Nov 20, 2017

This blog post was updated in March 2019. 

We’re not far from the day when robots perform many of the tasks done by humans today. We already have robotic vacuums cleaning our homes and robotic mowers cutting the grass in our yards. On factory floors, robots are building many of the products we use, from toothbrushes to cars. Robots are serving food in China and Japan, while drones are fertilizing farms and delivering goods.

So it won’t be long until robots will build our homes, lay our roads and drive us around. But one of the key requirements for the reality of such a future is for robots to have senses similar to humans.

One of the biggest challenges with robotic devices is how they find their way around without crashing into walls, furniture, equipment, humans or other robots. To avoid obstructions and do their job effectively, a robot should be able to detect obstacles from a few feet to a few centimeters away so that they have time to navigate elsewhere.

Common technologies for detecting obstacles include:

  • Ultrasonic sensing, which transmits ultrasonic waves and listens for the echoes that reflect back from any obstacles.
  • Optical time-of-flight (ToF) sensors, which use a photodiode to capture reflected light waves from obstacles.
  • Radar sensors, which use radio-frequency waves and the returning echoes from objects to determine the direction and distance of a moving object.

In this post, I’ll focus on ultrasonic sensing – a low-cost, slower-speed alternative to radar for robots that don’t need to reach high speeds in homes and factories. Ultrasonic sensing is more reliable than optical time-of-flight for obstacle avoidance, as ultrasonic sensing is not affected by the amount of available light reflected off of obstacles. Another benefit of ultrasonic sensing is the ability to sense glass or any other transparent surface since it uses sound waves as opposed to light to detect objects.

Many applications for robotics

Consider a robot vacuum that either on command or following a set schedule leaves its base and moves around a home to clean the floors. A good way to design this system would be to use ultrasonic sensors embedded on the sides of the vacuum to give it full 360-degree coverage. The spacing and number of sensors would depend on the shape of the vacuum and the field of view (FOV) of the ultrasonic sensor.

As the robot vacuum moves around, the ultrasonic sensor network maps obstructions, calculates the distance from obstacles and feeds this information to the central processing unit (CPU) to navigate around the obstacles. A similar approach with integrated ultrasonic sensors would work for robot lawn mowers, robot interactive toys, restaurant or retail service robots, and more, as shown in Figure 1.

Figure 1: Examples of service robots 

As a second example, consider assembly-line robots and robots that move raw materials or finished goods within and between a factory floor and a warehouse.

In today’s factories, robot arms assemble products by moving around to pick up and place parts and install nuts and bolts, as shown in Figure 2. A main concern among factory owners and robotic system manufacturers is installing sensors on robotic arms to prevent collisions between multiple robots on the floor. Ultrasonic sensors installed in appropriate locations on robotic arms or mobile robotic vehicles can provide intelligence about objects nearby, along with distance information that the CPUs of these robotic systems can use to avoid collisions.

Figure 2: Assembly line robotic system components

The components of an ultrasonic-based obstruction avoidance system used in robots would include:

  • Ultrasonic transducers. These are piezoelectric crystals that oscillate to generate ultrasonic sound waves when an AC voltage is applied and vice versa when the echo returns. There are two types of transducers: closed top, where the piezoelectric crystal is hermetically sealed (protecting it from the environment); and open top, where the crystal is exposed or covered by something similar to a speaker mesh. Closed-top transducers require a higher drive voltage that necessitates an additional component in the system: a transformer.
  • A transformer. A single-ended or center-tap transformer will generate the large voltage needed to drive closed-top transducers.
  • An ultrasonic signal processor and transducer driver. As an example, TI’s PGA460 drives the transformer, processes the returned electrical signals from the echoes and calculates the time-of-flight data for each of the relevant echoes in real time.
  • A CPU. This section of a robotic system uses the time-of-flight information from multiple ultrasonic sensors around the robot to map obstructions and either stop or help navigate away from them, depending on its programming.

An example of an ultrasonic transceiver module that combines an ultrasonic transducer and TI’s PGA460, ultrasonic signal processor and driver IC is shown below in Figure 3. The design files for the module are available as a reference from TI for customers. 

Figure 3: Example of ultrasonic transceiver module

Get started with ultrasonic sensing

Ultrasonic sensing is a cost-effective, reliable and practical solution for home and factory robotic systems. TI offers a few different devices to address the needs of different applications.

A wide variety of collateral covering technology evaluation modules, transducer selection guide, tuning the processor for specific transducers, optimal board layout for EMI, design files for small form factor modules etc. is available for quickly developing this technology for your specific product.

Additional resources