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Features

New technologies are coming along every day that are designed to make an engineer's life easier when attempting to fulfill his or her motion control system needs. This article reviews some of the latest advances in motion control and presents an application in which all of these components can be used together.

Creating the right motion control system for any application takes a lot of thought and planning. When selecting components, designers must be sure that the components will talk to each other and that they'll mount easily. Moreover, they must offer the right footprint, the right environmental capabilities, and the right accuracy and repeatability specifications within a long-life, minimal-maintenance package. These are a lot of requirements to consider.

An exoskeleton joint actuator for knee and hip exoskeletons. (Credit: maxon precision motors)

Sterilizable Drive System

The first steam sterilizable drive system has recently been introduced into the marketplace for use with medical equipment. In the medical device industry, sterilization has always been a key issue, placing additional wear and tear on the device. Accordingly, the ECX 13 and ECX 16, for example, are not only powerful brushless DC (BLDC) motors, but when combined with gearboxes, encoders, and position controllers, they are suitable for miniature surgical robotics and medical device applications.

Steam sterilizable encoders are available in incremental (1024 counts) and absolute (4096 steps) versions, both designed for 1000 autoclave cycles. The encoder can be integrated into the brushless drives. Most important to designers, though, is that this is done with no additional increase in body length to the product. Adding gear-heads (0.2 Nm max. continuous torque) opens up an entire new range of possibilities in compact and robust medical technology. This is especially true for power tools, which must maintain a small, easy-to-use footprint.

Further, frameless BLDC motors based on proven flat motor technology are also ideal for exoskeleton development. These frameless motors provide sets of stators and rotors in 45, 60, and 90 mm diameters. The outer runner motor technology supplies a tremendous torque and overload capability, while a large free inner diameter makes it ideal for applications where cable, optical wires, or tubes need to run through the center of the motor.

DC motors cannot always be optimally integrated into a structure. In robot joints, for example, space and weight are priority considerations that take standard solutions to their limits. For this reason, brushless flat motors (EC flat) as frameless kits have been developed as an alternative for the growing robotics market. A separate rotor and stator, without bearings and motor shaft, are connected only when the components are assembled. This approach offers robotics OEMs the best of both worlds: high-torque density and minimum volume. With outer diameters of only 43–90 mm, the brushless flat motors are extremely compact. Designed as external rotor motors, they offer plenty of space inside for cable glands. Hall sensors enable easy control.

Although an engineer may integrate a frameless motor into robotic joints to save space, doing so also provides additional benefits, such as a high level of integration in the structure of the machine, high torque through a multi-pole external rotor, low cogging torque, plenty of space for cable glands, and high overload capacity. Whether or not a solution with frameless motor is suitable needs to be determined on a case-by-case basis.

A number of new gear technologies have been developed that make larger torques, lower backlash, and lower noise possible. Many of these technologies are used in maxon's GPX standard program, for example, but the development of custom gearboxes and highly integrated gear trains may be necessary for some applications.

Closed-Loop Control Features

The ENX 16 RIO high-resolution encoder. (Credit: maxon precision motors)

A major technology trend is the integration of encoders (magnetic and optical) on the output of the gear train. This integration allows full position control without the need to compensate for backlash and nonlinearity issues that prevalent in many motor designs. For high-precision positioning, designers should consider an encoder that provides top-of-the-range, high resolution (up to 65,536 counts per turn). These encoders, based on optical reflection with a multi array sensor, are ideal for the precise position and velocity control of DC motors.

Requirements on encoders are becoming more and more demanding, especially in high-accuracy applications like robotics, medical analytics, and high-speed vision systems. This applies particularly to positioning applications where high precision must be delivered. Increasingly, these components need to be available in compact housings to accommodate an ever-greater number of electrical contacts. To solve this problem, new encoders have been developed that encompass all the requirements of a high-resolution optical encoder in a compact design. For example, maxon's ENX 16 RIO has an outer diameter of 16 mm and overall length of 7 mm. The housing is mechanically robust and protected from dust due to its injection-molded construction. The operating temperature range is –40° to +100 °C.

These new encoders can be combined and configured with matching drives online. They fit the new brushless EC-i 30 motors and the brushed DCX motors (diameters of 16 mm and up). The counts per turn and the electrical interface of the encoder are also configurable online.

The ENX EASY encoder is available as fully sterilizable. Combined with the sterilizable gearboxes, the complete drive units are designed for a minimum 1000 autoclave cycles (steam sterilization). The robust design makes these drive trains ideal for medical power tools.

The world’s first sterilizable drive system includes a brushless DC motor, gearhead, and encoder. (Credit: maxon precision motors)

The Right Controller for the Job

Positioning controllers are now available with additional connectivity options, including positioning controllers that can communicate via EtherCAT using the CoE standard (CAN application layer over EtherCAT). The EtherCAT card can be combined with the controllers in a housing. Compact designs with EtherCAT versions are in development. This expansion for the world of Ethernet-based communication protocols further expands the areas of application for the compact positioning controllers. For example, these controllers are useful for applications where short cycle times are important in synchronized multiaxis systems.

These new high-power density controllers are suitable for controlling both brushed and brushless DC motors. They include an intuitive user interface, libraries, and many application examples. By being available with the EtherCAT plug-in communications, the controllers make synchronized fast multiaxis systems possible for a wide range of applications.

Advances in Action

Exoskeletons are assistive devices that can be worn to work with a patient to restore function and mobility such as walking. Many exoskeletons use robotics, and these systems require a complex set of components to ensure that they provide the necessary functionality. Such devices require a powerful drive system, such a 90 mm frameless flat motor designed with custom low-backlash gearing, an integrated magnetic absolute encoder placed on the output, and an integrated position controller using a standard CANbus interface. When strategically designed into a combination that also includes a robust bearing system and a proprietary, easy-to-adapt aluminum housing concept, these technologies are ideal for knee and hip joints on exoskeletons.

This type of exoskeleton drive system supplies up to 120 Nm peak torque, which is state-of-the-art for such applications. The no-load backlash is rated with only 0.5 degrees. High-power density and lightweight housing technology minimize the overall weight of the complete unit to only 2.87 kg. Also ideal for an exoskeleton application is an absolute encoder located on the gearbox output for precision closed-loop operation.

Summary

Advances in motor technologies present many new options for designers of complex medical products such as exoskeletons and surgical robotics. Components must talk to each other and mount easily, but they must also offer the right footprint and the right environmental capabilities. Accuracy and repeatability are essential. Functionality such as closed loop control and connectivity and requirements such as sterilizability place greater demands than ever on the system's motion control components.

This article was written by Carsten Horn, Business Development Engineer for maxon precision motors, Fall River, MA. For more information, visit here.