P13202: TigerBot Humanoid Hip Servo

P13202: TigerBot Humanoid Hip Servo

Sponsor/Customer:Dr. Ferat SahinMulti Agent Bio-Robotics Lab Faculty Guide:Prof. George SlackTeam Members:Matthew LeStrange – Electrical EngineeringVu Nguyen – Electrical EngineeringBrandon Baker – Mechanical EngineeringPJ Haasenritter – Computer Engineering

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Problem:

• Several Senior Design groups have worked on building a “Tigerbot”, a 2-3 ft. tall walking Humanoid Robot.

• A continuing problem with off-the-shelf servo motors within their budget could not provide the necessary torque for the loads on the hip joints.

• The goal of this project is to create a low-cost servo that provides the necessary torque and speed for a walking robot.

• Additional goals are to have modular design a smart digital interface to communicate with the robot

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High Level Customer Needs

• Design a high torque/high speed servo motor suitable for a walking robot

• Make the enclosure design modular to allow servo to be used in different robotic joints

• Modular gear box design for variable speed vs. torque, optional dual output shaft

• Design a communication interface to between servo motor and the robot controller send feedback and receive commands

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High Level SpecificationMechanical• Provide sufficient torque for the hip joint (approx ~700 oz-in)• Torque specs based on size/weight of the servo

– (100-200oz-in/in^3 and 100-200 oz-in/oz)• 3 Gear Ratio Configurations

Electrical/Computer• Control Position using PWM or CAN bus interface• Provide Feedback for position, current and Read control loop

variables via CAN bus• Operate on a 6-8V range• Control Motor (CW, CCW, active brake and coast modes)

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Concept Selection Summary• Mechanical Enclosure, Aluminum vs. Plastic

– Aluminum is easier to machine for producing a prototype– Good for Thermal Dissipation– Low Cost

• Encoder Selection (Potentiometer, Optical, Magnetic)– Most servos use potentiometer– Simple analog interface to microcontroller– Relatively easy to create dual output shaft

• H-Bridge Selection– Use discrete IC, smaller space, easy to implement all functions

(CW,CCW, Brake, Coast)– Built in current sensing, short circuit and over temperature protection

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System Architecture

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Design Highlights (Mechanical)• Aluminum enclosure

– Multiple mounting options– Heat dissipation– Easily change between

ratios with no changes to the enclosure

• Modular gearboxes– 3 Gear Ratios for variable torque

output• 211:1, 264:1, 330:1

• Optional Dual Sided Output Shaft

CAD Model Cutaway View

Mounting Holes

CAD Model

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Design Highlights (Electrical/Computer)

• 4.5V to 9V Battery Supply (Motor Limiting Factor)• Small PCB Design (1.61 in2)• Robust H-Bridge

– Automatic Temperature and Short Circuit shutoff– Current Monitoring– Active Braking

1.4”

1.15”

PCB Top View PCB Bottom View Finished PCB

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Design Highlights (Electrical/Computer)

• Multiple Servo Position control interfaces– PWM – Standard Servo Interface– CAN – Smart Interface

• 12-bit position and current measurement feedback system

• Robust, fault tolerant control system

• PID Control System– Field programmable PID Gains values

• CAN Bus Interface– Current, position, and status feedback– Control up to 31 servos on a single 2-

wire bus– Servo configuration read/write: PID

gains, maximum power, address, servo mode

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Final System Results• Operates from 4.5 to 9V • PWM or CAN position control• Working CAN feedback/communication system• Initial Testing: 596 oz-in stall Torque• Gears for different gear ratios (211:1, 264:1, 330:1)

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Project Budget

Initial Budget $750Electronics (IC and other components, Development tools, motors, PCBs) $472.52Mechanical (aluminum for enclosure, gears, steel rod for shaft, fasteners) $207.00Total Spent $679.52

Note: Does not include Tax/Shipping charges for most orders

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System CostCurrent Design Material Cost

  Cost per Servo (qty 1)

Cost per Servo (qty 100)

Cost per Servo (qty 1000)

Electronics/Motor 25.80 16.26 12.45Mechanical 49.73 20.31 20.31Gears 66.32 40.24 34.64Grand Total 138.35 73.94 64.53

Current Labor Hours• PCB assembly: 1-2 hrs• Gears/Shaft: 3-5 hrs• Enclosure: 10-15 hrs• Assemble Servo: 1 hr• Program/Tune/Test Servo: 1 hr

Total: 16 to 23 hours

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Future Improvements

• Increase motor size/torque output for 5 ft. tall robot– Minimal changes to electronics needed to use 12V supply

• Continuous 360ᵒ rotation, use 2nd output from Potentiometer

• Additional Feedback (torque, velocity, temperature, stall warning)

• Add attachments to attach to standard servo horns

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Design For Manufacture Improvements

• Simplify Enclosure Design– Injection Molded Plastic– Extruded Aluminum ($4-15/each)

• Cut all gears from stock – $7-16/gear vs. $1.60-2.80/gear & increased machine time– Buy compound gears

Potential Future Cost after Redesign  Cost per Servo

(qty 1)Cost per Servo

(qty 100)Cost per Servo

(qty 1000)Electronics/Motor 25.00 15.00 10.00Mechanical 30.00 15.00 10.00Gears 65.00 25.00 15.00Grand Total 120.00 55.00 35.00