Tag Archives: piezo

What are Depth Sensors?

Ocean going ships typically use depth sensing techniques mainly for locating underwater objects to prevent running into them. This included gauging the distance of the sea floor. The principle involves measuring the time a burst of sound directed into the water takes to return after reflecting off an object. This time of flight gives a measure of its distance from the source of the sound, as the speed of sound traveling in water is fairly constant, depending on the water density and its temperature.

With the advent of peizo electronic devices it was possible to use ultrasonic sound frequencies to measure distance, using the same principle of measuring the time of flight. As better electronic components improved, engineers used the same technique for measuring distances using light waves in place of sound, as using light waves resulted in greater measuring accuracy as well as the ability to measure smaller distances.

Smartphone manufacturers are using depth-sensing techniques to enable facial detection, recognition, and authentication in their devices. However, this technology has far more potential, as Qualcomm is demonstrating. In collaboration with Himax Technologies, Qualcomm is promoting its Spectra image signal processor technology along with a 3-D depth-sensing camera module for Android systems. Very soon, we will be witnessing the emergence of a depth-sensor ecosystem, complete with firmware and apps.

Himax has expertise in module integration, drivers, sensing, and wafer optics. Qualcomm has combined their Spectra imaging technology with the technology from Himax and created the SLiM depth sensor suitable for mobiles. This has ample applications in surveillance, automobiles, virtual reality, and augmented reality. It took more than four years for developing the 3-D sensing solution.

The camera module from Qualcomm senses depth in real-time, and simultaneously generates a 3-D point-cloud of data in both indoor and outdoor situations. Qualcomm expects smartphone manufacturers to begin incorporating the computer vision camera module in their products in the first quarter of 2018.

Using infrared light, the camera module uses the well-known time of flight technique based on speed of light for resolving the distance from an object. The camera projects dots of infrared light onto the object, creating a cloud of points, which the sensor reads for the time of fight, thereby gathering depth information.

Approaches based on depth sensing techniques are gradually moving towards mobile handsets and head-mounted displays. Although mobile platforms may not be able to supply adequate power for room-scale 3-D sensing, they are certainly capable of managing the power required by the sensor and the image signal processor for running the complex software necessary for translating the point-cloud into an interactive and useful input.

The sensor packages use sub-half-watt range active laser illumination for providing high-quality point-clouds for short distances with structured-light solutions for applications involving facial and gesture recognition. However, for serving longer distances such as applications involving room-scale sensing involving a sensing range of 2-10 meters, the sensor packages will have to use high power lasers in the 5-W range.

As the power requirements for longer ranges are beyond those available from average mobile phones, designers are forced to adopt purely camera-based approaches for applications involving longer-distance image recognition.

How do Piezomotors Work?

Voltage applied to a piezoelectric material causes it to change its shape very minutely. Piezomotors such as Piezo LEGS are ceramic actuators that have four legs as its motors. These are designed cleverly such that the applied voltage can either elongate the legs or bend them sideways. It is also possible to synchronize the movement of each pair of its four legs such that it begins to walk just as an animal would – step by step. While walking, the legs can also stop at any instance on a nanometer level. The driving rod produces direct friction coupling with the legs. That means piezomotors can operate without any mechanical play or backlash. The direct drive, apart from providing full force, also offers power-off locking that does not require any power consumption.

However, the friction coupling between the drive rod and the internal piezo actuator legs does not allow counting the steps or knowing the position of the legs accurately. When they are under constant load, the legs face a certain vibration between the steps. As the load or temperature varies, so do the vibrations. Therefore, separate position sensors are required to know the accurate position of the legs of a piezomotor.

Piezomotors can move extremely slowly. When running in a closed loop system, you can make them achieve a continuous smooth motion at speeds under 1µm/s or 0.001mm/s. Since the speed of a motor depends on its step length and step frequency, a typical linear piezomotor is limited to a maximum speed of 10-15mm/s. In reality, the speed depends on both the external loads and temperature. Therefore, to run the motor at constant speed, you must have a closed loop controller.

Compared to conventional motors, piezomotors are very energy efficient. For example, when in a hold position, piezomotors do not consume any power. They also do not draw peak currents while starting or stopping. Power consumption of such motors is not dependent on inertia. That means the motor will consume the same amount of power under different external torque/load. When operated with a low duty cycle and for point-to-point applications, piezomotors provide excellent battery life.

Just as in regular stepper motors, one can define holding force and stalling force for piezomotors as well. While running, the highest load that the piezomotor can hold dynamically without slipping is called its stalling force. When powered down, the motor is able to hold a load statically and the maximum load that it can statically hold without slipping back is called its holding force. In general, the holding force of a piezomotor is about ten percent higher than its stalling force.

Although the operating principle of a piezomotor is very similar to walking, it can walk with full steps, reduced steps and it can even do micro stepping. Usually, the drive rod or disc will engage with the two or more actuator legs to move them forward and release. Then it will engage with a second set of legs to move them forward. This cycle repeats as long as the motor walks. Therefore, it is always possible to divide the full step into several smaller steps – also called micro stepping.