Rotary Encoder Improvements Save Power in Mobile Robotics

October 3, 2017 by Jeff Smoot

Rotary Encoder Improvements Save Power in Mobile Robotics

Commercial mobile robotics have the potential to spawn vast markets for equipment and services. Opportunities for aerial drones alone are expected to soar to more than $18 billion by 2022 according to recent research. On the ground, mobile robots are being utilized in a tremendous variety of scenarios – industrial, commercial, and domestic - to perform tasks faster and more cost-effectively than humans, to operate in environments unsuitable for humans, or in collaborative roles to relieve workers of repetitive or physically demanding tasks.

Some companies are already pioneering the use of mobile robotics to increase efficiency and improve customer service. One example is Amazon; in many of its warehouses, robots are doing the arduous fetching and carrying, to bring selected bins directly to human pickers. The picker then applies their finely evolved human cognitive and motor skills to select the wanted products, before the robot returns the bin to its correct location. As Amazon continues to grab share from brick-and-mortar retailers, and as they make their entry into the grocery market, the demand for mobile warehouse robotics in this space alone is expected to skyrocket.

Mobility drives the need for battery-powered operation, and as a result, energy efficiency throughout the entire vehicle becomes of critical concern in order to maximize operating range. Careful management of a limited power budget is essential to enable the robot to work longer or travel further on a full charge. This is true whether it is a drone needing to travel as far as possible to deliver food aid, a warehouse assistant that must deliver maximum uptime to boost the operator’s bottom line, or a domestic robot such as a vacuum cleaner or lawnmower whose market appeal is directly related to the amount of work it can do between charges. Greater efficiency also gives designers extra freedom to manage cost, size and weight by enabling robots to go further on smaller batteries.

Making Small Savings, Everywhere

Meaningful improvements in flying range or operating time require a multitude of small power savings across all on-board subsystems. Because mobile robots often have several electric motors to move actuators or positioning mechanisms, such as camera gimbals, a small increase in motor-controller efficiency can deliver a much larger overall gain. Controlling and commutating the motor is critically dependent upon knowing the rotor’s angular displacement at any time. Optical encoders are a popular choice to achieve this task, but they can more than double their current consumption from the lowest to highest resolution setting. They also require regular cleaning to ensure correct operation. Using magnetic encoders can help reduce maintenance demands, but these, too, can be power-hungry.

Capacitive-Based Rotary Encoders Save Power and More

CUI's AMT rotary encoders based on capacitive technology are a power-efficient alternative. Employing the same operating principle as a digital Vernier caliper, the sensor comprises a fixed body and one moving element that together form a variable capacitor. As the encoder rotates, this capacitor produces a unique but predictable signal that can be used to calculate the position of the shaft and direction of rotation.

The AMT10 encoder draws less than one-tenth of the current of a comparable optical encoder, which can yield significant power savings in a multi-motor system. The table below compares the AMT10 with examples of competing optical and magnetic encoders.

Encoder model Type Operating voltage Current at highest resolution Power in 4-motor system
AMT10 Capacitive 5 V 6 mA 0.12 W
Competitor 1 Optical 5 V 85 mA 1.7 W
Competitor 2 Magnetic 5 V 160 mA 3.2 W
Rotary encoders using capacitive technology demand less than 10% of the power consumed by optical or magnetic types

As well as saving enough power to meet the entire needs of other on-board subsystems, capacitive based rotary encoders require less servicing than optical encoders, which can require cleaning to restore accuracy. Magnetic encoders – although robust – often have less accuracy than optical or capacitive types, and can consume almost twice the power of an optical encoder.

If a robot has multiple motors for various tasks, replacing all of the optical or magnetic rotor-position encoders with capacitive encoders can trim enough from the overall power budget to run a subsystem like a GPS module, wireless communication, or sensing, so that more of the battery’s energy can be used to extend the vehicle’s working range.

Rotary encoders utilizing capacitive technology could play a small part in the exciting mobile robotics revolution, unlocking power savings much greater than some might imagine. More generally, they can also help producers of many different applications meet the stringent new efficiency regulations now coming into force: the EU’s IEC 60034-30-1 standard being one example. But thanks to these small, energy-efficient devices, motor-driven equipment everywhere could be running longer and using less energy ever.

Helpful Resources

Read our full length “Energy-Conscious Sensing for Mobile Motor Drives” technical paper
Learn more about capacitive rotary encoder technology in this Product Spotlight
View our entire line of AMT modular encoders

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Jeff Smoot

Jeff Smoot

VP of Application Engineering & Motion Control

Since joining CUI in 2004, Jeff Smoot has revitalized the company’s Quality and Engineering departments with an emphasis on developing, supporting, and bringing products to market. With a focus on the customer’s success, he also spearheaded the establishment of an Application Engineering team to provide enhanced in the field and online engineering design and technical support to engineers during their design process. Outside of the office, Jeff enjoys the outdoors (skiing, backpacking, camping), spending time with his wife and four children, and being a lifelong fan of the Denver Broncos.