P18318: Automatic Extend-Retract Hydraulic Restraint System for Amusement Rides
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Detailed Design MSD I

Table of Contents

This page documents follow on work from design work which was done during the Preliminary Detailed Design Phase. This means that analysis and other information presented on another page is not repeated here. Prior work can be found here. Detailed design work was concluded in MSD II and that can be found here.

Team Vision for Detailed Design Phase

Follow-Up Interviews

Our interview with Eric during this phase led to the modifying of a few engineering requirements. This has been reflected in our engineering requirements table on the problem definition page.
Interview Date Interview Notes
4/17/18 April 17 Notes

Bill of Materials and Budget

Bill of Materials

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Budget

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Component Prototyping, Engineering Analysis, Simulation

Throughout this section, there may be some content that has been hidden as our team and our customer felt some information was sensitive and we wished to protect for our own potential future uses. For those with proper permissions, all of our content can be found here.

Rod Design and Calculations

Calculations

The rod in current ACM cylinders is a solid steel bar. With our new cylinder design, this rod will need to be hollowed out to allow the lead screw to pass inside of it. Concerns arose about the stress that may be put on the rod, especially when the cylinder is resisting forces put on the restraint by extreme negative g-forces (airtime). We ran some calculations to give us an idea of what wall thickness we would need so that the rod would not buckle, bend, or shear due to various forces on the system. The free body diagram below was used to aid in the calculation of reaction forces induced on the system. A table documenting these force calculations is also shown below.

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Once reaction forces were found, the x direction reaction force was used to determine the appropriate wall thickness to avoid buckling. It was assumed that the boundary conditions for the system were one end clamped and one end pinned. This yields an equation for the critical force of

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where E is the modulus of elasticity of the chosen material (assumed AISI 52100 chrome steel), I is the moment of inertia, L is the length of the rod, and Pcr is the critical force.

This equation was rearranged to solve for I, as the inner and outer radii were the unknowns in this analysis. Pcr was set to be the reaction force into the system. For this analysis, we chose to have the inner radius set, and calculated an outer radius based on that input. A table of the calculations is below.

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These calculations made it clear that the rod is unlikely to ever buckle.

Next we considered the combined loading created by the y direction reaction force. This force produces both shear and bending moment at the "fixed" attachment point where the rod exits the cylinder casing. An analysis was done at this cross section for the fully extended rod, which would yield the worst case bending moment. See the diagram below.

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There are two elements where we will see stress on this cross section. These are stress elements labeled one and two in the hand sketch below. Element 1 will only experience the normal force produced by Mz while element 2 will only experience shear produced by Fy.

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Once again, for our analysis we assumed the inner and outer radii to be the unknowns. In this case, we set our outer radius, as well as a factor of safety, and solved for the inner radius which satisfied the conditions. The maximum allowable stress was determined by dividing the yield strength of the material by our fixed factor of safety. It was determined that the forces produced in shear were insignificant, so we only considered the bending moment in the results below. Unfortunately at the last minute, we realized we were considering our radii and diameters in our calculations, so some of our design choices elsewhere will have to be reworked as our current rod does not meet our design needs.

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Design

After initial calculations, a steel hollow tube with an inner diameter of 0.635 inches and an outer diameter of 7/8 inches was chosen as it was an available stock size. Unfortunately we believe this size may not work due to stress concerns. The rod will need external threading on both ends for connecting to the piston and the spherical bearing rod end.

Manual Release System

After speaking with our customer and doing some more research, it was determined that we wished to keep the same manual release system. This involves a button to manually open and close the valve allowing fluid to flow freely. This allows the passenger to then lift their restraint to evacuate. While this works easily in the current systems, there was a concern with our system that the patron would not be able to overcome the resistance in the new motor and lead screw system. Our system will be designed with a thread coarse enough that the restraint can be positioned without the need for motor input. The document below provides a summary of all calculations used to validate the manual release system design.
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The full pdf can be downloaded for viewing here.

Butt End/Manifold Design

Manifold

We originally thought the manifold was going to need to be redesigned, but can now most likely reuse current manifold from our ACM cylinder.

Butt End

Barrel Design

Nut, Screw, Piston System

Torque Optimization

When finalizing the design for our lead screw system, we had trouble deciding which of the variables to fix in order to design the overall system. We determined that for motor selection, it was best to pick system parameters which minimize the torque required by the motor. This led to the conduction of an optimization study. The equation used and its derivation are as follows.

