On the downside, drivetrain and coupling losses reduce efficiency. The mechanical elements compromise reliability. The latter point can be addressed by component choice — a planetary gearbox rather than a worm gear, for example — but that may affect cost, size, and audible noise.
Belt and pulley or chain and sprocket drivetrains tend to be vulnerable to contamination. They require regular cleaning and lubrication to prevent early failure, although wear is unavoidable.
Perhaps the biggest drawback to the discrete approach is the amount of additional engineering and assembly required of the OEM. In general, discrete designs tend to cost more than integrated packages simply because vehicle manufacturers may not have access to the economies of scale introduced by volume manufacturing. Integrated traction solutions provide a better alternative for most situations.
Integrated traction solutions. In an integrated traction actuator or powered wheel, all elements required are contained within a single assembly. The approach virtually eliminates engineering costs for the OEM compared to off-wheel designs. The OEM chooses the traction solution with the desired performance and features and installs it in their product as a single purchased component. Integrated traction solutions can be further subdivided into two types: in-wheel designs and on-wheel designs.
In an in-wheel traction solution, a brushless electric torque motor is directly integrated into the wheel. The stator windings are affixed to the wheel mounting structure and vehicle frame, sometimes through a suspension of some type. The rotor magnets (we’ll consider only permanent-magnet motors here) are integrated into the wheel hub.
Torque motors for in-wheel designs fit entirely in the wheel, and usually within the axial width of the wheel and tire, making for a compact solution (Figure 1). They do not require any sort of drivetrain or coupling. In-wheel solutions do exist that use housed, pancake-form motors, but these solutions also need gearing integrated into the wheel.
The power and torque capability of direct-drive, in-wheel torque motor designs are restricted mainly by available envelope size. While motor performance can be optimized by winding and magnet design, at the end of the day, diameter and wheel width set a practical upper bound on torque and power. In general, in-wheel traction solutions are reserved for applications in which space claim and/or audible noise are greater concerns than budget. Military and security applications, for example, may be good fits.
An on-wheel integrated traction actuator is built and delivered as a single element (Figure 2). It’s a hybrid design, combining the benefits of the off-wheel and in-wheel approaches. OEMs purchase a single part, pre-built to interface with the vehicle frame. The vehicle manufacturer just needs to bolt the unit to the chassis and connect the wiring harness. Performance and interoperability have already been validated by the traction actuator manufacturer.
The specifics of on-wheel designs vary, depending upon the application and manufacturer. At its most basic, an on-wheel integrated traction wheel incorporates a tire and wheel, mounting structure, motor, and gearbox; in the case of brushless motors, the unit also needs a commutation feedback device. More sophisticated versions may include onboard electronic drives, position feedback, and accessories such as holding brakes.
On-wheel traction actuators use standard motors, making the approach simpler and less expensive than in-wheel designs. The inclusion of a gearbox lowers torque requirements for the motor, which reduces its size, weight, and cost. Minimizing space claim is essential for logistics vehicles and robots, and cost control is a factor for every OEM. Incorporating a gearbox does increase complexity and points of failure, but because the gearbox is built into the assembly, it can be sealed to provide ingress protection and lubricated-for-life performance.
Probably the biggest advantage of an on-wheel traction drive is that it is built of components selected and integrated to address the challenges of industrial vehicles and optimize value. This approach enables the standard wheel to deliver a higher level of performance than discrete off-wheel designs. At the same time, these assemblies need less space and engineering and assembly costs for the OEM are lower. The on-wheel traction actuator also provides a standardized design platform that can be customized by the manufacturer to suit the OEM (Figure 3).
Although on-wheel integrated wheel actuators offer many benefits, they are not ideal for all applications. For budget-sensitive and/or low-volume vehicle projects, an on-wheel, integrated type traction assembly may be overkill. However, for OEMs wanting an all-in-one solution that involves less design and assembly work on their part, the on-wheel or integrated wheel drive is a very attractive approach.
In the fast-growing automated vehicle market, manufacturers need to get reliable products to market quickly. The various types of traction solutions offer a number of degrees of design freedom. In order to select the most appropriate assembly, OEMs should start by gathering as much detail about the application as possible, including budget and engineering development needed. By working closely with their vendors, vehicle manufacturers can determine the best solution for their needs.
This article was written by Will Hellinger, Engineering Manager at Allied Motion Technologies (Amherst, NY). For more information, visit here.