Lesson 2 : How DOES a Cycloidal Drive Works
Summary of the lesson:
In this lesson from the Archimedes Academy by IMSystems, we explore the working principle, key terminology, and advantages and disadvantages of cycloidal drives, a widely used speed reducer in robotics.
> Part 1: How a Cycloidal Drive Works
A cycloidal drive operates through a unique motion mechanism. The input shaft is connected to an eccentric bearing, which is not positioned at the center of rotation. This eccentric motion causes a cycloidal disc to rotate in an orbital movement.
The cycloidal disc features lobes, which engage with stationary pins around the perimeter. As the disc moves, the lobes progressively shift past the pins, transferring motion to the output shaft at a reduced speed. The final reduction ratio is determined using the following formula:
For example, if there are 11 stationary pins and 10 lobes on the cycloidal disc, the output speed is reduced by a factor of 10, meaning the output torque is increased tenfold.
> Part 2: Terminology - TORQUE DENSITY, EFFICIENCY, BACKDRIVABILITY.
Before discussing the advantages and disadvantages, it is important to understand three key terms:
TORQUE DENSITY – Torque density refers to the amount of torque a system can generate relative to its weight. It is often described as the torque-to-weight ratio and is critical in robotic applications where minimizing weight is essential.
EFFICIENCY – Efficiency in a drive system is defined as the ratio of output power to input power. Some energy is always lost due to friction and heat dissipation. However, friction isn’t always bad—it has been used throughout history for useful purposes, such as making fire or sharpening blades.
In mathematical terms:
This means that power losses in a system directly impact overall efficiency.
BACKDRIVABILITY – A system is backdrivable if it allows the output shaft to rotate freely when force is applied externally. This depends on how much internal friction is present and whether it is amplified through the reduction ratio.
- Highly efficient drives are typically more backdrivable because internal friction is low.
- Less efficient drives resist backdrivability because internal friction prevents reverse motion.
Backdrivability is useful in applications like robotic arms where external forces might need to push back against the system.
> Part 3: Advantages and Disadvantages of a Cycloidal Drive
Advantages:
- High Gear Reduction in a Compact Package
Cycloidal drives offer very high reduction ratios while maintaining a small size. This makes them particularly useful in robotic joints, where space and weight are limited. - High Torque Density:
Due to their internal design, cycloidal drives can generate significant torque relative to their weight, making them well-suited for robotic arms and industrial automation. - Robust Against Shock Loads
If a robot using a cycloidal drive collides with an obstacle, the drive can withstand sudden force spikes without immediate failure. This is a significant advantage over traditional gear mechanisms, which might break under similar conditions. - High Precision Cycloidal drives offer high positioning accuracy, making them suitable for applications where exact motion control is required, such as industrial automation and robotic arms.
- Low Operating Noise
Compared to other speed reducers, cycloidal drives produce less noise, making them beneficial in applications where quiet operation is necessary. - Potential for Backdrivability
At lower gear ratios, some cycloidal drives can be backdrivable, allowing for energy recovery and compliance in robotic systems.
Disadvantages:
- Higher Cost
Cycloidal drives are generally more expensive than conventional gear systems, making them a less economical choice for applications where cost is a primary factor. - Induces Vibration
The orbital motion of the cycloidal disc creates vibration in the system. This can be problematic for high-precision applications. Engineers often compensate for this by using dual-disc setups to cancel out imbalances, but this increases system size and weight. - Compromise on Specialization
While cycloidal drives perform well in many areas, they are not the absolute best in any single performance category. Other drive types may outperform them in specific aspects, such as efficiency or smoothness.
Conclusion:
Cycloidal drives provide a well-balanced combination of high torque, precision, and robustness, making them a preferred choice for robotic arms, industrial machinery, and automation systems.
While they come with some trade-offs—such as vibration and higher cost—their ability to withstand shock loads, provide high efficiency, and maintain low noise levels makes them a strong contender for many applications.
In the next session, we will explore harmonic drives, another widely used speed reducer in robotics. If you have any questions or comments, feel free to leave them below!
