Eliminating Backlash in Mechatronic applications Without Compromises

Backlash can have a significant impact on the accuracy, repeatability, and efficiency of a mechanical system, which can be particularly detrimental in high-precision mechatronic applications. It can also pose significant challenges when it comes to controlling a system, and the noise it generates proves to be a challenge for people-centric working environments. There are a few ways to reduce its effects, however, they don’t fully eliminate it, not to mention they each have drawbacks. Nonetheless, all hope is not lost, as we will examine a new way to fully eliminate backlash by tackling it at its main source – the transmission drive.

 

1 – What is backlash?

Backlash (also called slop, play, or free-play) refers to the amount of movement between two mating gears or components, occurring when the driving members is not directly connected to the load (Fig. 1). When a force F is applied to move the free-floating mass m, but the components are not in constant direct contact backlash can occur between the components.

Figure 1. Backlash in a system

Backlash most commonly occurs due to the distance between the teeth of engaging gears (Fig. 2) in the case of bidirectional movement in the system.

Figure 2. Backlash between gear teeth

 

Backlash  is one of the factors of lost motion, even though they are often erroneously used interchangeably. Lost motion is the amount of movement that is lost due to the clearance between parts, or the angle of movement in both directions where a load is applied at the output, without a corresponding movement at the input (Fig. 3) (Tang, 2020).

Figure 3. Lost motion

2 – Why does backlash occur?

Manufacturing gears is not a perfect process, with small deviations appearing during production, which can add up to a larger deviation in a system with several gear stages. Environmental factors also come into play once the gears are in use, with thermal changes or impurities affecting the precision of the system. 

Generally, some space also needs to be present between the gears to ensure the system can run without too much friction and to allow space for lubrication and thermal expansion (Eliana Giovannitti, 2022). Excessive friction reduces efficiency and creates unwanted noise, and the vibrations created impose a challenge on controllability. Having to consider both needing to reduce friction by leaving space between working parts, and trying to minimize inefficiencies leaves a small window for a local optimum scenario.

3 – Backlash issues in Delta robots

In the case of high-speed Delta robots, backlash affects the trajectory and performance of the operation, be it pick-and-place sorting, crafting, or assembly. Higher running speed results in higher vibrations and jerk, which may reduce accuracy, and stability, and even shorten the lifecycle of the robot. (Pu Wu, 2022)
With time, gear teeth deteriorate and get worn down, increasing the amount of backlash. This becomes a critical issue in controlling endpoint motion in delta robots, as some encoders attached to the motor can have a limited resolution of sensing the angle of the output, exhibiting a false positive (Kevin M. Lynch, 2015). If not managed properly, backlash can cause significant deterioration of motion control or even loss of stability. Eventually, this can lead to system failure, causing production and/or economic losses. 

4 – Attempts to reduce backlash

In some systems, the effects of backlash and lost motion can be reduced through the use of specialized components or design modifications. For example, anti-backlash gears can minimize the amount of play between gears, while preloading can reduce lost motion by applying a slight force to the system to remove any clearance between parts. The use of sensors is an option as well, where encoders are placed on the motor to control position accuracy and make necessary adjustments. (Bright Hub Engineering, 2022)
Backlash is “one of the most important nonlinearities that limit the performance of speed and position control in industrial, robotics, automotive, automation, and other applications”. (Ambaye, 2021)
Where Delta robots are concerned, there have been many attempts to attenuate the effects of backlash by developing complex algorithms to reduce the jerk, increase efficiency, or make the trajectory smoother and more accurate. Those algorithms do however take a long time to develop and can be trapped in a local optimum, where some of the factors are improved while others are sacrificed.
However, in many cases, these solutions can be costly or impractical on a larger scale. One solution would be to get rid of what is causing the backlash in the first place: gear teeth. Using a smooth surface for contact would eliminate the gaps required for the gears to mesh together.
 

5 – True zero-backlash speed reducer

The Archimedes Drive is a new type of speed reducer that provides true zero backlash by utilizing a so-called “compound planetary traction drive” principle (see Fig. 4). At its core, the Archimedes Drive is based on a unique mechanism that instead of using gear teeth like traditional gear systems, it 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.
Figure 4. Compound planetary traction system inside the Archimedes Drive

Backlash is “one of the most important nonlinearities that limit the performance of speed and position control in industrial, robotics, automotive, automation, and other applications”.

G. A. Ambaye (2021)
The design of the Archimedes Drive 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. This means that all the movement is transferred from the motor to the output application with an efficiency of over 90%.

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. This is especially beneficial in high-speed Delta robots 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 can help improve the performance and repeatability of these systems.

FAQ: backlash and lost motion

What is lost motion in a speed reducer?

Lost motion is the total angular play measured at the output when the input reverses, expressed in arcminutes. It combines backlash, torsional wind-up and hysteresis into one number, which makes it the honest spec to compare.

What is a good lost motion value for robot joints?

Precision reducers typically specify around 1 arcmin. The Archimedes Drive holds a maximum lost motion of 0.2 arcmin, and because its traction contact has no gear teeth to wear, that number holds for the life of the drive: zero backlash for life.

Is zero backlash the same as zero lost motion?

No. A drive can have zero backlash at the teeth yet still show lost motion from wind-up and hysteresis. Always compare lost motion numbers, not backlash claims.

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