[Robot Hardware 03] - Cycloidal Drives
Robot hardware from a Physical AI perspective - cycloidal drives
This note covers cycloidal drives, which have recently received renewed attention in legged robots and compact actuators because of their shock resistance and high torque density.
A cycloidal drive uses an eccentric input and the resulting cycloidal motion to reduce speed and amplify torque. The gear ratio is set by the cycloid tooth geometry. In a compact package, it can deliver high torque, and more importantly, it is mechanically robust against impact.
Operating Principle and Gear Ratio
A cycloidal drive works through three main mechanisms:
- Eccentric Input: The input shaft connected to the motor carries an eccentric bearing. As the shaft rotates, it creates motion offset from the center axis.
- Cycloid Motion: The cycloid disk mounted on the eccentric bearing rolls and wobbles along a cycloidal path while engaging with the ring pins of the housing.
- Output Extraction: Pins connected to the output shaft pass through holes in the disk, extracting only the slow rotation of the disk as output motion.
Gear Ratio for Different Fixed Elements
The gear ratio and output direction change depending on which element is fixed. If the housing has $N$ pins and the cycloid disk has $N-1$ lobes:
-
Outer ring fixed / disk output: When the input shaft rotates once, the disk rotates backward by one tooth pitch. Gear Ratio = $(N - 1) / 1$
Cycloidal Drive - Fixed Ring (short) -
Disk fixed / outer ring output: When the input shaft rotates once, the outer ring rotates forward by one pin pitch. Gear Ratio = $N / 1$
Cycloidal Drive - Fixed Shaft (short)
2. Contact Design: Rolling vs. Sliding Contact
Unlike ordinary gears that transmit power through gear-tooth meshing, a cycloidal drive is governed by kinematic constraints. Depending on the design goal, it can use either rolling contact or sliding contact.
Rolling Contact
Bearings or cylindrical rollers are added at the housing pins or output pins to minimize friction.
- Strength: Very low friction loss, high transmission efficiency, and less heat.
- Limitation: It is harder to miniaturize. Higher reduction ratios require more rollers, which increases part count and weight.
Sliding Contact
Instead of adding rollers, the cycloid disk surface itself slides directly against the housing or output pins.
- Strength: The structure becomes much simpler, allowing extreme miniaturization and weight reduction.
- Limitation: Sliding friction is large, so efficiency drops and wear becomes a major issue.
In practical actuator design, rolling contact is often used near the central eccentric shaft, where the load is high. Sliding contact may be used between the disk and the outer housing, where the relative surface speed is lower.
To reduce friction and wear in the sliding-contact region, the cycloid disk can also be machined from low-friction engineering plastics such as Delrin (POM) or nylon instead of metal.
3. Advantages
1. High Torque Density
Like a Harmonic Drive, a cycloidal drive can generate high torque in a compact volume, making joint packaging easier.
2. High Shock Resistance
Where a spur gear may carry torque through only one or two teeth, a cycloidal drive can distribute load across roughly 30 to 50% of its tooth profile at the same time.
That load distribution makes it resistant to shock loads. It is a good fit for systems that see repeated impact, such as legged robots striking the ground or manipulators colliding with the environment.
3. High Single-Stage Reduction Ratio
A single-stage cycloidal drive can reach reduction ratios from 50:1 to above 100:1, reducing the volume and complexity that would come from multi-stage reducers.
4. Limitations and Design Considerations
1. Manufacturing Difficulty
The part count may look small, but the required manufacturing precision is high. The cycloidal profile must be machined accurately, usually requiring precise CNC work and careful assembly.
2. Tolerance Sensitivity
Tolerance errors or assembly mistakes directly degrade performance. If clearances are too tight, the drive can jam. If they are too loose, backlash increases and positioning accuracy drops.
3. Profile Modification
Eccentricity is a key variable that determines the curvature of the cycloid profile and the kinematic constraints of the drive.
If the theoretical cycloid curve is machined exactly, assembly tolerance and thermal expansion can easily cause interference. Designers therefore modify the profile slightly, for example by shrinking or offsetting the curve. This profile modification step is necessary, but it also increases design complexity.
4. Mass Imbalance and Vibration
Because the cycloid disk moves eccentrically, its center of mass continuously shifts during rotation. At high speed, this can create serious vibration.
To cancel this imbalance, many designs use two cycloid disks with a 180-degree phase difference. This improves balance, but it adds weight and volume.
Next post: [Robot Hardware 04] - Actuators (3): QDD Actuators
References
[1] https://mevirtuoso.com/become-a-member-cycloidal/
[2] https://www.youtube.com/watch?v=KX9Mx8ghtio
[3] https://www.youtube.com/watch?v=yBckAoqNQx4