Drive Precision In Robotics: Tackling The Issues Of Backlash And Lost Motion
The evolution of robotics and mechatronics has brought forth the need for precise motion control. Speed reducers, being integral components in these systems, have their performance affected by backlash and lost motion. So let’s delve into the definitions, effects, and consequences of these phenomena, particularly in the context of robotic and mechatronic applications. We also compare the backlash and lost motion figures in two prominent types of gearboxes: precision planetary and strain wave, and explore how the Archimedes Drive can help alleviate the current drawbacks of high precision speed reducers.
Defining The Concepts
Let’s first look at what each of the concepts represents:
Backlash (also called slop, play, or free-play) refers to the amount of movement between two mating gears or components, occurring when the driving member is not directly connected to the load. It’s the gap between the trailing and leading edges of adjacent gear teeth.
Lost Motion is the amount of output deviation, without a corresponding movement at the input, as a result of a torque being applied to the output. It’s a consequence of the cumulative effect of factors like backlash, compliance in the system, and material windup (torsional deflection).
Precision Planetary vs Strain Wave Gearboxes
These speed reducers are known for their low or even “zero” backlash, but how do these claims hold up, especially in the long run?
Precision Planetary Drives are known for their high torque and precision. Typically, they have a backlash range of 3-8 arc-minutes. Over time, wear and tear can increase this range by a further 1-3 arc-minutes.
Strain Wave Drives are prized for their zero-backlash characteristics, but these drives are not immune to wear and tear. Over prolonged use, they can develop a backlash of up to 1 arc-minute.
However, engineers working on servo applications generally consider “zero backlash” to be between 0.5-5 arc min. Another classification makes a differentiation between:
- Micro accuracy: < 1 arc-minutes
- Increased accuracy: < 3 arc-minutes
- Standard accuracy: < 6 arc-minutes
As can be seen, true zero backlash is difficult to find. Which is not great news if you need an application to produce microchips or assist in surgery, or other uses where precision is of utmost priority.
Effects on Robotic and Mechatronic Applications
Backlash and lost motion directly affect the precision of servomechanisms, robotic arms, and other mechatronic devices. An increase in either characteristic results in a decrease in the accuracy of the system, leading to errors in positioning and movement. In high-speed applications like Delta robots, backlash affects the trajectory and performance of operations, be it pick-and-place sorting, crafting, or assembly. In industrial robots, backlash errors affect the robot’s repeatability. Higher running speeds result in increased vibrations and jerk, which may reduce accuracy, stability, and even shorten the lifecycle of the robot. Over time, gear teeth deteriorate and wear down, increasing the amount of backlash. Without proper systems in place, backlash can cause significant deterioration of motion control or even loss of stability. Eventually, this can lead to system failure, causing companies major losses.
In 2012, a study found that surgeons were still able to perform laparoscopic surgeries even with increasing amounts of backlash in the surgical robot (1, 5, 10, 15, and 20mm of backlash specifically), but the surgery time increased by 5% for every 1 mm of backlash added to the system. This proves that while operations are still able to run, efficiency greatly drops due to backlash.
Next up, we will discuss the Archimedes Drive, a speed reducer solution, which through its design provides an inherent “true zero backlash”.
The Archimedes Drive: A Revolutionary Solution
The Archimedes Drive is a different type of speed reducer that provides true zero backlash by utilizing a patented “compound planetary traction drive” principle. At its core, the Archimedes Drive is based on a unique mechanism that, instead of using gear teeth like traditional gear systems, employs compressed, smooth, hollow cylinders (also called traction rollers or flexrollers) to generate tractional torque transfer from the motor to the output, similar to the tractive contact of train wheels on a track.
The design of the Archimedes Drive thus eliminates backlash through the use of constant tractive rolling contact. The operation can run without any vibrations from backlash impact, lowering the jerk significantly, and making it much easier to control the system. Because the traction rollers have smooth contact, there is no clearance between parts, meaning that all the movement is transferred backlash-free from the motor to the output application with an efficiency of over 90% and full transparency. The lost motion of under 0.2 arc minutes is due to material windup on the compressed flexrollers.
Furthermore, the tractive contact, as opposed to using meshing gears, makes the Archimedes Drive more efficient, precise, and quiet than traditional speed reducers, leading to improved overall lifetime and system performance. The lack of gear teeth also removes over torque damage and failure, as the drive will keep delivering its maximum rated torque in case of an over torque, and not suffer from critical failure or teeth breakage. All these characteristics are especially beneficial in high-speed and high-precision applications, such as electronics, aerospace, or medical devices, where even small amounts of backlash can have a significant impact on accuracy and performance. By eliminating these factors, the Archimedes Drive helps improve the performance and repeatability of these systems.
Conclusion
Backlash and lost motion are critical considerations in the design and maintenance of robotic and mechatronic systems. Their increase over time, due to wear and tear, can have significant repercussions on the efficiency, safety, and cost-effectiveness of operations. The Archimedes Drive, with its innovative design, offers a promising solution to these challenges, potentially revolutionizing the field of robotics and mechatronics.
