P17227: Walking Soft Robot
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Detailed Design

Table of Contents

Team Vision for Detailed Design Phase

Summarize:

Progress Report

Team Progress report

Prototyping, Engineering Analysis, Simulation

Final Bone Design

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The final design for the Inflatable Bones is shown above. It is divided into three parts: the rod, the two caps, and the hinge. The two caps close off the ends of the rod to create a sealed compartment, which is pressured using a through wall connector. The hinges at each end are used for the joints, allowing a simple pin hinge to be used between the two bones and between the bones and body.

Bone version 1

Bone Version 2

Bone Version 3

Final Soft Robotic Muscle Design

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The final version of the soft robotic muscle is altered to account for both attachment to the robot and ease in pneumatic assembly. The choice of "Smooth on" Dragon Skin 30 for the gel muscles in front of the knee, is due to the flexibility of the gel and the direction of motion that the design creates in making the classic knee bend. Because the bend will occur when the leg is lifted the power behind the bending motion is only need to lift the lower bone. There is a limiter on either side of the knee to prevent the bones from bending too far in either direction while also relieving the weight from the body away from the gel muscles.

Old muscle actuator

McKibben Muscle

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The McKibben muscle design utilizes coupling hex nuts and a quick-disconnect for tubing. Furthermore, a screw-in hook may be implemented for mounting purposes.

Interchangeable connections

Bone model with connection points, displays the interchangeability of the leg design where all muscles and bones can be isolated and replaced. Change in muscle placement from before, with the muscle behind the "knee" bend removed since the elasticity of the gel muscle deflated will keep the leg at resting position but bent when activated.Each leg will now have 3 muscles (A,B, and C) that will be attached in these general locations.
public/Photo Gallery/elbow bone.PNG
public/Photo Gallery/Tee bone.PNG

Previous push to connect

The push to connect will allow the "Bone" to be detached from the air flow system with ease.

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Hook and loop strips with adhesive backs can be attached to the bones or the body, and used to secure the muscles. This way, muscles are easily removed from the system.
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The MettleAir quick disconnect provides convenient detachment from the air system if necessary.
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Barbed tube fittings could allow an easy disconnect from the air flow system for the soft robotic muscles. The pressure sensor shown above, SSCDANN100PGAA5, has been selected to measure the pressure of the air tank and the two separate air pressures going to the bones and muscles.

Body Design

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The final body design includes two plastic - potentially ABS or acrylic - 'shelve ends', with plexiglass inserts. The bottom panel will house the solenoid valves and pressure regulators, while the top shelf will provide mounting for the electrical components, the battery, and the air tank.

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Weight Estimation

Center of Mass

The analysis used for estimating balance relies on support polygons. While the center of mass projects onto the support polygon created by the contacts with the ground, balance should be maintained. Thus, in the final design, it is optimal to center the mass for the robot.

By arranging the components a certain way on the body, it is possible to center the weight. This will provide the best chance at stability. Moreover, the use of the lower shelf will provide a lower center of mass - below the top shelf, where the heavier components are placed - on the robot, which should provide further stability when walking.

Mechanical

Pneumatic System This is a system level diagram of the air flow that includes the basic components. All solenoid valves will be normally closed valves. Air flows from the compressor to the tank through a check valve and 3-way solenoid valve 1. During operation solenoid valve A will be in the actuated position. This ensures that if there is any electrical failure to the system the air safely vents to atmosphere instead of exploding and components. Solenoid valve 2 will be in the un-actuated position when the system is on in order to allow the air tank to fill before walking. Once optimal pressure is reached solenoid valve 2 will remain open for the duration of robot use unless there is a critical decrease in pressure. Once solenoid valve 2 opens air will pass through regulator one in order to reduce air pressure and flow. Solenoid valve 3 will open allowing the reduced airflow into the bone structure. The valve will then close once the desired PSI has been reached(TBD). Solenoid valves 4 through 11 will remain closed until the bone structure is inflated. Air will flow from the tank through regulator 2 and produce a reduced airflow and pressure. Solenoid valves 4 through 11 will then open allowing the muscle system to inflate to base pressure(TBD). Solenoid valves 4 through 11 will then open and close based on the user input from the controller and allow the robot to walk. Detailed preliminary schematic of the air flow system. Appropriate pressure fittings, various connectors, and additional valves will be added once they are determined. ABS has a yield stress of 7 kpsi, so using a diameter of 1/4" for the joint will provide an FoS of 3.8 for yield.

