This article provides an example of how to properly integrate encoder, motor and gear assemblies into a robotic joint design employing precision zero backlash gearing, two high-resolution encoder kits, and a brushless frameless direct drive motor kit.
As robotic and robotic-assisted products proliferate the commercial, industrial and medical markets, new design trends are emerging that capitalize on more compact and smaller assemblies with high reliability and precision.
To achieve this, one design solution is to create 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 a single common housing. This method of component integration leads to a low weight and extremely low axial height, that is, low profile, compared to that of prepackaged motors, encoders, and gearboxes assembled together.
Integrated Robot Joint
Figure 1 below presents an integrated robot joint. This design has capitalized on components featuring low axial height, making the assembly extremely compact. The assembly also comprises of an encoder with high resolution and accuracy on the output, as well as a medium resolution encoder on the rear of the motor.
Figure 1. Integrated Robot Joint.
The cross-section provided below reveals the key components in this integrated robot joint. It contains the following features:
- Medium resolution encoder kit on the motor side
- High-resolution encoder kit on the output side
- Front, rear, and center housing components
- Axial through hole for simplified robot joint wiring
- Precision low profile gearing with zero backlash
- Frameless brushless motor kit
- A flanged output shaft for interfacing with nearby assemblies
- Precision bearings for the input shaft, gearing, and output shaft
Figure 2. Cross-Sectional View of Integrated Joint.
Robot joints have differing reflective loads and inertia based on position. Using a gear reduction boosts output torque, alleviates the servo tuning implications of a large change in inertia with position, and enables the use of smaller, more efficient motors.
One problem that arises from using standard gear reductions is backlash. Even though a higher gear ratio solves some torque and inertia quandaries, the resulting backlash will cause positioning errors and possible tuning issues. There are two commonly available varieties 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. Latest improvements 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. Similar products are offered by Cycloidal gear suppliers.
Figure 3. Harmonic Drive Component Set.
Frameless Brushless Motor Kit
The assembly above employs a frameless brushless motor kit, also called a torque motor kit. This kit comprises of a permanent magnet rotor and an electromagnetic stator 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 are capable of satisfying high torque requirements all 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 AgilityTM slotless, low profile, large through-hole, frameless 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 have large through holes and low magnetic core losses.
Proper choice and sizing of a frameless motor kit is vital to the whole robotic joint design, and because of this Celera Motion offers online tools and performance prediction software in order to allow accurate and fast component selection that will help fulfill the 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 only controlling torque, then lower values are sufficient, however, position control and velocity considerably enhance with higher resolution in this range.
The above-integrated robot joint employs an optical encoder kit capable of over 200,000 counts/revolution 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 higher accuracy, measured in arc-seconds, when compared to other lower performance encoder technologies like capacitive and magnetic encoders, measured in arc-minutes. While high accuracy on the input may not appear to be as important, it can certainly impact performance. For instance, if the control system is differentiating position to produce a velocity signal, inaccuracy in the position information will produce a velocity ripple.
Presented below is an example of a Celera Motion OptiraTM series configured as a low profile, medium resolution, optical encoder kit comprising of a read head and glass grating. This kit uses PurePrecisionTM technology, and is capable of a resolution of 250,000-500,000 counts per revolution along 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 permitting 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 contain 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 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 could depend on the output encoder in order to compensate for deflection and stiffness of all the joints working together and variations 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 PurePrecisionTM optical technology. These encoders are capable of < 2 arc-sec of accuracy and resolutions well into the millions of counts per revolution in rotary form. Interpolation for a digital output is developed into this tiny package and there is an option for 1-volt pp sine/cosine output for interpolation 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 above example, the housing comprises of three sections. Two shafts are available: one internal for the input encoder and motor, and one external for the output encoder and output. All parts are precise in nature, following guidelines of the encoder, bearing, and motor suppliers.
Housing design must consider the following:
- Relative precision of the housing will have to match the motor, bearing, and encoder requirements.
- High-resolution encoders require extremely tight axial and radial runout specifications. Any runout will decrease absolute accuracy. It is common to use ABEC 7 or better bearings.
- Material selection should achieve both mechanical accuracy and account for temperature fluctuations.
- In the case of a robot joint, weight is vital, thus minimizing the number of parts advised.
In this article, the most compact, lowest profile robot joint is designed along with a combination of low profile gearing, encoders, and a direct drive frameless motor kit. 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 include current and voltage inputs, speed and torque requirements, and temperature limits on the outside and inside of the assembly. It is essential to take into account the thermal, electrical and mechanical integration of all components, including manufacturability of the whole assembly.
This information has been sourced, reviewed and adapted from materials provided by Celera Motion.
For more information on this source, please visit Celera Motion.