Motion detectors have been steadily watching us for more than a decade. These ubiquitous pyroelectric eyes can detect everything from thieves breaking into your house to when customers enter a supermarket.
Early designs were unreliable. Sensor-based automatic lighting, for example, would often time-out, leaving you in the dark attempting acrobatic maneuvers to re-trigger and turn a light back on. But better materials, sensors, processors, algorithms, lenses and system designs are now making motion detectors much more usable, lower in cost, and more reliable than in the past.
This article discusses the technology and design considerations behind Passive Infrared (PIR) detectors, which are at the heart of virtually every low-cost motion detecting system. It will review the types of detectors that are available, examine their issues and benefits, and provide design hints essential to successfully adding motion detection functionality to your designs.
The pyroelectric effect
The pyroelectric effect was first documented in Greece in 314 BC, when it was observed that when heated, tourmaline attracted bits of straw and ash. This was re-discovered in the 18th century and the phenomenon was attributed to electricity, but without an understanding as to how. Continued research in the 19th century by Pierre Curie and Jacques Curie uncovered the mechanisms behind the pyroelectric effect and a new technology was born.
There are 32 classes of crystals based on rotational axis and planes of reflection. Of these 32 classes, 21 are non-centrosymmetric. That means they have no center of symmetry, which imposes a fundamental asymmetry in the structure. Of these 21, 20 classes exhibit piezoelectric properties and will either induce electric charge when stressed, or stress and deform when exposed to electric charge. In turn, of the 20 piezoelectric class structures, 10 of these contain a dipole in their unit cells and, as a result, also exhibit pyroelectric properties.
A pyroelectric device will produce a vector product of net polarization in response to a change in temperature. Since all matter above absolute zero emits radiation energy in the form of heat, a pyroelectric sensor can be created and used to detect any changes in heat radiation it is exposed to. This results in changing voltage levels that are amplified and used to indicate a change in temperature in its "field of view."
It is important to note that since the position of the atoms in a pyroelectric crystal move slightly in response to temperature changes, the voltage induced is temporary. Even if the change in temperature remains, leakage currents will eventually dissipate the charge separation to become electrically neutral again. This means that the pyroelectric sensor is not detecting heat; rather, it is detecting a change in heat.
A passive role
Motion detectors based on PIR technology do not need emitters to stimulate and paint a target or an area of detection. This is an attractive benefit of using the pyroelectric effect. But because there are no emitters, care must be taken to expose the crystalline detector surface to the desired field of view.
A window that is transparent to IR but translucent to visible light is used as a rudimentary filter to help eliminate false triggering and thus, false alarms. Glass or plastic can be used for this filter, which also serves to protect the face of the sensor from dust, dirt, moisture, or other contaminants that can affect the performance of the detector. Polarizing lines can also be etched into the glass or plastic to further limit heat radiation to a given plane orientation, if desired.
Another common practice is to use a lens assembly to focus distant IR radiation onto the detecting surface of the pyroelectric detector. Optically pure crystals and glasses work best, but because of the cost, these crystals are unlikely to appear on products for the consumer market. As a result, lower cost plastic is most common because facets and Fresnel type lenses can be scribed, etched, or imprinted into the IR passing plastic membrane (Figure 1).
Figure 1: Translucent plastics provide good optical characteristics for passing IR and can be formed to make a variety of Fresnel lens arrangements. Segmented parabolic reflectors can also be used.
A different approach that can be employed instead of, or in addition to, plastic lenses is to use a segmented parabolic reflector arrangement to direct reflected IR radiation to the sensor face. This may be a more expensive and design-intensive approach, and, what is more, if manufacturing repeatability is uncertain, it could lead to wide variations in performance from manufacture run to manufacture run.
It is important to note that when choosing a plastic sheet focusing lens, the design engineer should select a material that will limit or attenuate the spectral energy to pass 8 – 14 µMeter wavelengths. A Gaussian pattern with a 9.4 µMeter center frequency is ideal in this case (Figure 2).
Figure 2: A desired optical filter will center at around 9.4 µMeter wavelength and follow a Gaussian distribution.
Also important is the shape of the lens assembly that provides the three-dimensional sensitivity pattern to which the sensor will respond. A horizontal swath or three-dimensional dome pattern is possible depending on the patterns etched into your lens (Figure 3).
Figure 3: A variety of coverage patterns can be created to optimize a motion detector for specific applications. Linear swaths as well as hemispherical patterns make contrast boundaries for better detection of changes in heat or movement.
Very localized or even long narrow path sensing can be achieved this way. Note that the transition regions (areas of non-coverage) provide contrast to the detection zone. This helps maximize the effect when motion through the sense pattern takes place.
Typically, pyroelectric sensors are available as single element, dual element, or quad element. Using more than one sensing element allows any two sensors to be wired differentially. Here, the sensor's outputs are applied to a differential amplifier, which has the effect of canceling out the average temperatures in the fields of view. An increase in energy across the entire sensor cancels itself out so false alarms do not occur with slow or even rapidly changing light levels or temperature rises/falls.
Central to any PIR-based design is the choice of sensor material. As previously noted, 10 types of crystals exhibit pyroelectric properties but not all are cost or performance effective for a motion detector application. If you are designing your own sensor from the ground up, then a detailed understanding of each crystal type, its cost, benefits, and shortcomings will be helpful. Most engineers, however, will use fabricated sensors packaged into a ready-to- use device.
If you are designing from the sensor material up, you can take advantage of an integrated PIR Sensor preamp IC from ROHM. The BD9251FV-E2 is ideal for human body detection applications such as security systems and automated lighting. It implements standby modes and power saving features to maximize battery life for low power applications.
