Sensors, Smart Sensors and Sensor Control Electronics

Tamara Schmitz, Intersil Corporation


Promise that improvements in efficiency and productivity will continue

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Figure 1. Block Diagram of the ISL29028A ambient light and proximity sensor

For safety, accuracy and convenience, automation is providing improvements in everyday applications. Consider the precision and advantages of computer-assisted surgery or the array of automation now available in automotive applications like blind-spot detection, automatic bright light adjustment of rear view mirrors, back-up cameras or parallel parking assistance. The first sector to embrace automation, however, is the industrial segment. Major advances in robotics and factory automation have lead to production efficiency and factory safety. The four major technology sectors that have enjoyed these investments are sensors, transducers, motors and control electronics. Here we will discuss the role of sensors and their development. We'll provide an example to help you understand the distinction between simple and smart sensors. In addition, we provide a few examples of support devices whose high quality enhances the sensors they accompany. As any and all of these devices improve, the entire robotic system benefits from their enhancements. Smart Sensors and Simple Sensors Since they lack human-like powers of observation, robotic systems need sensors to view and interpret their environment. Consider the seemingly simple act of picking up an object. First, the system must locate the object. After deploying an arm, a sensor is needed to ensure the approach toward the object. Often, another sensor signals when contact is made. Gripping is an art all by itself. How much pressure is needed to secure the object without crushing it? These actions of motion control typically involve accelerometers in conjunction with continuous position feedback from other system sensors. This simple example assumes the item is stationary and doesn't exhibit certain material behaviors such as magnetism or heat that can be detected by specific sensor types. Typically, the information from this network of sensors is processed in a centralized microcontroller. This setup is straightforward and simple. It exhibits the clear advantage of centralized control, centralized power requirements and simpler algorithm development. This type of system may enhance flexibility, but may also limit maximum performance. Consider that there is a risk that the microprocessor may not be able to calculate fast enough or even to provide service interrupts quickly enough. These limitations can cap the achievable robotic performance. The use of smart sensors moves the decision making process to the point of interest, in this case the arm and "the picker," and thus provides opportunities for improved capabilities. It also allows multiple events to be easily controlled. Remember that distributed control makes the system design much more complex, but removing the central bottleneck allows the system's capabilities to be significantly enhanced. Sensor Interfaces It is no surprise that technology companies have been and will continue to develop and deploy devices to propel the field of robotics to the next level. It is of particular interest when the sensor and related electronics can be integrated into a simple to use, monolithic solution. One of the most popular types of sensors is the proximity sensor. These sensors are found in all sorts of consumer and communications products, ranging from vending machines, ATMs, security systems, cell phones and leading-edge personal computers. They not only can provide positioning information, like in the robotic example above, they are used in safety systems and intrusion detection. Proximity sensors operate with infrared wavelengths which are invisible to the human eye. This allows a system to monitor how close objects are without constantly blasting visible light in a variety of directions, possibly even distracting people nearby. In cell phones, a proximity sensor allows the phone to detect when the user has brought it to his or her ear to engage in a phone conversation. During this time, the screen can be disabled, saving power and increasing battery life. It also prevents accidental hang-up or muting. A popular proximity sensor is the ISL29028A. It houses an ambient light sensor and a proximity sensor along with a built-in IR LED driver and I2C interface. The ambient light sensor allows the system to reduce the screen brightness in lower light situations, which is more pleasant for the user as well as a power saving technique. The IR LED driver sends out short bursts of current to an IR LED. An object within a few centimeters will reflect this signal to the proximity sensor, alerting the system to a nearby object. An example of increasing the smarts of a sensor is the interrupt scheme of the ISL29028A. The microcontroller does not have to continually pole the sensor to search for an object approaching the phone. Instead, the ISL29028A provides an interrupt signal. This interrupt signal allows the microcontroller to power down and wait for the sensor to inform it that an object is approaching. This allows for minimal loading on the microcontroller and an obvious savings in power.

Smart Sensor Support To function appropriately, smart sensors require strong support chips, including very low power and low noise signal conditioning elements. High input impedance instrumentation amps such as the ISL28274 from Intersil provide the rail-to-rail inputs and outputs required in many sensor applications. In addition, they amps exhibit extremely low input bias current and high CMRR needed for the strain and pressure sensing used in tactile robotic applications. Another support IC sensor amplifier example, which is an important contributor to advanced product design, is the ISL28133 micropower chopper stabilized op amp. This breakthrough design is optimized for single supply operation from 1.65 to 5.5 V, with only 18 ?A quiescent current and 8 ?V, max. input offset voltage. Since it is a chopper stabilized op amp, it continually measures and cancels input offsets, so the entire offset over temperature is just 0.075 ?V/°C maximum. While showcasing rail-to-rail inputs and outputs, the ISL28133 still can provide strong noise specifications for this type of IC: 1.1 ?VP-P, typ. noise (0.01 Hz to 10 Hz). Robotics in various forms has stimulated the imagination of many science fiction writers as well as investment bankers over the last forty years, in addition to creating amazing gains in efficiency and productivity. And this trend is in full force, through both fast-growing and slow growing economic conditions. The right combination of smart or simple sensors like ambient light and proximity sensors along with the optimal support devices such as instrumentation amplifiers and sensor amps, assures the industry of continuing innovation and productivity enhancement.