Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (leaf 11).An investigation into the failure of pneumatic actuators on the HypoSurface was conducted to provide information on the current HypoSurface prototype. Using a systematic approach throughout testing, piston components were found to have been subjected to fatigue-failure, causing pressure leaks from the piston. Through further analysis, the increase in wear on the piston gaskets was caused by exceeding the recommended loads during operation of the HypoSurface with its skin. Furthermore, vibrations from piston actuation may be the cause of loose electrical connections in the control panel, resulting in voltage problems in the solenoids.by Darren M.K. Chun.S.B

Chun, Darren M. K. (Darren Masayasu Kekoa)

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Investigation into the Cause of Pneumatic Actuator Failure on theHypoSurfacebyDarren M. K. ChunSUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING IN PARTIALFULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OFBACHELOR OF SCIENCEAT THEMASSACHUSETTS INSTITUTE OF TECHNOLOGYJUNE 2007© 2007 Darren M. K. Chun. All rights reserved.The author hereby grants to MIT permission to reproduceand to distribute publicly paper and electroniccopies of this thesis document in whole or in partin any medium now known or hereafter created.Signature of Author:I V//Department of Mechanical EngineeringMay 11, 2007Certified by:Accepted by:MASSACHUSETTS INSTITUTEOF TECHNOLOGYJUN 2 1 2007Mark GoulthorpeAssociate Professor of ArchitectureThesis SupervisorJohn H. Lienhard VProfessor of Mechanical EngineeringChairman, Undergraduate Thesis Committeerrb~CnffJE-SInvestigation into the Cause of Pneumatic Actuator Failure on theHypoSurfacebyDarren M. K. ChunSubmitted to the Department of Mechanical Engineeringon May 11, 2007 in partial fulfillment of therequirements for the Degree of Bachelor of Science inMechanical EngineeringABSTRACTAn investigation into the failure of pneumatic actuators on the HypoSurface was conducted toprovide information on the current HypoSurface prototype. Using a systematic approachthroughout testing, piston components were found to have been subjected to fatigue-failure,causing pressure leaks from the piston. Through further analysis, the increase in wear on thepiston gaskets was caused by exceeding the recommended loads during operation of theHypoSurface with its skin. Furthermore, vibrations from piston actuation may be the cause ofloose electrical connections in the control panel, resulting in voltage problems in the solenoids.Thesis Supervisor: Mark GoulthorpeTitle: Associate Professor of Architecture1. Introduction:HypoSurface is the world's first dynamic surface display system. Using a powerfulinformation bus technology, more than 500 moving actuators are controlled to deform a pliablesurface, allowing high-speed movement of the screen surface. A wide-range of effects, such aswaves, patterns, images, and text, can be generated and choreographed for entertainment andinformation display. HypoSurface is made to also be responsive to sound and movement tocreate an engaging and interactive environment with its audience.' The current prototype of theHypoSurface uses pneumatic pistons to actuate the surface. During each prototype stage,information is essential for improvement, such as actuator limitations. This purpose of this thesisis to research into why the pneumatic components are failing within the current prototype, and todevelop a systematic approach to finding problems.2. Pneumatics background:A pneumatic system is a system that uses pressurized gas, usually air, to generate power.With a variety of advantages, pneumatic powered systems are generally used in a wide range ofapplications. Pneumatic advantages include high reliability due to few moving parts, low cost,the availability of a large selection of sizes and capacities, easy installation and maintenance, andthe capability of being easily integrated with other systems. Also, with air as the working fluid,pneumatic systems have advantages over hydraulic systems, such as having relativelylightweight supply hoses, not needing fluid return lines, and having leaks that tend not to bemessy. However, pneumatic systems still have a few drawbacks compared to other systems.Pneumatic systems always need at least a compressor and distribution lines, which takesaway from spatial efficiency for large systems with hundreds to thousands of actuators. Inregards to speed of response to control signals, electronic systems are much faster thanpneumatic systems by bypassing the speed of airflow and valve shuffling speed. By using a fluidthat is nearly incompressible, hydraulic