Electrical

Initial Solenoid Test Schematic

The purpose of this schematic is to test one instance of interfacing with the solenoids used to control air flow. Using a supply of 24 V to power the solenoid and a 5 V supply to act as the Arduino, the 5 V signal shall be toggled on the gate of the NMOS to control the solenoid. This shall be accomplished via breadboard and once verified manually, the Arduino shall be used to control one solenoid before moving to the expected number of needed solenoids.

Ardunio Solenoid Example

Low Voltage Cutoff Simulations

Due to the sensitivity of lithium polymer batteries, they can not be discharged below 2.8 V per cell absolute minimum. In order to prevent this, a low voltage cutoff circuit was developed to disable any load from further depleting the battery. Using the voltage comparator LTC1540, the circuit will turn off at 3.2 V per cell for safe measure. The circuit tested may be seen below.

In the simulations, the relay was replaced with a equivalent resistive load to provide the required current to activate the relay. By sweeping the voltage V1, the effect of a depleting battery can be observed below.

At a voltage of 13.2 V, the equivalent voltage per cell of a 4 cell battery is 3.2 V, which verifies the concept.

Updated Power Supply Design

The power supply will consist of three stages which include a 24V boost stage, an 8V buck stage, and a 5V LDO stage. These will all be achieved through a suppply voltage of a 4 cell LiPo battery with a voltage range of 13.2V to 16.8V.

24V Stage

The 24V boost stage consists of LTC3786 and its corresponding components. The LTPowerCAD calculations can be seen below:

Knowing these calculations, the LTSpice schematic was realized:

This resulted in the following simulation results:

Through these calculations and simulations, this stage was deemed justified.

8V Stage

The 8V boost stage consists of LTC3624 and its corresponding components. The LTPowerCAD calculations can be seen below:

Knowing these calculations, the LTSpice schematic was realized:

This resulted in the following simulation results:

Through these calculations and simulations, this stage was deemed justified.

5V Circuit

The 5V LDO stage was realized through the use of LT1129-5. By combining the 5V stage and the 8V stage, a more accurate analysis could be determined based on how they would be connected in the detailed design. This can be seen below in the schematic along with the low voltage cut off circuitry.

This resulted in the following simulations:

Through these calculations and simulations, this stage was deemed justified.

By combining all components of the circuits above, the completed detailed design for the electrical system minus the air compressor drive can be observed below.

Due to a limited budget, and hardware constraints, a relay was no longer able to be used to shut off in the event of a low battery. In lieu of this will be status LEDs, which include a green "good" LED and after an inverter is the red "bad" LED which denotes that the robot should be turned off and the battery recharged. Also in the schematic are the three pressure sensors which send data back to the arduino which is powered on the 8V bus.

Software

Bill of Material (BOM)

A live document for the BOM can be found here: Bill of Materials
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Test Plans

Purpose

Demonstrate objectively the degree to which the Engineering Requirements are satisfied

Bone Load Bearing Test Plan

To be performed by Avery Becker.

Objective/Data to Acquire: Determine the relationship between the pressure of the “Inflatable Bones” and load required for cause the bone to buckle.

Equipment:

Procedure: Normal Stress

Per table one below, the bone prototype will be inflated to the desired psi. After inflation, the bone will be place onto the compression rig. Weights will be added in 5 lb increments until the prototype buckles and fails. The weight before buckling will be recording and any other observations (buckle location, air leakage, etc) will be noted.

