Electromechanical Design

Slides from a graduate course lecture given on the topic of electromechanical design for humanoid robots.

Transcript

1. Electromechanical Design Safwan Choudhury ECE 780 Lecture
2. Lecture Overview • Design Challenges • Electric Motor Designs • Hydraulics-Based Designs • Pneumatics-Based Designs • Compliance-Based Designs • Design Comparisons • Summary
3. Design Challenges •Energy Efficiency •Power Density •Effects of Walking •Compliance •Backdrivability
4. Electric Motor Designs • Basic Principles • Commonly Used Actuators • Gear Reduction • Application Examples • Advantages • Disadvantages
5. Electric Motor Designs Basic Principles •Principle of operation is electromagnetism. •Coverts electrical energy into mechanical energy. •Can be either powered by AC/DC. •Force control via current manipulation.
6. Electric Motor Designs Common Actuators •Brushless DC Motors (BLDC) •Brushless Servomotors •Often custom built.
7. Electric Motor Designs Direct Drive •Motor shaft mounted directly to load. •High torque and low speed. •Not really used in humanoid applications due to motor size required.
8. Electric Motor Designs Geared Drive •Meet design torque/speed requirements via gear reduction ratio. •Planetary/Spur Gears •Sprocket/Chain Drive •Backlash problem.
9. Electric Motor Designs Harmonic Drive •Eliminates backlash while maintaining high torque capabilities. •Available in high gear ratios and compact assemblies. •Reconfigurable ratios within the same housing.
10. Electric Motor Designs Harmonic Drive •Eliminates backlash while maintaining high torque capabilities. •Available in high gear ratios and compact assemblies. •Reconfigurable ratios within the same housing.
11. Electric Motor Designs Example Applications •Brushless Servomotors Honda ASIMO (link) •Harmonic Drive Waseda WABIAN-2R (link)
12. Electric Motor Designs Advantages •High precision in position control of output shaft. •Can be sized to handle variety of loads (via gearing). •Easy to model •Inexpensive
13. Electric Motor Designs Disadvantages •Not a compliant mechanism (high mechanical impedance). •Large mass which increases inertia of swinging limbs (i.e. leg). •Drivetrain losses when coupled with gearing.
14. Hydraulics-Based Designs • Principle of Operation • Commonly Used Actuators • Application Examples • Advantages • Disadvantages
15. •Principle of operation is fluid dynamics, uses oil. •Hollow cylinder with piston which is pressurized/depressurized. •Pressure controlled via oil from a hydraulic pump. •Force control via pressure manipulation. Hydraulic-Based Design Basic Principles
16. •Linear Hydraulic Actuator •Flow control valves are used to regulate the hydraulic pressure, which in turn manipulates the linear force. •Linear force applied to moment arm and length of moment arm determines resulting torque at the joint. Hydraulic-Based Design Hydraulic Actuators
17. •SARCOS Humanoid (link) Hydraulic-Based Design Application Examples
18. •High power density. Small sized actuators provide large amounts of power. •Compliant by nature of actuation. •Smaller mass when compared to electric motor designs with equivalent power. Allows for light weight limbs. Hydraulic-Based Design Advantages
19. •Oil must be routed to each actuator via hydraulic hoses. •Requires a pump to supply the high pressure oil. Portable pumps have limited capacity. •Increased design complexity and more potential points of failure (oil leaks). •Expensive. Hydraulic-Based Design Disadvantages
20. Pneumatics-Based Designs • Basic Principles • Commonly Used Actuators • Application Examples • Advantages • Disadvantages
21. Pneumatic-Based Designs Basic Principles •Principle of operation is fluid dynamics, uses air. •Force control via pressure manipulation within the membrane by an air compressor. •Pneumatic Artificial Muscle (PAMs) are most commonly used in humanoid applications.
22. Pneumatic-Based Designs Pneumatic Actuators •Most commonly used: McKibben Muscle (Braided PAM) •Many other variations: Pleated PAM, Netted Muscles. •Need to be used in “antagonist” pairs for bi-directional motion.
