Smarter, better, faster, less expensive — those words are the mantra of today’s global economy. As organizations seek to streamline operations and do more with less, they increasingly focus on automated guided vehicles (AGVs) and similar vehicles to help get the job done. By 2022, analysts expect the global AGV market sector alone to hit $2.65 billion. To meet demand, OEMs will need traction technologies that perform unquestionably while remaining rugged, economical, and easy to install and maintain. Electric traction solutions deliver on all counts.
As in most aspects of engineering, there is no single ideal solution. Options include brush DC motors, brushless DC motors, stepper motors, and AC synchronous or asynchronous (induction) motors. In this article, we will review the special requirements of traction applications and discuss the categories of electric traction solutions with a focus on the pros and cons of each.
Understanding the Application
Choosing the best traction or steering solution for a logistics vehicle or mobile robot starts with a detailed review of the requirements. For the types of specialty vehicle applications described above there are some special nuances.
Load: The load for an industrial vehicle includes both the weight of the unit load and the weight of the vehicle itself. These vehicles are typically supported on some combination of active wheels (drive wheels) and passive wheels (casters). Choosing the right traction solution requires understanding how the weight of the vehicle and load will be shared among the drive wheels and casters. This distribution may change, depending on whether the vehicle is loaded or unloaded.
Wheels and operating surface: In addition to determining the number and location of drive wheels and casters, OEMs need to analyze the characteristics of the wheel’s tire, the operating surface, and the interface between them. A vehicle with a narrow, slick tire running on a smooth surface will need to generate considerably less torque to achieve a desired performance than one with a wide tire running on loose gravel, for example.
Turning radius: The required turning radius of the vehicle has a direct impact on vehicle design and the characteristics of the traction actuators. Also, the clearance available for the wheels can limit wheel diameter, which in turn affects the wheel-drive motor speed and torque required.
Peak torque and average torque: Peak torque is calculated using the acceleration and friction torques required for the application. Actuators need to be sized to the worst-case scenario, taking into consideration factors like incline and rolling friction (surface texture, tire material, bearing friction, etc.) but mostly the inertia of the loaded vehicle. The motor needs to be sized to generate the breakaway torque required to get the vehicle moving but also sufficient running torque, or average torque to keep it traveling at constant speed.
Braking and holding torque: It is essential to know the requirements for static and dynamic performance, both in a power on and power off condition. Does the vehicle need emergency stop capabilities? Does a holding brake need to be applied; for example, in the event of power failure? Brakes for emergency stopping and holding while stopped are sized differently.
Power source: Is the vehicle battery-operated or is it fed from a generator? What are the voltage rails of the system? Are there current limitations? What is the desired operating time per charge? The average power consumed by the vehicle is determined by the speed and acceleration profiles, and the duty cycle (frequency of vehicle starts and stops).
Environment: Will the vehicle be used inside, outside, or both? What is the operating temperature range? Will it be exposed to contaminants such as dust, moisture, or corrosive chemicals? What about salt spray?
Shock and vibration: The higher the operating speed, the greater the shock level introduced by collision and rough terrains. Uneven surfaces can introduce vibration, especially when traversed at high speeds. It is important to evaluate these factors, which can be surprisingly high. Some warehouse applications can subject equipment to shock loads of up to 80 g.
Electric Traction Solutions
Traction actuators provide motive power to the vehicle drive wheels (we won’t consider tracked vehicles in this article). The process of choosing the best solution for the application begins with the wheel motor. Key considerations include torque, speed, efficiency, and size (see Table 1). That said, the most common motor selections tend to be brush or brushless DC motors, and AC synchronous motors, with brushless motors increasing in popularity. Stepper motors are effective for very specific, low-power niche applications. AC induction motors are also used in wheel drives.
AGVs and other autonomous vehicles or mobile robots are most commonly driven by one or more independent powered wheels. We can divide powered-wheel traction solutions into two classes: discrete and integrated.
Discrete traction assemblies consist of a collection of individual components that are typically located away from the wheel itself (off-wheel designs). In an integrated design, the motion components are integrated into the wheel assembly itself, whether as part of an integrated package (on-wheel designs) or connected directly to the wheel (in-wheel designs). Let’s consider each class in detail.
Off-wheel traction solutions. In an off-wheel traction solution, one or more electric motors transmits power to the drive wheels. Locating the motor away from the wheel assembly can provide useful degrees of freedom in the structural design of the vehicle.
Off-wheel traction solutions use a drivetrain to transfer power from the motor to the wheel. The most common drivetrains include belt and pulley, chain and sprocket, or gearbox and coupling. All three can be used to confer a mechanical advantage, decreasing the size and cost of the motor. As with the motors, the different types of drivetrains involve tradeoffs (see Table 2).
Depending on the cost of the drive-train, the architecture may lower cost of ownership. On the downside, adding a drivetrain increases complexity, maintenance, points of failure, and space claim. For best results, the options need to be carefully analyzed in the context of a given project.
Off-wheel designs offer several benefits. Without the constraint of building the motor into the wheel or directly adjacent, OEMs have greater ability to modify performance, size, and configuration. The choice of drivetrain adds another degree of design freedom. The distributed architecture makes it easier to add accessories such as encoders, holding brakes, and drives.
Off-wheel designs enable the motor, mechanical components, and electronics to be in protected areas of the vehicle. This location can reduce exposure to shock and contamination, increasing lifetime and reducing the chance of unplanned downtime.
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 .