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An Engineering Analysis
of the Pulse Width Modulation Method
of Controlling Output Pressure of a Hydraulic Power Unit

By Jack L Johnson, BSEE, PE, EH Engineer
Brian S Johnson, BSMET, Hydraulic Specialist
PUBLISHER: IDAS Engineering, East Troy, WI, USA, 1996

 


                                                       ABSTRACT

Pulse width modulation has been used successfully in electronic control circuits for at least forty years. It is an efficient method for controlling a large amount of output power while expending a relatively small amount of power in the controller itself. The sources of inefficiency in the electronic PWM circuit are well-known, and are reviewed in this book: Finite switching time of the solid state controlling devices and the finite conducting resistance of the switching device when it is in its ON-state. The extremely short switching times of electronic devices allows the efficient use of PWM at frequencies in the kilohertz range, and so the range of applications is fairly broad.

On the other hand, using pulse width modulation to perform hydraulic switching necessarily uses valves, the switching times for which must be measured in tens or even hundreds of milliseconds, not microseconds or nanoseconds as in the case of electronic circuits. The result is that the PWM hydraulic circuit too often produces unwanted vibrations and pulsations at frequencies which are in the ranges at which they are palpable in the output actuators. That possibility exists in the case of the PWM regulated pressure source that is the subject of this book.

The authors constructed a PWM controlled constant pressure power supply such as those that would be used for electrohydraulic servo mechanisms and motion control systems. The test results are displayed and analyzed, and a practical design methodology is outlined which can be used by others wishing to apply the PWM method to regulate pressure from a power unit that is to supply more than a trivial amount of power to a dynamic load. The suitability of the system design was evaluated by comparing the servo system performance with the PWM power supply with the same tests with a conventional pressure compensated pump.

 

The system that was studied consists of an engine driven pressure compensated pump which was operated as a fixed displacement pump for the PWM tests and was also operated in its normal pressure compensated mode for purposes of comparing the two pressure regulation methods. Accumulators were engaged and disengaged at both the pump and control valves sites. This book summarizes the results of the engineering study regarding the effectiveness of the Pulse Width Modulation method of controlling pump output pressure.



                                              OBJECTIVES

There were two objectives to this applied engineering analysis:

1. To determine the extent that a regulated pressure source could be constructed with PWM technology using a fixed displacement pump and augmenting accumulator as compared to the conventional method using a pressure compensated pump.

2. To formulate a design strategy for such hydraulic power units, especially analytical methods for determining the proper sizes for the accumulator and pump.



                                                   FOREWORD

The technical staff of IDAS Engineering undertook an effort to study the characteristics of a constant pressure power source that uses pulse width modulation in order to effect the pressure regulation. Many details of that study are contained in the report that follows.

It is stated in the CONCLUSIONS section that this is a viable method of pressure control and cites the test results as justification. It is hoped by the writers that the hydraulic industry will see the merits of this method of pressure control and use it in those applications where it makes economic and performance sense to do so. The results, then, were intended to yield benefits in the short run, and as such, represent an engineering study aimed at being able to design and build such systems for use in many industrial and mobile systems.

In short, the efforts represent the kind of thing that most engineers in the real industrial world would like to do, if they had the time and resources. Too often, the pressures of the competitive business world force engineers to accept their own successes without necessarily knowing precisely why, or not having the wherewithal to test their theories, or without understanding the full range and impact of their own efforts.

The project started out with an ambitious agenda, which would have involved, initially, over 1000 tests, if all the combinations and permutations were executed. Perhaps we should have explored all of them, pursuing our own dream of knowing all the salient features of our own creations. Alas, we too, are victims of the clock and economics. Nonetheless, this report, we feel, represents more study than can normally be given to a specific development project in so many industrial venues.

We sincerely hope that the studies are both useful, and even a bit tutorial. All inquiries should be directed to the writers at the address and phone number below.
Good luck!
Jack L Johnson  &  Brian S Johnson
East Troy WI 53120
September 1996

 


                   TABLE OF CONTENTS

ABSTRACT


OBJECTIVES


BACKGROUND ON MOTION CONTROL AND CONSTANT PRESSURE


EQUIPMENT AND PRINCIPLES OF OPERATION

   THE LOAD AND THE PROFILE
   INSTALLATION DESCRIPTION
   PWM ON-OFF DUMP VALVE
   COMPUTER AND DATA LOGGING
      Data Logging Descriptions
   THE COMMAND PROFILES
      Velocity and Position Command Profiles
      Cylinder And Profile Constants
       Load Flow Demand Profiles
   BACKGROUND PERSPECTIVE ON PWM
      Ideal Electronic PWM Control
      Practical PWM Control
      Hydraulic PWM
      Hydraulic PWM Inefficiencies
      Valve Pull-In and Drop-Out Times


METHOD OF INVESTIGATION
   FIXED TEST PARAMETERS
   PROCEDURE
   POST PROCESSING OF THE DATA
   PRESSURE REGULATION CRITERIA
      Baseline Testing
   REAL TIME PROCESSING
   SHORTHAND NOTATION FOR
   CIRCUIT CONFIGURATION


TEST RESULTS
   EXAMPLE MOTION CONTROL SERVO RESPONSE
   TANK PORT PRESSURE TRANSIENTS
   VALVE SHIFT TIME EVALUATIONS
   ABOUT THE PUMP RIPPLE
   PRESSURE COMPENSATION AND THE BASELINE
   TEST RUNS
   PRESSURE COMPENSATION DYNAMIC DATA
   ANALYSIS OF THE P-COMP DATA
      Servo Loop Error Evaluation
      Observations On Pressure Compensation Results
      Steady-State Pump Characteristics
      Pressure Compensator Without Accumulators
      Variable Displacement and Pressure Compensation
   PWM PRESSURE REGULATION TESTS
   PWM DYNAMIC DATA GRAPHS
   ANALYSIS OF PWM RESULTS
      Dump/Undump Frequency
      Effects Of Accumulator Location
     More About Pressure Ringing
     Oddities Of The PWM Recharge Frequency
   Observations Using The 1 Second Motion Profile
   Comparison Of Profile Flow To Measured Flow
   Reducing The Dump/Undump Set Point Spread
   Results Of Comparative Efficiency Tests


DESIGN METHODOLOGY
   THE FIXED DISPLACEMENT PUMP
   THE PWM ON-OFF VALVE
   ACCUMULATOR SIZE


CONCLUSIONS


APPENDIX A -- IDAS ENGINEERING LAB EQUIPMENT


APPENDIX B -- STATISTICAL DATA SUMMARIES


APPENDIX C -- CORNER POINT PROFILE VALUES


APPENDIX D -- LIST OF FIGURES
 


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