A Guide to Sensors
A crucial aspect of any robotics project is the ability for the robot sense objects around itself, the environmental conditions, or its relative position. Then report back this information or use it for its own purposes. Below is a general listing of the types of sensors we carry and their uses. As always, if you find this information to be incomplete in any way, please post your questions on our forums.
These sensors work as a pulse of light (wavelength range of 850nm +/-70nm) is emitted and then reflected back (or not reflected at all). When the light returns, it comes back at an angle that is dependent on the distance of the reflecting object. By knowing the angle, distance can then be determined. These sensors are commonly used to prevent robots from driving into walls and general object avoidance. Please note that accuracy will be reduced when used outdoors. Be sure to check out the we use to gauge distances from our robots. Sharp Analog Distance Sensor
LIDAR, also called Light Detection And Ranging, uses near infrared light to image objects. The narrow laser-beam that’s emitted is capable of mapping physical features at high resolutions (). Offering precise positioning while targeting a wide range of materials, LIDAR is a very powerful feedback system that can be used on many robotic applications. At SuperDroid Robots, we offer both a scanning and non-scanning variant. Where the scanning version ( click here for an example) uses spinning mirrors to redirect the laser on a 2D plane, and the non-scanning system ( Hokuyo UST-20LX) offers a static, less expensive option. LIDAR-Lite v3
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GPS Receiver Module
GPS is a blending of positional and movement detection. It can return your position along with how fast you are moving. GPS is a space based satellite navigation system that provides location and time information anywhere on the planet. A GPS receiver calculates its position by timing the signals sent by GPS satellites. The receiver compares the time stamp between multiple satellites. These time stamps are compared and the position of the receiver is then extrapolated by the delays. The expected accuracy of the position is expected to be between 10 and 20 meters depending on various atmospheric conditions and the location of the receiver. Combining the data from an accelerometer, gyroscope and GPS allows you to know where you are, what direction you are moving and what orientation you are in. These are critical components in many autonomous systems. These features can be found in our . 66-Channel LS20031 GPS Receiver
High Precision RTK GNSS Receiver
When it comes to obtaining accurate location feedback outdoors, an RTK GNSS is hard to beat. Let’s go over what this setup can do, and how it does it. This is a global navigation satellite system (GNSS) configuration that provides positioning accuracy up to 1 cm. Compared to the 10 meter accuracy of normal GPS setups, RTK systems greatly improve the viability of integrating satellite navigation on autonomous robots.
So, how does this work? The current, and most popular, method to obtaining a real-time kinematic (RTK) lock is to have one GNSS module as the designated “base” station and another as a “rover” station. The base receiver is responsible for calculating error offsets while keeping the rover station updated. The rover station applies this offset to its own position readings and provides the user with very precise position feedback. The base station is more interested in the phase of the signal rather than the content of the signal. This is because Earth’s atmosphere (ionosphere and troposphere in particular) is an error source in the form of signal delays and phase changes. Since the base station has to make these precise calculations, it must remain stationary in operation.
Along with the base station remaining stationary, two conditions need to be met in order to achieve an RTK lock. Both GNSS receivers need to have a 30 degree view of the horizon, and they also need to be linked to at least four of the same satellites with signal to noise ratios (SNR) of 40 or higher. Of course, the second is more critical than the first. This should make sense given how the error is calculated.
In areas with tall buildings and dense vegetation, it can be very difficult to obtain and keep a RTK lock. However, there’re some modifications that will increase the overall efficiency. Antenna placement is critical in order to reduce multipathing errors. Multipathing occurs when the satellite signal is reflected before it reaches the receiver. This causes the signal to take multiple paths and therefore increases the delay since the distance traveled is increased. This is usually the culprit of massive outliers in position data that we see all too often. Mounting the antennas on ground planes can also help with this issue. A ground plane is a typically a flat symmetrical metallic plate under the antenna that will create a more consistent reception pattern as well as filtering out satellite signals close to the horizon. The optimal size of the ground plane usually depends on the antenna design itself. Luckily, most RTK GNSS manufacturers make setup rather simple by selling the two modules and corresponding antennas as a package while providing sufficient “how to” documentation.
WAAS Enabled GPS
More often than not, 1 cm accuracy is overkill for applications that require global positioning feedback. Fortunately, SuperDroid Robots now stocks the Garmin 18x GPS modules in and 1 Hz variants. This offers a cheaper and easier to use alternative to the RTK systems that are viable for autonomous development platforms. The driving force behind the 18x series compared to a normal GPS is the Wide Area Augmentation System, or WAAS. By promoting a reasonable cost and less than 3 meter error, this technology is a must have on outdoor autonomous robots. 5 Hz
The underlining methodology of WAAS is very similar to RTK as the onboard receiver uses correctional data for improved positioning. Multiple ground reference stations positioned across the U.S. that monitor GPS satellite data. Two base stations, located on either coast of the U.S., collect data from the reference stations and create a GPS correction message. The corrected differential message is then broadcast through 1 of 2 geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.
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An inertial measurement unit (IMU) is an electronic device that measures and reports a body's specific force, angular rate, and the magnetic field surrounding the body. These systems typically use a combination of accelerometers, gyroscopes, and magnetometers.