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The torque of the motor was optimized using the parameters in the “Torque Calculation Parameters” table as well as the variables for motor speed (omega) and restraint movement time (t).

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Both motor speed and restraint movement time were constrained based on engineering requirements and technical feasibility. Active restraints are included on the graph. Due to the simplicity of the equation and constraints, the optimal was obtained graphically. The minimized torque lies on the intersection of the constraints. Values for torque, motor speed, threads per inch, and restraint movement time presented in the “Torque Optimization Results” table.

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The code used for our Matlab analysis can be viewed in the pdf file found here.

Piston Design

Force Sensor

It was originally stated that our team would be using a pressure sensor to help us determine when a restraint makes contact with a passenger. We have now shifted to using a force sensor at the recommendation of our customer. This is a more more reliable option than the previous pressure sensor. The force sensor is still within budget and is easier to implement than having our team research the threshold pressures for sensing contact with a passenger. The force sensor that was decided upon was the FSR 406 (photo below). This sensor was chosen for multiple reasons. One is that it can be used seamlessly with the microcontroller. It also has a wide sensitivity range, and this range includes our ideal 5 pound maximum force on the passenger. It also has a wide surface area that will allow for better accuracy of force values. In theory, when placed on the restraint bar, it will make direct contact with the passenger and send a signal to the microcontroller, stopping the motor from continuing to spin and advancing the restraint any further.

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FSR 406

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Motor Selection

Calculations were done to determine the required gear ratios based on a few different types of motors. Some of the calculations can be found below. For those with access, our spreadsheet can be found here. It was determined that we would use a Crouzet Brushed DC Motor (8989B104). This is a 24V, 3800 RPM motor. It was also chosen as it was in our budget and had a reasonable lead time.

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Drawings, Schematics, Flow Charts, Simulations

Main Circuit (Microcontroller, Motor control, Force Sensor, PLC controls)

To control the motor, and continuously read inputs from PLC(Ride Operator Controls), as well as a force sensor, Teensy, L293D and Logic Level Converter. The Logic Level Converter is used in this system to drop incoming signals from the PLC output’s to the Teensy which can only handle a 5V input signal. The L293D (IC) integrated circuit is used to take control of a motor from a microcontroller. This will work together with the Teensy to control motor speed, and motor direction. It is also used with motors of higher voltage, for this system the motor needs a supply of 12 volts. What can be seen below besides the IC components are the controls and feedback signals/notifications that will be implemented in this electrical system.

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24V DC to 12V DC Buck converter

This PTN78020H IC is used as a buck converter component to lower the incoming 24V DC to 12V DC. The 12V DC will be used for the motor control for this system, which can be seen in the main circuit. This buck converter provides a consistent 12V DC output.

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24V DC to 5V DC Buck Converter

This MP1584 IC is used as a buck converter component to lower the incoming 24V DC to 5V DC. The 5V DC will be used for the microcontroller for this system, which can be seen in the main circuit. This buck converter provides a consistent 5V DC output.

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Top Level Pseudocode

This is a basic diagram of how the motor will be controlled by the microcontroller. The three control buttons below labeled “Close Restraint”, “Open Restraint”, “Stop Restraint” will implement the overall control of this system. The close restraint having the most features due to safety factors and unexpected circumstances that will help prevent any harm to the passenger. As well as the multiple speed controls for the motor. If the motor is disengaged before time 2 in the decision block below, it will send a signal to notify the ride operator of an error/fault.

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Test Plans

Most of our testing was discussed on our Preliminary Detailed Design page. However, we have recently obtained an oil sample and some steel mesh and wish to test out these components in our system.

Filter Flow

A filter will be cut to size and placed in the expelling end of a syringe. Then, the syringe will be filled with the hydraulic fluid. The fluid will be expelled at different speeds, and the expelling will be timed to calculate the flow rate. Expulsion speed will increase until filter failure and maximum flow rate will be recorded. This will be done for each filter mesh.

Manufacturing and Purchasing Plans

Risk Assessment

New Risks

Major Risks

Failure Modes

Design Review Materials

Our design review materials contain sensitive information. Those with proper permissions can view them on our private page here.

Plans for next phase

(Use the individual 3-week plan template for this)


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