23. Pneumatic-Based Designs Example Applications •Delft Denise (link) •VUB LUCY (link) •Shadow Dextrous Hand (link)
24. Pneumatic-Based Designs Advantages •Compliant by nature due to the compressibility of gas. •Safer for man-machine interaction when compared to hydraulics. •High power density which allows for lightweight limbs. •Easy to replace individual muscles.
25. Pneumatic-Based Designs Disadvantages •Inherently difficult to control for position (dry friction and threshold pressure). •Non-linear response and generally not possible to get uniform and constant speed. •Performance depends on the compressibility of air.
26. Compliance-Based Designs • Basic Principles • Compliant Actuators • Intrinsic Compliance • Application Examples • Advantages • Disadvantages
27. •Adds compliance to electric motor based systems. •Can be intrinsic by way of adjusting string tension to emulate muscle tightening. •Can be purchased as self-contained actuators which use springs to get compliance. Compliance-Based Designs Principle of Operation
28. •Electric/Hydraulic Series Elastic Actuators (SEA) •A self-contained unit with a spring between the output shaft of the motor and the load. •Adds compliance to the overall electric motor design. Compliance-Based Designs Compliant Actuators
29. •Some advanced mechanical designs have unique actuators to adjust wire tension which changes joint stiffness. •Standard electric motors are still used to actuate the joints. •Coupled with a musculoskeletal structure, this type of humanoid can replicate human- like motion Compliance-Based Designs Intrinsic Compliance
30. •Compliant Actuators IIT (SEA) Compliant Actuator (link) •Intrinsically Compliant Musculoskeletal Humanoid Kojiro (link) Compliance-Based Designs Application Examples
31. •Inexpensive, uses traditional electric motor parts along with springs. •May keep design complexity relatively simple if self-contained compliant actuators are used. •Improved efficiency by storing and reusing energy in spring elements. Compliance-Based Designs Advantages
32. •Use of compliant actuators may make it more difficult to relocate motors to achieve higher COM. •May significantly increase design complexity if compliance is built into the mechanical design. •Low power density when compared to other compliant mechanisms (hydraulics, pneumatics). Compliance-Based Designs Disadvantages
33. Design Comparisons Compliance Actuation Compliance Electric Motors Hydraulic-Based Pneumatic-Based Compliant-Based Depends* Compliant Compliant Compliant
34. Design Comparisons Force Capabilities Actuation Maximum Force Electric Motors Hydraulic-Based Pneumatic-Based Compliant-Based Medium/High High Medium Medium/High
35. Design Comparisons Speed Capabilities Actuation Maximum Speed Electric Motors Hydraulic-Based Pneumatic-Based Compliant-Based High Highest Medium High
36. Design Comparisons Position Control Actuation Position Control Electric Motors Hydraulic-Based Pneumatic-Based Compliant-Based High High Low High
37. Design Comparisons Backdrivability Actuation Backdrivability Electric Motors Hydraulic-Based Pneumatic-Based Compliant-Based Low High Medium High
38. Design Comparisons Overall Expense Actuation Cost Electric Motors Hydraulic-Based Pneumatic-Based Compliant-Based Low High Low High
39. Design Comparisons Design Complexity Actuation Complexity Electric Motors Hydraulic-Based Pneumatic-Based Compliant-Based Low High Medium High
40. Design Comparisons Overall Weight Actuation Weight Electric Motors Hydraulic-Based Pneumatic-Based Compliant-Based Heavy Light Light Heavy
41. Summary • As with most engineering decisions, the overall electromechanical design choice depends on the application. • Compliant systems provide a fairly good balance between traditional use of electric motors and fully compliant systems like hydraulics/pneumatics. • Hydraulics are a promising design alternative for cases where fast speed and large power requirements are present. • Pneumatics are a promising design alternative for cases where light weight and lower power requirements are present (i.e. humanoid hands).
42. Questions?