systems are able to have higher energy densities, leadingto much higher working pressures than pneumatic systems. For example, for applicationsneeding power on the order of magnitude of ten horsepower, the increasing size and cost ofpneumatic devices begin to exceed the basic cost of a hydraulic power generation andtransmission system.Despite some disadvantages, a pneumatic system is more than adequate for testingprototype concepts of the HypoSurface. The physics behind the pneumatic actuators in thecurrent HypoSurface prototype are straightforward. The thrusting force of the pistons is directlyrelated to the cross-sectional area within the piston and the pressure within the tube. Since thecross-sectional area will always remain constant, forces will vary relative to varying internalpressures. Speed of actuation is dependent upon mass and duration. Given the thrusting force, FT,the mass of a skin element, ms, the mass of the piston shaft, mp, and the total time of actuation, t,the average velocity, v, of the actuation is shown below in Equation (1).F .t(m + m (1)With the thrusting force and average velocity of the piston, the power generated from a singlepiston is given in Equation (2).P= v.F (2)where P is the average power generated in a stroke, v is the average velocity, and FT is thethrusting force. These calculations are good estimates to describe the system and define certainpower and pressure requirements. More precise calculations for stroke velocity would need toinclude sliding friction and static friction generated from beam bending.3. Apparatus and Schematics:The current HypoSurface prototype consists of ten modules, with each module housing56 pneumatic pistons. Each module has a height of 12 ft 2 in, a width of 3 ft 3.5 in, and a depthof 2 ft 3.5 in. Air is supplied for the entire pneumatic system through a compressor into a maintank and a dryer, which is used to remove the moisture accumulated from air compression. Fromthe main tank, each module is supplied high pressure, low volume air through is suppliedthrough a hose with a diameter of 0.175 inches. The air pressure is then regulated down to supplyevery module with low pressure, high volume air through a 2 inch diameter hose. Within amodule, both high pressure and low pressure air is shared with every valve-piston assemblythrough a series of loops. Figure 1 below is a flow chart showing the air flow from thecompressor to a piston.~100 PsiHigh Pressure/Compressor Main Tank - LowVolume RegulatorValveExhaustLow Pressure/Control System II High Volume24 V Input L- Val Valve~50 PsiPistonFigure 1: Flow chart of the air flow throughout the pneumatic system.The fabrication of the HypoSurface modules and its components is done by a Britishcompany called Univer. A valve is manufactured out of die-cast aluminum, and contains aninternal piston, which is actuated indirectly by two solenoids. There are five main external portson a valve, one for the low pressure air supply, two for air consumption by the piston, and twofor exhaust, that are used to transport air to and from the piston. Also, to minimize leaks, thethreads of the valve connections are wrapped with Teflon tape. Figure 2 shows a valve assembly(a) and the cross-section of a valve during the extension of a piston (b).(a)2 4*3 1 5(b)Figure 2: Valve assembly. (a) Photo of a full valve assembly with numberscorresponding to the ports shown in the cross-section diagram. (b) Diagram 2 of the cross-section of a valve showing how the ports are controlled by the internal piston. Port 1 isthe low-pressure supply, ports 3 and 5 are the exhaust, port 2 provides air to the back ofthe piston, and port 4 provides air to the front of the piston.High pressure in the valve is used to shift the internal piston. During the activation of asolenoid, a magnetic field is generated, pushing a cap and displacing the piston just enough to letthe high pressure air through. Figure 3 reveals the high pressure air path to the internal piston.Figure 3: Revealing the high pressure path to the internal valve piston.The pneumatic pistons are 26 inches long and have a 22 inch displacement. The outercasing is made out of aluminum and houses a stainless steel shaft, which has an 8 mm OD and a5 mm ID. Pistons on a module are radially and axially constrained in the back with a threadedrod and nut assembly. The front of a piston is free to displace, but will return to its originalposition due to 4 springs. Low pressure, high volume air is sent from the valve to the front andthe back of the piston for retracting and extending the piston shaft respectively.4. Piston and Valve Failures with General Troubleshooting Procedure:During testing, pistons that don't