An accelerometer is a sensor that measures proper acceleration. Proper acceleration is not necessarily coordinate acceleration (∆v/∆t). Instead, an accelerometer measures the forces acting on the test mass relative to free fall. This means that at rest, an accelerometer will read an acceleration upward of 9.81m/s^2. There are multiple methods of measuring acceleration. Typically, it involves some sort of test mass that either deforms or causes another part of the circuit to deform. This deformation can either cause a change in voltage, capacitance or resistance. This change is then measured and the sensors direction of travel is then calculated.
The gyroscope is a device for measuring angular momentum. A mechanical gyroscope consists of a spinning disk suspended within two rings with the ability to move freely in any direction. Modern MEMS gyroscope devices that you will find in modern electronics are based on a vibrating structure. Combining a gyroscope and an accelerometer provides greater precision in determining the orientation and motion of a device.
A magnetometer is an instrument that measures both the strength and direction of the Earth’s magnetic field to provide compass readings for the IMU. Combining a gyroscope, accelerometer, and magnetometer provides orientation and motion of the device relative to the Earth’s magnetic poles. These devices are susceptible to magnetic noise produced from DC motors and other electronics. Therefore most manufacturers offer an offset variable to calibrate to maintain precision.
We offer the as a complete IMU system. The board ships fully populated with its SMD components, including the LSM6DS33 and LIS3MDL, as shown in the product picture. Pololu MinIMU-9 v5
To elaborate on the above description on accelerometers, there are three methods that most accelerometers use in order to measure acceleration: piezo-electric, strain gauge, and capacitive.
Piezo-electrical based accelerometers contain crystal structures that become stressed due to acceleration forces. Piezo based designs are unable to measure DC but are able to measure higher frequencies.
Strain gauge based acceleration detection uses adhesive backings that deform when forces are applied. This deformation alters the resistance of the object. That resistance can then be measured and the acceleration can be calculated. Strain gauge based designs are able to measure DC but are unable to measure at high frequencies.
Capacitance based designs utilize a mass attached to the plates of a capacitor. When the mass vibrates the capacitance is augmented. This change in capacitance can be measured and then the acceleration applied to the sensor can be calculated. A capacitance based design performs well in the same frequency ranges as a strain gauge but is generally more rugged.
The from Dimension Engineering is the most advanced analog accelerometer solution to date, featuring a triple axis ±3g accelerometer chip. It also has integrated op amp buffers for direct connection to a microcontroller's analog inputs, or for driving heavier loads. Buffered 3D 3g Accelerometer
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These range sensors test how close you are to an object by using sonar waves. These will bounce off an object and the time it took for the wave to get back to the sensor is used to calculate the distance from the object. These come in all distances and strengths but can be very accurate. Mounting these sensors on a servo can be a great way to get a wide range of readings without needing very many servos/range sensors. More details on Range sensors can be found on this items description page, . HRLV-MaxSonar-EZ3 Ultrasonic Range Finder
Depending on the application, a wide or narrow beam wave might be a better fit. The has the widest and most sensitive beam pattern of any unit from HRLV-MaxSonar-EZ sensor line. This makes the HRLV-MaxSonar-EZ0 an excellent choice for use where high sensitivity, wide beam, or people detection is desired. The HRLV-MaxSonar-EZ0 is the narrowest beam width sensor that is also the least sensitive to side objects offered in the HRLV-MaxSonar-EZ sensor line. The HRLV-MaxSonar-EZ4 is an excellent choice when only larger objects need to be detected. HRLV-MaxSonar-EZ4
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Optical encoders use a rotor disc made of plastic or glass that is patterned with transparent and opaque areas that can be detected as the disc rotates between a light source and a photodetector. Like the magnetic encoder, the simplest configuration might use just one sensor and have one half of the disc transparent and the other half opaque. But for higher resolution, the disc is usually divided into many more segments (often in concentric rings) with two or more sensors.
Magnetic encoders use a combination of permanent magnets and magnetic sensors to detect movement and position. A typical construction uses magnets placed around the edge of a rotor disc attached to a shaft and positioned so the sensor detects changes in the magnetic field as the alternating poles of the magnet pass over it.
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Common sensors to prevent damage to the robot are related to object detection as discussed above. Other sensors such as current, temperature and humidity can also be employed to better protect the robot from itself as well as the environment. Knowing what's around your robot is normally the first step in damage prevention. The next step is to ensure that the characteristic of that environment, , are not able to cause a catastrophic event. such as humidity and ambient gas
Once you have the appropriate sensor array to protect the robot from the environment you need to protect the robot from itself. and encoders are often used to set firm limits on robot movement. When a robot has attachments such as a pan-and-tilt camera or an arm there are typically ranges of movement that are save and ranges that would cause the robot to harm itself. Identifying these limits is key to developing a solid robotic solution. Once the movement limits have been addressed, it's important to Contact sensors going to the motors monitor the current. Knowing the specifications of the motors and the motor controller will provide you with the safe limits of your drive system. Dynamically responding to these limits will help prevent motor stalling and overheating which can leave your robot stranded in a potentially dangerous environment and cause expensive permanent damage. as well as their temperature
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