Table 1

Pressure Load Comments
0 PSI
2 PSI
4 PSI
6 PSI

Procedure: Shear Stress

Per table two below, the bone prototype will be inflated to the desired psi. After inflation, the bone will be place onto the shear rig. Weights will be added in 1 lb increments on the free end of the rig until the prototype buckles and fails. The weight before buckling will be recording and any other observations (buckle location, air leakage, etc) will be noted.

Table 2.

Pressure Load Comments
0 PSI
2 PSI
4 PSI
6 PSI

Leg Load Bearing Test Plan

Leg Testing Plan

Muscle Force Output Test Plan

Muscle Testing Plan

Electrical Hardware/Software Testplan(s)

Software Test Plan

Objective: To make have a safe first interface with the hardware

Plan:

  1. To be tested by: Naveena Shanmugam
  2. Equipment to be used: Arduino, Xbox controller, LEDs, project hardware
  3. Test all hardware connected to digital I/O pins of Arduino using dc voltages to ensure correct implementation
  4. Connect LEDs to used digital I/O pins of Arduino and test the various software commands one by one
  5. Connect Arduino to project hardware setup and ensure that the various software commands interface with the hardware properly and work as expected

Air Compressor Control Test Plan

Due to current limitations in the power electronics, the air compressor will no longer be powered via power electronics. Due to extreme instantaneous currents, this is no longer seen as viable. The air compressor test plan will simply consist of determining if the air compressor operates at a higher voltage.

Solenoid Driver Test
  1. To be performed by: T. Brudz
  2. Equipment to be used: power supply, breadboard, necessary components, oscilloscope, digital multimeter.
  3. Data to be acquired: waveforms of applying voltage to gate of NMOS device, resulting current of applying voltage to gate of NMOS device

The purpose of this test is to develop and test the control circuitry for the solenoids and their power supply. Using the schematic below, the circuit shall be realized while monitoring the supply current for a given supply voltage to the gate of the FET. The power supply shall also be varied to test for fluctuations in the 24V power supply.

The circuit below is the unadjusted power supply demo board donated by Linear Technologies.

The following equipment shall be utilized:

  1. Power Supply (16V - 36V)
  2. Power Supply (3.3V or 5V)
  3. Digital Multimeter
  4. Oscilloscope
  5. DC1687A (Linear Demo Board)
  6. Various cables

^^^Due to changes in the scope of the project the demo board above will no longer be used.

The following data shall be captured:

2V025-1/8 (STC Valve)

Supply Voltage (V) Load Current (A) Control Voltage (V) Control Current (mA)
23.5 0.000211 3 0.5
23.5 0.25 4 1
23.5 0.25 5 1
23.5 0.25 6 1
24 0.000225 3 0.5
24 0.25 4 1
24 0.25 5 1
24 0.25 6 1
24.5 0.000225 3 1
24.5 0.25 4 1
24.5 0.25 5 1
24.5 0.25 6 1

AVP-31C1-24D (Automation Direct)

Supply Voltage (V) Load Current (A) Control Voltage (V) Control Current (mA)
23.5 0.00022 3 0.5
23.5 0.135 4 1
23.5 0.135 5 1
23.5 0.135 6 1
24 0.00022 3 0.5
24 0.135 4 1
24 0.135 5 1
24 0.135 6 1
24.5 0.00023 3 1
24.5 0.138 4 1
24.5 0.138 5 1
24.5 0.138 6 1

Initially, a power supply will be utilized and further developing will require the use of demo board DC1687A or applicable substitution. One solenoid at a time will be tested.

Design and Flowcharts

This section should continue to be updated from your systems level design documentation.

Predetermined pattern of Movement

public/Photo Gallery/Walking.PNG

Risk Assessment

A live version of the Risk Assessment document can be found here:

Risk Management

Plans for next phase

As a team, what do you need to accomplish between now and the end of the semester?

As a team, what do you need to do to prepare for MSD II?

Individual Plans before MSD II

Trevor's Six Week Plan

Avery's Six Week Plan

Amanda's Six Week Plan

Nicole's Six Week Plan

Cameron's Six Week Plan

Naveena's Six Week Plan


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