as a function of Load Pressure are examples of A.L. in the Power mode (Fig. 14 to 17). Facilities for analysing systems with multiple variables are also included.
In contrast to the Bode and Nyquist diagrams and the Nichols chart which show system frequency response at a set Loop Gain, the plot of Stability Margins gives an overview over a wide spectrum from a negatively damped to an underdamped, adequately damped and overdamped system (Fig. 4 to 7).
A.L. provides the facility to determine the significance of any chosen parameter with respect to steady-state and dynamic system performance.

Performance Specification for a typical system

A table moving in a horizontal plane is required to be accurately positioned over a total range of 300 cm at a maximum velocity of 50 cm/sec by means of a single ended cylinder.
The total mass of the table is 3000 kg and the actuator is subject to an external resisting force of 12 kN.
An additional requirement is the ability of the system to respond to a sinusoidal stimulus of 5 hz at an amplitude of 1 cm.
Steady-state and dynamic performance charac-teristics are to be investigated.

System Design and Analysis

The task set out in the Performance Specification was performed using the simulation package ‘HYDRO ANALYST’. The program group contains four interactive programs:

Flow Control
Pressure Control
Power Efficiencies
Help

For this application a proportional flow control valve will be selected as the electro-hydraulic interface.

Flow Control

The program is arranged in a modular structure comprising 12 inter-active modules:

Hydraulic Transmission (Cylinder)
Hydraulic Transmission (Motor)
Components
Options
Frequency Domain
Time Domain
Power Efficiencies and Dissipation
Graphics Display
Summary
Examples
File
Help

Hydraulic Transmission (Cylinder)

Since the hydraulic system, when subjected to a sinusoidal stimulus, has to control both accelerating and decelerating loads, a system configuration capable of handling negative loads has to be selected. In this case a 4 way meter-in/meter-out valve was chosen. All independent variables, consistent with the general performance specification, are entered into the text boxes provided, and dependent variables, e.g. flow rate, load pressure, valve pressure drop, are automatically entered by the program. This module performs two functions:

Hydraulic system integrity check, i.e. a di-agnostic for faulty system design.
Derivation of hydraulic transmission trans-fer function.

All entries, together with the h.t. transfer function, are displayed on the ‘Hydraulic Transmission’ screen (Fig. 1).

Fig. 1: Hydraulic Transmission Module

Components

This module provides a number of manufacturers’ databases in which the steady-state and dynamic characteristics of the valves are stored, and which are available for selection to complete the hydraulic circuit. Selection can be either automatic or manual. The percentage spool travel gives an indication of the size of the valve relative to the required duty. There is also the facility to create customized databases using normal catalogued data. The need for constant reference to catalogues is thus eliminated. Steady-state errors can be viewed in this module.
For the worked example specified, automatic selection and ‘Vickers Systems’ were selected. The ‘Components’ screen shows the valves enabled and the model chosen from the listing (Fig 2).

Fig. 2: Components Module

Frequency Domain

The main tasks of this module (Fig. 3) are the derivation of the system transfer function and the setting of the loop gain compatible with the given feedback mode and the stated stability criteria. Transfer functions of up to 15th order and 7 alternative feedback options can be accommodated. The default values for the stability criteria gain and phase margin are 7dB and 45 degrees respectively. Loop gain settings for closed loop systems can be established by one of three options: Manual, semi-automatic and automatic selection. Semiautomatic selection makes use of the plot ‘Stability Margins; automatic selection is the fastest and pre-ferred option.

Fig. 3: Frequency Domain Module

An additional task of this module is to generate frequency based graphs (Fig. 4 to 7):

Open and closed loop Bode diagrams
Nyquist diagram
Nichols Chart
Stability Margins

It can be seen from the screen that the system is represented by a 7th order transfer function with an optimal Loop Gain of 18.17/sec.

Fig. 4: Closed Loop Bode Diagramm (Amplitude and Phase)

Fig. 5: Nyquist Diagram

Fig. 6: Nichols Chart

Fig. 7: Stability Margins

Time Domain

In this module (Fig. 8) the ability of the system to respond to a sinusoidal input stimulus of 5 Hz at an amplitude of 1 cm has to be investigated. The system dynamic performance characteristics are automatically transferred from the Frequency module, the Continuous Function Generator and wave form are selected and the specified input data are entered.

Fig. 8: Time Domain Module

5 graphs are generated (Fig. 9 to 13):

Transient Response
Velocity Derivative
Velocity-Displacement
Accelerating Load
Transient Response to a customised duty cycle

Fig. 9: Transient Response	Fig. 10: Velocity Derivative
Fig. 11: Acceleration Load	Fig. 12: Velocity Displacement

Fig. 13: Customised Transient Response

Power Efficiency and Dissipation

Some typical bar charts and line graphs covering five alternative power unit arrangements are shown (Fig. 14 to 17). The line graphs plot power efficiency and dissipation as a function of load pressure. Line graphs as a function of flow are also included in the program.

Fig. 14: Power Dissipation Chart	Fig. 15: Power Dissipation Diagram
Fig. 16: Power Efficiency Chart	Fig. 17: Power Efficiency Diagram

Conclusion

Steady-state performance S-s errors, as displayed in the ‘Component’ screen, are:

Load error @ 12 kN = 0.47 mm
Hysteresis error @ 2% of command signal units = 1.29 mm
Velocity error @ 50 cm/sec actuator velocity = 27.52 mm

Dynamic performance The system can respond to a sinusoidal input of 5Hz and 1 cm amplitude without attenuation.

The time lag is approximately 0.06 sec.
The maximum velocity required to sustain this duty cycle is 30 cm/sec.
The maximum accelerating/decelerating load is 30 kN.

RW DH

Facts about HYDRO ANALYST

Vendor & Location	FLOTRON Ltd.
Contact Person	Don Harrison, Ronald Walters
Address	Unit 3, The Royston Centre Lynchford Road, Ash Vale Hants GU12 5PR, England
Phone	+44 (0) 1252 - 511003
Fax	+44 (0) 1252 - 519003
E-mail	d.harrison@flotron.co.uk
Internet	http://www.flotron.co.uk
Educational Version	Available
Platforms	Windows 3.1x/95/98/NT/2000

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