From moving raw materials to sorting parcels for shipping, sensors play key roles in material handling. Magnetic, ultrasonic and photoelectric technologies are commonly used for these sensing applications. However, these technologies are all susceptible to certain environmental elements:
--magnetic sensors to electric currents,
--ultrasonics to wind interference and
--optical sensors to dust and dirt.
For many challenging material handling and logistics applications—such as vehicle approach, presence and departure detection—a radar-based sensor can withstand the elements and deliver reliable performance.
Material handling & logistics application examples
Due to their weather-resistant design, versatile modes and flexible installation options, Frequency Modulated Continuous Wave (FMCW) radar sensors can work in numerous material handling and logistics applications—even those where traditional sensors have failed. Applications in which these sensors allow detection—either to initiate a required action or for tracking purposes—include the detection of vehicles at loading docks, shipyard crane detection, and cargo positioning and detection on a trailer.
Exterior loading docks have traditionally proved challenging applications for photoelectrics and ultrasonics, due to the wind, rain and even sunshine that interfere with their sensing ability. However, a radar-based sensor designed to withstand the elements can prove effective in sensing a truck or semi-truck at an outside loading dock, sending a signal to workers inside the warehouse for loading or unloading.
Although the trailer or bed of a truck is often composed of aluminum or steel, many are simply made of fiberglass, wood or plastic—which are radar-absorbing targets. To ensure detection of all trucks, a retroreflective-mode sensor should be used. The sensor can be positioned above the loading dock, aimed straight down and perpendicular to the ground. If a large wood or fiberglass trailer backs over the ground, the sensor will lose sight of the ground and thusly detect presence. A large area—approximately a one or two meter diameter circle of a concrete, brick or asphalt ground—can serve as the retro-target for loading dock detection application. For dirt or sand ground applications, a flat metal plate or buried radar target accessory can serve as the retro-target.
Shipyards contain dozens of giant mobile cranes that transfer cargo between ships and land. These cranes require proximity-warning systems to alert operators to objects within the path of the cranes. Current technologies such as lasers and ultrasonics are inhibited by the high winds, constant vibrations and mists associated with the sea. However, a weather-immune FMCW radar sensor can resist these extreme conditions to deliver accurate sensing of the cranes, improving material handling and logistics for these applications.
Proximity systems often require two distance zones: a far zone and a near zone. Using the adjustable-field mode and background suppression features with dual-zone technology allows a sensor to output a far distance zone on one wire and a near distance zone on the other wire, detecting not only presence of a crane but also the distance from the sensor.
Sometimes the cranes require a narrower field of view. Radar sensors equipped with a high gain antenna have a beam pattern of ±10 degrees, instead of the standard ±35 degrees, and can be used for these applications.
Cargo positioning and detection on a trailer
A radar-based sensor can also sense large objects on a flatbed trailer and send a payload status output to the cab. This status from an adjustable field sensor can ensure that cargo is present before driving away, or ensure the truck is empty before a container is loaded. Sensors should be mounted to receive perpendicular reflection off cargo for the best signal. Wooden cargo would require a metal plate or radar target accessory to assure detection. Augmented with an analog output, the radar sensor can provide coarse positional information as well.
How Radar Sensors Work
FMCW radar sensors emit a well-defined beam of high-frequency radio waves to detect moving or stationary objects. The waves are emitted from an internal antenna, which processes the signal that is reflected back from the target objects to the receiving antenna.
Since electromagnetic waves reflect from any large change in relative electrical permittivity (the dielectric constant), any object in air will typically reflect some amount of radar. In order to reliably reflect waves for accurate presence/absence detection, the target object typically must be composed from or covered with electrically conductive materials, such as metals. Large solid walls or floors of brick, concrete or asphalt contain trapped water, which is a conductor, so they will also reflect radar. However, raindrops and snowflakes are invisible to the radar-based sensors due to the very small quantity of water they are composed of and since they are smaller than the wavelength of the radar, making the sensors a reliable solution for outdoor applications—especially in inclement weather conditions.
Adjustable-field radar sensors operate on this basic concept—with vehicles that are composed of sufficient metal or concrete walls or floors typically serving as the target object. For sensing poor or non-reflective, radar-absorbing targets, such as objects composed of plastics, fiberglass and any organic materials, such as wood or fabrics, a retroreflective mode sensor can be employed. Retroreflective mode sensors effectively detect stationary targets with poor or no radar reflection through the use of a reference signal, or retro-target.
Radar Sensor Characteristics
Whether using an FMCW sensor in adjustable-field or retroreflective mode, it must be properly set-up to ensure accurate performance, with attention given to the following characteristics: dead zone, distancing, field of view and weather deflectors.
A typical FMCW radar sensor may have a dead zone of approximately one meter. The actual dead zone varies from 0.25 to 1.50 meters, depending on the size of the target. The dead zone can be eliminated in two ways: through use of the retroreflective mode or a specially designed adjustable-field mode. To eliminate the dead zone in the first mode, the retro-target itself must still be positioned beyond the dead zone. Since the sensor looks for the background rather than the target object, the target objects can be located all the way up to the face of the sensor and still be detected. Because of this, there is no effective dead zone. The special adjustable-field mode is only applicable if targets will only be located in front of the sensor for relatively short intervals. It requires a regular teaching operation via the remote wire, which must be re-taught every time the sensor is power-cycled.
The far distance in which a FMCW radar sensor can accurately function depends on the size, shape and material of the target. Additionally, the sensor has a near-distancing limit. From the dead zone to three meters, the sensor only detects presence—not the actual distance. The resolution of the distance is ±0.2 meters for most targets, although strong, stable and stationary targets may have a resolution down to ±0.05 meters.
Field of view
The key to implementing a successful application using a FMCW radar sensor is in understanding its field of view. Unlike traditional photoelectric and ultrasonic sensors, radar sensors are not simply line-of-sight sensors. Instead, these sensors may typically see ±10 to ±35 degrees from the centerline, depending on the model. This results in a full 70-degree field of view. Targets closer to the center of the beam pattern will provide stronger reflections than targets at the edges. However, it is still possible for a strong target in the peripheral field of view to be detected more accurately than a weak target in the center field of view.
FMCW radar sensors can only range in one direction. They can distinguish radial distance from the face of the sensor to the target, but not up/down and left/right. Due to this limitation, there is no way for the sensors to differentiate a target at the edge of the field of view from a target at the center. For applications employing the adjustable-field mode, users should check the surroundings to ensure there are no unwanted peripheral targets within the wide field of view that could create unwanted signals. In the retroreflective mode, users only need to make certain the retro target is the dominant reflection within the wide field of view.
While the nature of FMCW radar sensors allows them to function accurately even in wind and when falling rain or snow are in its field of view, water build-up on the face of the sensor can eventually lead to false detections. A weather deflector accessory can be used with any outdoor application if no other shielding mechanism is employed. The accessory partly covers the sensor, shielding it from the elements and allowing continued presence and absence detection.
One of the fist applications of FMCW radar sensors was for train detection applications that proved difficult due to pressure waves from passing trains, electric currents, dust clouds and dirt buildup. Today, these robust sensors are used to see right through these environmental hazards for a broad range of applications, with many more applications to come.
Ashley Wise is a development engineer at Banner Engineering.