48 n SENSORS AND ENCODERS Encoders help to eliminate backlash in precision motion systems Many robotic applications, such as surgical robots or industrial robots handling semiconductor wafers, share the same goal. They all require precision control of a load under real operating conditions. The common challenge is the discrepancy between the commanded motor position and the position of the load. A primary reason for this is mechanical transmission error, introduced by components such as gears and couplings between the motor and the load. These errors can include backlash – the play when a mechanical system reverses direction before its components re-engage – and compliance – elastic deformation under load. Combined with other factors such as inertia, as well as structural flex across the machine, this can heighten challenges in position tracking. As a result, for applications that demand precision, accuracy, and dynamic performance, it is essential to resolve any mechanical transmission errors. This can be done by improving mechanical power transmission, or using compensation techniques involving control software. The two approaches can be used with each other, but some applications may be better suited to refining mechanical transmission, while others will gain more from software-based motion control. Strainwave gears If the motor and gear axis can be mounted directly to the final load with no additional power transmission stages between, backlash and compliance can be reduced by using a strainwave gearhead. These devices are based on elastic deformation and ensure continuous engagement of the gear teeth, achieving almost zero backlash to enhance precision and repeatability. Strainwave gearheads incorporate a flex spline – a flexible cup with external gear teeth. A rotating disc, known as the wave generator, compresses the flex spline into an elliptical wave that engages continuously with the teeth of an outer ring. This design also delivers high stiffness because, although the flex spline undergoes controlled elastic deformation to enable tooth engagement, it is highly resistant to torsional deformation under load, ensuring predictable motion transmission. With multiple teeth engaged simultaneously along the elliptical contact zone, the load is distributed across a larger contact area than in conventional gearheads, reducing compliance. This distributed loading also contributes to smooth, quiet operation, as torque transfer occurs continuously rather than through the discrete tooth impacts and clearances typical of conventional gear designs. Importantly for applications such as robotics that require a compact and lightweight footprint, the strainwave design also enhances torque density. For example, maxon’s GSW 70 A strainwave gearhead delivers up to 57Nm of intermittent torque and up to 95Nm of overload torque in a package measuring 70mm diameter and weighing 473g. Reduction ratios up to 100:1 are available in a single stage. The smallest gearheads are just 20mm in diameter. While a strainwave gearhead’s compact dimensions simplify integration, it can also manage with a single encoder because the motor position closely matches the output position. This offers the potential to remove a second encoder that may otherwise be needed on the output, and to simplify the design. Although strainwave gearheads are more expensive than typical planetary gears, eliminating the need for a second encoder can cut the total cost. Despite these advantages, strainwave gearheads only deliver full performance when mounted directly to the final load. If additional mechanical transmission – such as belts or couplings – are required downstream, this can reintroduce the challenges the gearhead was intended to resolve. Dual-loop control If further mechanical power transmission is involved between the motor and the output, or if a planetary gearbox is used, further compensation for backlash and compliance is likely to be necessary. In this case, a system known as dual-loop control is preferable. This involves two cascading control loops – an inner loop with an encoder that provides feedback on motor torque and velocity, and an outer control loop with an Precision motion systems can be limited by mechanical transmission errors. Designers can reduce these errors mechanically, or compensate for them actively using control software and encoders to achieve accurate load positioning, as maxon technical engineer, Amir Janjua, explains. July/August 2026 www.drivesncontrols.com A strainwave gearhead can avoid the need for a second encoder on the output because the motor’s position matches the output position closely
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