The BD9251FV-E2 contains an internal voltage regulator that drives a sensor with a stable voltage (typically 2.3 V) from the up to 7 volt power supply. Internally, a hard limiter is amplified, integrated, and processed to actually allow you to determine motion and the direction of motion in the field of view of the dual differential sensor. Analog and digital outputs are available to add more flexibility.
The modular approach
Because of the complex design factors discussed, many motion detector devices are integrated modules that encompass the actual semiconductors, lenses, amplifiers, and microcontrollers. They are available with a variety of sensitivities, lenses, sense patterns, and output types needed to create a variety of fairly robust and reliable designs.
Starting with the sensor, you have a choice of using a two-dimensional surface-sensing element, or a sensor with an integrated uni-directional or multi-directional lens. With a flat surface detector, you can design Fresnel plastics for specific coverage zones and patterns (including areas, swaths, conical and spherical types of detection patterns).
For example, the Panasonic EKMA1302120 is a single element, through-hole mounted, low profile, polyethylene PIR detector with digital output and an integrated silicon lens incorporated into the outer package (Figure 4).
Figure 4: Encapsulated sensors with flat surface lenses allow you to design the focal configuration while providing high levels of electronic noise immunity.
The sensor and the company's PaPIRS sensing circuit are both encapsulated in the metallic case to provide designers with a sensor module that is inherently shielded from most EMI and EMF fields that can also create false trigger conditions (Figure 5).
Figure 5: More than just a sensor, the modular detectors can contain lenses, amplifiers, power conditioning, and output drivers.
Panasonic makes a series of PaPIR sensors with integrated polyethylene lenses for standard and log distance sensitivities. For example the EKMB1101111 is a PIR sensor with an integrated white standard distance-sensing plastic lens while the EKMB1203112 is a long distance version with a domed black lens.
The company also provides the NaPiOn family of integrated PIR sensors. These also feature a variety of lenses and form factors, and, like the PaPIRS series, can become part of the faceplate. Panasonic also provides training for its NaPiOn sensors. These, again like the PaPIR sensors, incorporate amplifiers, filters, optics, and logic into a compact TO-5 case.
NaPiOn sensors come with either an analog or digital output. The digital logic level is fed by an internal comparator resulting in a threshold-based output that can be fed into a processor pin or drive a solid state relay directly. The analog output allows the designer to control the thresholds.
Nicera also has a line of integrated sensors and lenses for PIR-based motion detection. In partnership with Zilog, PIR solution packages include the lens, sensor, and a microcontroller with functions and peripherals optimized for PIR motion detection (Figure 6).
Figure 6: The Zilog approach bundles lenses, sensors, microcontrollers, and software support to provide a highly effective and well engineered solution you can modify for your specific needs.
Zilog offers a complete system level approach with much of the engineering already completed and available to Zilog customers who use parts such as the Z8FS021 MCU. For example, the microcontroller can implement a variety of algorithms to reduce white light effects.
White light will cause a DC shift over the area of the sensor, so the microcontroller can detect an entire DC shift and compensate to look for changes above and beyond the white light level shift in a rather clever way. Zilog uses a white LED as both a status indicator and as an ambient light level sensor. LEDs will respond to light levels and can be used as sensors. Zilog takes advantage of this quality to switch the white LED between a driver, and an A/D stage. In this way the LED is used as the sensor for the ambient light discriminator.
Zilog also provides algorithms to discriminate against false triggers set off by pets. Note that this can be dependent on the placement of the finished sensor, the shape of a room, the type and size of the pet, and whether or not you have a sleeping dog vs. an overactive cat. These algorithms can be implemented directly, or used as a starting point for your sharp pencil to refine.
The above illustrates the point that the Zilog CPU is acting as a decision engine. In their approach, an AC-coupled pyroelectric sensor is processed through a gain stage and bandpass filter. The resulting low frequency signal is analyzed by the processor in the amplitude and time domain to determine if the signal represents valid motion.
To simplify our lives, Zilog has bundled detectors, lenses, and microcontrollers into single part numbers that you can directly place on your BOM. For example, the ZMOT1AHH0E0AG is a kit that is optimized for an 18 meter sensing distance that runs from 2.6 to 3.6 V and feeds a digital output directly to the 20 pin SSOP ZMOTION microcontroller to detect a wide area.
Similarly, the ZMOT0BSB0C0AG comes with a domed type filter and includes a smaller 8 pin, SOIC-based controller. Zilog also has a training module that can help you quickly understand the benefits and tradeoffs of its different parts and approaches.
Zilog makes available a prefabricated 8 pin SIP module that you can use as a reference design or OEM into a prototype or low volume application (Figure 7). Zilog expects these will find their way into more than just security and lighting systems. As an enticement to vending machine customers and to automate the operations of kiosks, localized motion detection may emerge as a prime candidate for efforts aimed at drawing people in as they get close.
Figure 7: For use either as an evaluation module, prototyping, or even for limited OEM applications, the Zilog SIP module demonstrates a compact and effective solution.
Zilog also offers two kits to help speed understanding and development of your specific application. The ZEPIR000102ZCOG development kit is a serial-based motion detector test, development, and evaluation system that includes the SIP module, power supply, carrier and prototype boards, and cable. The ZMOTIONS200ZCOG is similar, adding USB functionality. Both will allow you to quickly put your PIR design-based application in motion.
This article has examined low-cost pyroelectric detector-based motion-tracking systems. We first discussed the design requirements of creating a system for motion detection applications, then looked at the characteristics of the detector and the Fresnel lenses and other elements used to modulate the visibility of these detectors. Finally, we presented complete modules that encompass the actual semiconductors, lenses, amplifiers, and microcontrollers needed. All parts, tools, data, and tutorials mentioned here are available online on the Digi-Key website and links have been provided.