Getting Started with Sensors

[InFlight]

From our Engineering Document

Digital Sensors

Micro-Switches:
A simple switch can be used to determine the travel limits, or whether a game piece is acquired. There are literally thousands of different switches sold. Our team generally uses through-hole mount, SPDT, lever action switch with gold contacts. This style has a normally closed contact, a normally open contact, and a shared common. The lever action switches can be bent slightly bent to adjust the switch point. The Honeywell P/N ZM10D20D01 would be an example of this type of switch.


Magnetic Switches
There are three types (Reed, Hall Effect, & Magnetoresistive) of magnetically operated switches that are activated by the presence of a magnet. Unlike the micro-switch, these switches are a non-contact switches. The reed switch modules are really the simplest interface, and the ones I recommend.

The reed switch uses a magnetically operated switch is a sealed capsule. The Little Fuse 59150 series with the matching 57150 magnetic are very economical. The window alarm switches sets sold in the big-box hardware stores have the same basic operation.

Hall Effect sensors are omni-polar solid state devices that activate (change output state) when a particular magnetic field is near (South or North). The Littlefuse 55140 series is one reasonable option.

There are also Magnetoresistive devices that change resistances as a function of the magnetic field. The device includes electronic to process the output of the device. The Honeywell SM451R is an example, there aren’t many packaged conveniently for immediate use.

Inductive Proximity
An Inductive proximity sensor is another non-contact sensor that detects the presence of steel and too much lesser degree aluminum. It has an oscillator that through a loop of magnetic wire that creates a magnetic field. When a ferrous metal is within the sensing distance, the total induction of the coil is changed. The inductance change is detected as a sensor frequency shift and output as a state change. The Omron E2B series is an example of this type of sensor.


Analog Sensors

Potentiometers:
Potentiometers are a variable resistance device that changes value with rotation. They can be directly used as a low cost method of measuring rotation angle, or to measure the length of travel using a string pot set setup. Our team keeps a supply of ten-turn linear potentiometers for use as sting pots (TT Electronics P/N 7276R10KL25 ). Single turn linear potentiometers are readily available locally in any electronics shop.

Photo and Infrared Reflectance Sensors
These sensors provide a narrow frequency source LED of either visible or infrared light. There is a paired receiving photo transistor that provides an output when there is reflected light within the matching frequency range. These non-contact sensors would be used in parallel with a mechanism or game piece detect the presence or lack presence of a reflecting adjacent surface. These sensors provide low current at large distances, and a peak current at their detection distance.

The TT Electronic P/N OPB700ALZ sensor is a nice prewired package with an adjustable distance slot for its 0.2 inch detection distance. (>$9.00). The Honeywell HOA1406 is nearly identical. (Both of these require a ~ 220Ω resistor in-line with the photo diode.)

Pololou.com also carries a range of Sharp sensors with a range of activation distances.


Photo and Infrared Transmissive Sensors
These are much like the Photo or Infrared Reflectance sensors where the Source LED and Photo transistor are separated and detect if there is a clear line or a blocked line. (Very much like a Garage Door sensor set). There are combined sets that sets such as the Honeywell Series HOA6980 that would detect a vane type device. You can also get a separate IR emitter source and a matching Photo-detector sensor in either the 880 nm or 935 nm range sets. The Honeywell SPX1189 series is a matched set that is prewired in modules.


Range Finders
There are several methods available to determine your distance from an obstacle (shooting target, wall).

There are some infrared reflectance sensors (discussed above) that have a considerable range (Sharp GP2Y02A02YK0F with 8 inches to 150 inches of range)

Ultrasonic range finders can measure distance 0 inches out to 20 feet. Matbotixs has a range of sensors with different ranges/sensitivities. The newer HRUSB series offers a USB interface.

Garmin also markets a LIDAR range finder that uses near infrared laser source, the receiver measure the time delay allowing measurement of distance and velocity.


Encoders
There are two fundamental classes of encoders; incremental and absolute. Incremental encoders are normally used for speed detection. An absolute encoder provides the 360⁰ position data, and maintains that calibrated position data through a power interrupt. Some absolute encoders can be used for speed as well. Speed encoders normally provide two “quadrature” signals that are 90⁰ phase apart to indicate both speed and rotation direction. The normal encoders encountered in first are of the Optical, Hall Effect or Capacitive operating types.

