Guidelines for Robot Joint Designs

This article provides an example of how to properly integrate encoder, motor, and gear assemblies into a robotic joint design by employing precision zero backlash gearing, a brushless frameless direct drive motor kit and two high-resolution encoder kits.

Background

As robotic-assisted and robotic products proliferate the commercial, industrial, and medical markets, new design trends are developing that capitalize on more compact and smaller assemblies with high precision and reliability.

To achieve this, one design solution is to develop an integrated robotic joint that comprises of a high-resolution encoder kit, direct drive frameless torque motor kit, and precision zero backlash gear set in one common housing. This method of component integration leads to a low weight and very low axial height, that is, low profile, compared to that of prepackaged motors, encoders, and gearboxes assembled together.

Integrated Robot Joint

Figure 1 presented below depicts an integrated robot joint. This design has capitalized on components with low axial height, making the assembly extremely compact. The assembly is also available with an encoder with high resolution and accuracy on the output, as well as a medium resolution encoder on the rear of the motor.

Integrated Robot Joint.

Figure 1. Integrated Robot Joint.

The cross-section below presents the key components in this integrated robot joint. It comprises of the following features:

  • Frameless brushless motor kit
  • Medium resolution encoder kit on the motor side
  • High-resolution encoder kit on the output side
  • Front, center, and rear housing components
  • Precision low profile gearing with zero backlash
  • Axial through hole for simplified robot joint wiring
  • A flanged output shaft for interfacing with nearby assemblies
  • Precision bearings for the input shaft, gearing, and output shaft

Integrated Joint

Figure 2. Cross-Sectional View of Integrated Joint.

Precision Gearing

Robot joints have changing reflective loads and inertia based on position. A gear reduction results in increasing output torque, alleviates the servo tuning implications of a large change in inertia with position, and allows for the use of smaller, more efficient motors.

One problem that arises from using standard gear reductions is backlash. Although a higher gear ratio solves some torque and inertia quandaries, the resulting backlash will lead to positioning errors and potential tuning issues. There are two commonly available types of gearing with zero backlash: cycloidal drive and harmonic drive. Both these solutions utilize a unique mechanical design that keeps sub-components in contact at all times. Recent enhancements in packaging and design have produced extremely low profile gearing sets compared to earlier offerings from the supply base.

Presented below is an example of a low profile harmonic drive component set. Cycloidal gear suppliers offer similar products.

Harmonic Drive Component Set.

Figure 3. Harmonic Drive Component Set.

Frameless Brushless Motor Kit

The assembly above makes use of a frameless brushless motor kit, also called a torque motor kit. This kit is available with an electromagnetic stator and a permanent magnet rotor operating as a standard, synchronous motor through a three-phase servo motor controller.

A rapidly emerging design trend is to use a motor kit intended for direct drive systems within the integrated robot joint in order to drive a high ratio gear set. Direct drive motor kits have greater pole counts that enhance torque output and large through holes in order to optimize mechanical packaging. These kits are shaped like a ring, and satisfy high torque requirements while conforming to low profile constraints.

Robot joint output is usually slow. For instance, 20 rpm would be a fast robot joint move. After a typical gear ratio of 150:1, the input speed, (motor rotating speed), is 3000 rpm. This is not significantly high for an electric motor as long as the proper impedance is selected to match the available voltage.

The figure below presents an example of an Agility slotless, low profile, large through-hole, frameless motor kit.

Agility Motor Kit.

Figure 4. Agility Motor Kit.

Slotless motors prevent cogging torque and make the fine motion of the robot predictable and smooth. They also contain low magnetic core losses and large through holes.

Since proper choice and sizing of a frameless motor kit is critical to an entire robotic joint design, Celera Motion provides performance prediction software and online tools to allow accurate and fast component selection that will help accomplish design requirements.

Encoders for the Input and Output

Encoder: Input (Motor) Side of Joint

Most motor controllers benefit from medium resolution encoder feedback, that is, 100,000 to 250,000 counts/revolution. If the motor controller is just controlling torque, then lower values are sufficient; however, position control and velocity greatly enhance with higher resolution in this range.

The above-integrated robot joint employs an optical encoder kit capable of over 200,000 counts/revolutions with an installed accuracy of 20-50 arc-seconds. It is a low-profile, diffraction-based, interpolated encoder that uses a glass grating. Optical encoders usually have greater accuracy, measured in arc-seconds, compared to various other lower performance encoder technologies such as magnetic and capacitive encoders, measured in arc-minutes. While high accuracy on the input may not appear to be as vital, it can influence performance. For instance, if the control system is differentiating position to produce a velocity signal, inaccuracy in the position information will create a velocity ripple.

Below is an example of a Celera Motion Optira series configured as a low-profile, medium-resolution, optical encoder kit comprising of a read head and glass grating. This kit uses PurePrecision technology, and is capable of a resolution of 250,000-500,000 counts per revolution with accuracy in the 20-50 arc-sec range. This is almost 2 to 5 times more accurate than magnetic encoders or resolvers, while offering medium resolutions and allowing higher motor speeds of the input shaft.

Figure 5. Optira Read Head and Glass Grating Scale.

Encoder: Output Side of Joint

Robot controllers or motion controllers comprise of algorithms for trajectory control and coordination of multiple robot joints. These algorithms rely on high-resolution feedback at each joint, that is, resolutions greater than one million counts per revolution.

The output encoder is considered to be one of the most vital components of the integrated robot joint. The accuracy and performance of the robot greatly rely on the absolute accuracy of each joint. In certain cases, the robot controller may rely on the output encoder in order to compensate for deflection and stiffness of all the joints working together and changes in environmental conditions such as temperature.

The same Optira series encoder can also be configured as a high accuracy, high resolution, read head with the same grating. It uses Celera Motion PurePrecision optical technology. In rotary form, these encoders are capable of < 2 arc-sec of accuracy and resolutions well into the millions of counts per revolution. Interpolation for a digital output is developed into this small package and there is an option for 1 volt pp sine/cosine output for interpolation at in the host controller.

Mechanical Housing and Output Shaft Components

The general form factor of a robot joint is driven by complete robot operational requirements. In the example above, the housing comprises of three sections. There are two shafts: one external for the output and output encoder, and one internal for the motor and input encoder. All parts are accurate in nature, following guidelines of the encoder, bearing, and motor suppliers.

Housing design should consider the following:

  1. Relative precision of the housing must match the motor, bearing and encoder requirements.
  2. High-resolution encoders need very tight axial and radial runout specifications. Any runout will lower absolute accuracy. It is common to use ABEC 7 or better bearings.
  3. Material selection should be able to accomplish both mechanical accuracy and account for temperature fluctuations.
  4. In the case of a robot joint, weight is vital, thus minimizing the number of parts advised.

Conclusion

In this article, the most compact, lowest profile robot joint has been designed with a combination of low profile gearing, a direct drive frameless motor kit, and encoders. This combination comprises of the fewest number of components, and provides the highest torque output in the smallest size. While the final external packaging will differ by application, the internal components of the integrated assembly shown above are considered to be common, and the complete strategy is capable of benefiting all segments of the robotics market.

Each robot joint is available with a set of conditions that comprise of voltage and current inputs, torque and speed requirements, and temperature limits on the outside and inside of the assembly. It is essential to consider thermal, electrical, and mechanical integration of all components, including manufacturability of the complete assembly.

Celera Motion

This information has been sourced, reviewed and adapted from materials provided by Celera Motion.

For more information on this source, please visit Celera Motion.

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