move or pistons that respond slowly were the firstindications of a problem. With pneumatic systems, it is currently not feasible to eliminate allleaky sources because of the lack of perfect seals or sealing techniques available. Therefore, thenext step was to check all of the connections, since the source of the problem in either case couldhave still been the piston, the valve, or even both of them. After all the connections have beenchecked, meaning there was nothing loose or disconnected, the first thing to check is the piston.The first thing to look for, while checking the piston, is whether or not it has a bent shaft.In most cases, a bent shaft would explain why the piston during testing was either really sluggishor unable to move. However, aside from bent shafts, the most common problem with the pistonwas a faulty head gasket or internal gasket, which both caused unwanted pressure leaks withinthe system. With the piston extended, a faulty head gasket would let out a loud hiss whiledisplacing the piston shaft in the radial direction. A faulty internal gasket was detected when thepiston shaft could be displaced axially without a significant restoring force, while both highpressure and low pressure was still active throughout the system. Figure 4 shows the wear andtear of an internal gasket from general use of the HypoSurface.Figure 4: Wear and tear of a gasket inside of a faulty piston.After checking the piston for problems and the piston is still "dead," the only thing left tobe checked was the electrical connections to the valve. To check the electrical connections, oneneeded to disconnect the solenoids from the valve and expose the wires from the housing. Usinga multimeter, set to measure DC voltage, the voltage across each solenoid was measured. Inorder for the internal valve piston to properly shift, only one solenoid should measure 24 V,while the other solenoid should have 0 V. If this was not the case, then the problem liedanywhere along the wiring between the control system to the module's control panel or along thewiring between the control panel and the valve solenoids. However, if the voltages were fine,then the solenoids needed to be checked whether or not they do their job in shifting the valvepiston. This could be done by applying 24 V, one at a time, to both solenoids and listening for a"click" sound from the valve piston shifting.5. Discussion and Conclusion:The current HypoSurface prototype is not capable of handling the demanded loads andcycles, shown by the high failure rate in both the skin components and the pneumaticcomponents. For future prototypes, improving the piston and piston shaft constraints andreducing the mass of the skin will greatly improve the life of the components. Piston and pistonshaft constraints are important because with future prototypes having larger and larger pistonstrokes, the deflection at the end of the shaft and moment felt on the piston housing willdrastically increase if not properly constrained. Then again, increasing the diameter of the pistonshaft will also help to reduce deflection. Mass could be effectively "reduced" by having morepistons per given area of skin, which will minimize the end load on the piston shaft.The next generation module, with the valve assembly separated from the piston andplaced in the control panel, will be an improvement in maintenance and troubleshootingdifficulty and will eliminate problems caused from lengthy electronic wiring. Also, pistonactuation speed and timing will not be affected by the variable length tubing. Using a fluiddynamics model, with the current running pressure and as long as the diameter of the tubes iskept small, the time it takes the air to travel the extra distance is negligible and on the order ofmagnitude of a millisecond.For the next generation skin design, the current durability problems with the current"squids" should be taken into account. If rubber is still to be used, it should be flexible enough towithstand stretching between a piston at its maximum stroke and a surrounding piston at itsminimum stroke. This is most important when an adjacent piston is unable to move to relieve thestress on the skin.6. AcknowledgmentsThis thesis was conducted with the help of thesis advisor and founder of HypoSurface,Prof. Mark Goulthorpe.7. References'from notes and materials provided by Prof. Mark Goulthorpe2UNIVER homepage <http://www.univer-group.com/eng/indexeng.html>

Investigation into the cause of pneumatic actuator failure on the HypoSurface

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Investigation into the cause of pneumatic actuator failure on the HypoSurface

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