Hall Effect encoders work by having a North & South poled magnet rotating near the Hall Effect sensor, providing a sinusoid waveform as the shaft rotates.

Optical encoders have an alternating series of clear and solid sections on a rotating disk. Typically pair of 90⁰ infrared sensors detect the waveform and create the quadrature signal. Many speed encoders will also have a once per revolution index signal.

Capacitive encoders have a disk with a wire trace pattern on it. The rotation of the disk causes a change in the local electromagnetic field strength which is picked up by hall sensors and processed by an ASIC.

Across the Road Electronics SRX Magnetic Encoder (Vex P/N 217-5046). This is a Hall Effect based sensor that provides both standard 1024 cycles per revolution quadrature and absolute PWM positioning. Seems like a very flexible unit at a low cost. It’s fully compatible with the TALON SRX and used in the VersaPlanatery Integrated encoder (Vex P/N 217-5046)

AndyMark AM-2816 is a two channel quadrature encoder that fits on the back of some of their motors. There is a N/S ring magnet that fits on the shaft and two 90 degree mounted hall effect sensors under it.

CUI AMT 10 & AMT11 series are through-shaft (2 m to 8mm) capacitive speed encoders with resolutions available between 48 to 2048 (to 4096 w/ AMT11) quadrature cycles per revolution . There are 102/112 models with a radial connector, and 103/113 models with a 90 degree axial Connectors. The newer AMT11 series has a serial port that allows the index position to be set via a serial port with proprietary software.

CUI AMT20 series is an absolute encoder which is mechanically very similar in physical attributes to the AMT11 series. It provides the normal speed quadrature resolution of 2046. Absolute position is transmitted on a Serial Peripheral Interface bus (SPI) that is compatible with the roboRio. Only offered as a 203 model with an axial connector.

Greyhill 63R Series is a very popular and reliable ¼ inch shaft optical quadrature encoder. Available between 25 and 256 cycles per revolution. An excellent choice for any type of tank drive system. (Team 3574 has used these.)

TT Electronic 6127V1360 is a very low cost (<$15) analog Hall Effect absolute position sensor. Another alternative to the MA3 analog absolute sensor.

US Digital E4T (AM-3132) is a miniature through-shaft optical encoder typically used in the 0.250 inch shaft option. Available between 100 and 500 cycles per revolution. This is a very common encoder used on the AndyMark Gearboxes. In this application it is fine; however as any shaft run-out or axial movement can affect the reliability. (Team 3574 has used these.)

US Digital S1 Series is reliable ¼ inch shaft optical quadrature encoder (equivalent to the Greyhill G3R). Available between 32 and 4096 cycles per revolution. An excellent choice for any type of tank drive system or other application that needs higher resolution. (Team 3574 has used these.)

US Digital MA3 (AM-2899) is a low cost ¼ inch shaft absolute encoder. It is available as either an analog output or 10 or 12 bit Pulse Width Modulation Signal.

Navigation
With integrated navigation and compass functions becoming basic in almost all smart phones; there are a number integrated navigation chips coming out each year. Adding an integrated navigation board allows more consistent autonomous modes operation, and allows field centric driving control. I’ve listed a few of the current offerings, but the navX board is by far the simplest solution.

Analog Devices ADXRS450 Evaluation Board. Simple gyroscope only board with an SPI interface for the roboRio.

navX MXB 3.0 (AM-2080b) uses a Ivensense MPU-9250 chip; that has features an accelerometer, a MEMS gyroscope (angular rate sensor) and a hall effect Magnetometer (compass) in each of the X, Y, an Z axis’s. This chip also has motion processing to provide integrated output data. This navX MXB directly interfaces with the roboRio, making it a plug in navigation solution.

LSM9DS0 Board with a Bosch BN005 chip navigation chip. This chip features an accelerometer, a MEMS gyroscope (angular rate sensor) and a hall effect Magnetometer (compass) in each of the X, Y, an Z axis’s. This chip also has motion processing and data fusion to provide integrated output data.