Embedded Files
THE PROJECT
This was a case study in the design of a rotary servo actuator for application in multi-axis robotic arms including rotary encoder, CSD-20 HarmonicDrive, and stator/rotor kit. I designed the entire assembly with a single cross-section reference image.
A significant amount of time was spent conceptualizing motor operation with the reference image prior to working on the design.
All of the interfacing areas including fastening thru holes were designed with tolerancing in mind. An important next step would be to create 2D technical drawings with manufacturing tolerance specifications (ISO 2768 for CNC housing). This assembly is otherwise ready for fabrication and testing.
SPECIFICATION
Hollow Shaft: 19mm OD, 16mm ID
HarmonicDrive: CSD-20
TQ Stator/Rotor Servo Kit ILM70x18
AksIM, 53mm Rotary Encoder
35mm Ultra-Thin Ball Bearings, Crossed Roller Bearings (20mm)
MOTOR DESIGN CONSIDERATIONS
COOLING
Cooling is generally done at the stator, often extracting heat from the outer surface (that originates at the inner).
A small form factor mechanical heat exchanger might be necessary depending on the application.
TORQUE DENSITY
Dependant on copper fill, shape/position of magnets in rotor assembly
GEAR REDUCTION
In robotic arm and humanoid applications, movement must be precise and, in many cases, maintain zero-backlash (extraneous movement at the joint). The HarmonicDrive (Strain-Wave gear) is widely used in practice due to zero backlash and reliable gear reduction.
Input to the HarmonicDrive starts at the 'Wave Generator'. This rotates against the 'Circular Spline' which respectively drives an elliptically shaped compliant mechanism referred to at the 'Flex Spline'.
STATOR/ROTOR
INVERTER
DESIGN ASSUMPTIONS and DECISIONS
Rotor is bonded to shaft by way of an adapter (shown in Black in above cross section). Adapter is fastened to Wave Generator
Stator is bonded to outer CNC casing in some way
Left-most housing is rotating (in reference image)
16mm inner shaft diameter (HarmonicDrive ID 20mm & 2mm thick hollow shaft walls)
Adapter for hollow shaft is needed so that Wave Generator ID does not make contact with OD of shaft
Because of ID similarity in HarmonicDrive model
Zero-tolerance for the sake of initial 3D model and cross-sectional showcase
15mm shielded single row deep-groove ball bearings
Smaller Diameter holes on FlexSpline are threaded
MODEL BASED DEFINITION (MBD) and GD&T
Part: stator/rotor interface for wave generator on strain-wave gear
2D Drawing of Rotor interface for Wave Generator on Strain-wave gear (supplemental 2D drawing for above MBD)
TOLERANCE STACK-UP ANALYSIS
Design Requirements
Hollow shaft must be centered in such a way to minimize error between the redhead and magnetic ring of the rotary encoder
Encoder accuracy: ±0.05° before installation (not including errors caused by mounting inaccuracy)
General Information on Tolerancing standard for CNC machined housing (DFA)
Mounting Surfaces: ±0.01 mm
Critical for ensuring that the HarmonicDrive and motor components are properly aligned.
Bearing Fits: H7/g6 (ISO tolerance system)
For standard ball bearings, the hole tolerance (H7) should be ±0.015 mm for holes 6-30 mm in diameter.
The shaft tolerance (g6) should be +0.004/-0.006 mm for shafts 6-30 mm in diameter.
Bolt Holes: ±0.05 mm
Ensures proper alignment and mounting of the casing to other components.
Mating Parts: ±0.02 mm
Ensures that parts fit together snugly without excessive play.
Overall Dimensions: ±0.1 mm
Ensures that the entire assembly fits within the designed space constraints.
Encoder assembly and tolerance documentation for MB049 redhead (D) & magnetic ring
Encoder 'Ride height' allowable variation is ± 0.18 mm
Worst-case based 1D tolerance stack-up analysis
Tolerance not completly within allowable variation - some changes should be made
Residual Sum of Squares (RSS) 1D tolerance stack-up analysis
Tolerance analysis is within allowable variation - no worries for assembly
CONCLUSIONS
If going by worst-case tolerance stack-up analysis, either lower tolerances must be achieved for the CNC-machined aluminum casing, or this case geometry must be simplified
2D Drawing of case part #1 for production
BLENDER RIG
ISOMETRIC CROSS SECTION
EXTERNAL PARTS USED IN ASSEMBLY
SOME MECHANICAL/FASTENING DECISIONS
16mm ID hollow shaft diameter (HarmonicDrive ID 20mm & 2mm thick hollow shaft walls)
M3 x 8mm Socket head Screws for HarmonicDrive Circular Spline interface to Case#2
M2.5 bolts to mount encoder Redhead to motor housing. 2.05mm diameter tapping hole size
Google Sites
Report abuse