How Rotary Encoders Will Serve Mobile Robotics
Autonomous robots and drones first got their start for large corporations and government agencies in areas such as manufacturing and the military. However, commercial mobile robotics have exploded in recent years, as costs have lowered and made them more accessible to a range of markets and services (Figure 1). Recent research suggests that the market for aerial drones alone is expected to soar to more than $18 billion by 2022.
On the ground, affordable, agile, mobile robots are attractive for a variety of duties such as warehouse automation and autonomous agricultural applications. For example, in many of its warehouses, Amazon has robots performing the labor-intensive task of fetching and carrying items to human pickers to increase efficiency. In consumer markets, the Roomba is a prime example of mobile robotics’ surge with millions of these autonomous vacuum cleaners in homes worldwide.
With this mobility comes the need for battery-powered operation, which means that energy efficiency throughout a system becomes crucial as users demand increased performance and operating limits. Careful management of a limited power budget is essential to enable the robot to do more work and operate for longer on each full charge.
Whether an autonomous warehouse robot must maximize uptime, a delivery drone must boost its travel distance, or a domestic robot must perform for long periods between charges to meet user demand, power efficiency will be a priority. A focus on power savings will also allow engineers to specify smaller and lighter batteries as the miniaturization of electronics comes to the forefront of product design.
ENERGY SAVINGS EVERYWHERE
The mobile and aerial platforms now emerging incorporate large numbers of electric motors. Motor power consumption may range from several watts in a small positioning mechanism to tens of watts or more for traction or lift. Each motor has an associated driver/controller unit that also dissipates significant power. Minimizing the power consumed by each motor system opens more battery energy for other systems, and the cumulative effect across multiple motors can deliver a valuable advantage in the quest to build robots that can go further on smaller batteries.
A perhaps overlooked aspect of a motor and overall system efficiency is the encoder used to capture position information. Because motors are often used for driving wheels or rotors as well as positioning mechanisms and actuators, closed-loop control and commutation of the motor is critically dependent upon knowing the shaft’s angular position at any time.
For many years optical encoders have been the popular choice to achieve position feedback on motors, but they can suffer from high current consumption as well as susceptibility to dirt and grime collecting on their code wheel that lessens accuracy and reliability. A closer look at an optical encoder’s energy efficiency shows that current consumption can more than double from the lowest to highest resolution with some optical encoders drawing as much as 85mA at their highest resolution. When put into perspective, this 85mA at 5V results in power consumption of 0.425W, which in a four-motor system would total 1.7W of power consumed.
Magnetic encoders are a common alternative to optical encoders due to their increased resistance to environmental contaminants. However, they are typically less accurate than their optical counterparts and can also be a significant power drain with current consumption upwards of 160mA.
Used for years as the key technology in the digital Vernier caliper, capacitive-based rotary encoders, such as CUI Devices’ AMT encoder series, are a power-efficient, durable alternative to other encoder technologies. Capacitive encoders are comprised of a fixed body (or bodies) and one moving element, each with two patterns of bars or lines. These combine to form a variable capacitor configured as a transmitter/receiver pair, so as the encoder rotates, this capacitor produces a unique but predictable signal that is interpreted by an onboard ASIC to calculate the position of the motor shaft and direction of rotation.
In addition to being power-efficient, capacitive encoders are able to maintain accuracy in dusty or dirty environments and can also operate fully submerged in nonconductive fluids such as gear oils. This can save expensive sealing of the code wheel enclosure and minimize demand for routine cleaning or replacement of the disk, which is often needed when using optical encoders. At the same time, capacitive encoders provide equivalent or superior accuracy compared to other encoder technologies, with a typical accuracy value of 0.2 degrees.
When it comes to power consumption, capacitive-based encoders draw as little as 6mA of current at 5V, corresponding to 0.12W consumed in a four-motor system (Table 1). A capacitive encoder’s operating current is also independent of the resolution setting, which allows designers to optimize the encoder for the intended system without compromising power consumption. As mentioned earlier, an optical encoder can as much as double its current consumption from its lowest to highest resolution. This power savings could allow an application to run other subsystems such as an onboard camera, GPS module, or wireless communication, while extending the system’s working range.
|Type||Operating voltage||Current at highest resolution||Power in 4-motor system|
Capacitive encoder technology demands less than 10% of the power consumed by optical or magnetic types.
Lastly, when paired with a brushless dc (BLDC) motor, capacitive encoders also allow faster and easier digital “zeroing” to align the encoder U/V/W signals with the rotor windings. This is typically an iterative and time-consuming process for optical encoders that the digital nature of capacitive encoders reduces to seconds, while ensuring perfect alignment every time. By allowing accurate, repeatable alignment, capacitive encoders ensure that the motor can run smoothly at optimum efficiency thereby delivering the best performance and maximizing battery life in mobile applications.
SENSING FOR THE FUTURE
Sometimes forgotten in an overall system design, encoders will play an important role in the continued rise of mobile robotics, while capacitive encoder technology has the power to drive the future even further forward with their energy efficient, durable and accurate design. The digital nature of these capacitive encoders also opens the door to a range of programmable features and diagnostic capabilities, giving designers added flexibility in the age of IoT and IIoT.
CUI Devices | www.cuidevices.com
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • April 2020 #357 – Get a PDF of the issue