EL6483: Sensors and Actuators EL6483 Spring 2016 EL6483 EL6483: Sensors and Actuators Spring 2016 1 / 15
Sensors Sensors measure signals from the external environment. Various types of sensors Variety in terms of what the sensors measure (what physical quantity, e.g., voltage, current, force, pressure, temperature, etc.) Variety in terms of how they provide their measurements to other devices (e.g., analog and digital interfaces, etc.; many sensors measure analog signals, but provide output data through a digital communication interface) Variety in terms of sensor specications (e.g., sampling rate, resolution, accuracy, noise characteristics, etc.) EL6483 EL6483: Sensors and Actuators Spring 2016 2 / 15
Some examples of types of sensors Position, velocity, acceleration, etc. Local position and velocity measurement, e.g., using ultrasonic range detectors, vision, etc. Global position and velocity measurement... GPS (Global Positioning System) Linear acceleration measurement... accelerometers (measure gravity + inertial acceleration), three axes of measurement EL6483 EL6483: Sensors and Actuators Spring 2016 3 / 15
Some examples of types of sensors Angular position, angular velocity, etc. Angular position measurements for motors, etc.... potentiometers (analog), optical encoders (quadrature outputs) Angular position measurements... magnetometers (compass)... measures magnetic eld of earth (thus, can be used to estimate yaw/heading); accelerometers can measure direction of gravity and thus can be used to estimate roll and pitch... combine accelerometers and magnetometers for three-axis angle estimation Angular velocity measurements... tachometers (for motors, etc.), gyroscopes (for general rotating objects) EL6483 EL6483: Sensors and Actuators Spring 2016 4 / 15
Some examples of types of sensors Sensing objects in the environment Vision (electro-optic/infrared EO/IR) Sonar uses sound waves to sense objects (measure distances, estimate object type/density from intensity of sound echo, etc.) Laser range measurement and LIDAR (Light Detection And Ranging) uses light to measure distances to objects (and estimate their light reectivity properties) Radar (Radio Detection And Ranging) uses radio waves to sense objects (measure distance, estimate object type, etc.) RGB-D (RGB color + depth) provides both visual image and distance (depth) data, e.g., Microsoft Kinect EL6483 EL6483: Sensors and Actuators Spring 2016 5 / 15
Some examples of types of sensors Kinematic measurements (position, velocity, acceleration, angular position, angular velocity, etc.) Sensors to sense objects in the environment (e.g., vision, laser ranging, sonar, Radar, RGB-D) Voltage and current sensors Pressure sensor, airspeed sensors, air ow sensors, etc. ; pressure sensors also can be used to measure altitude (since air pressure changes with height) and also underwater depth Force and torque sensors, etc. Chemical sensors (e.g., ph, conductivity, oxygen, etc.) Humidity, temperature, etc. Audio sensor (microphone) For a large list of types of sensors, see http: // en. wikipedia. org/ wiki/ List_ of_ sensors EL6483 EL6483: Sensors and Actuators Spring 2016 6 / 15
Actuators Actuators output signals that inuence (in some way) the external environment. Some examples of types of actuators: electric, e.g., motors mechanical (actuators and actuation devices): gears, mechanical linkages, etc. hydraulic and pneumatic Motion actuators: linear, rotary; mechanical linkages to convert between linear and rotary actuations In general, actuators are any components that output signals to the external environment. e.g., audio output (speakers), display devices, force actuators, torque actuators, etc. EL6483 EL6483: Sensors and Actuators Spring 2016 7 / 15
Variety of sensor and actuator interfaces Analog (e.g., through analog-to-digital converters and digital-to-analog converters) Digital Serial (UART), SPI, I2C, CANBus, GPIO, Ethernet, USB, etc. PWM (Pulse Width Modulation), PDM (Pulse Density Modulation) Many sensors support/require multiple types of interfaces Examples: a sensor may provide both an analog and a serial interface to read sensor data; a sensor may provide output through a serial interface but may utilize an analog pin for some conguration (e.g., to set a reference voltage); a sensor may provide output through an analog interface but may utilize a digital communication interface for conguration (e.g., to set sampling rate); a sensor may provide two dierent types of digital interfaces. EL6483 EL6483: Sensors and Actuators Spring 2016 8 / 15
Methods for sensor and actuator interfaces Registers : memory addresses within the sensor/actuator device that can be read or written to; the actual reading from and writing to memory addresses uses some protocol for the corresponding communication interface format of bit structures within registers (bytes/words), e.g., 2 least signicant bits set range with four possible settings; 2 register addresses (2 bytes) together form a data value (low byte and high byte), etc. Control bytes : bytes that can be sent to the sensor/actuator device through a communication interface; control bytes do not necessarily correspond to a specic memory address in the sensor/actuator device format of bit structures within the control bytes (e.g., 2 least signicant bits of a control byte that is sent over a serial port set the range with four possible settings) similarly, data bytes (e.g., sensor readings provided through a serial port as a sequence of data bytes) Combinations of control/data bytes and registers are used in general. EL6483 EL6483: Sensors and Actuators Spring 2016 9 / 15
Typical sensor/actuator specications Sensors Range max and min of the physical signal that is being measured Accuracy how closely does the sensor output value that is read correspond to the actual physical signal? Resolution what is the smallest possible dierence (increment) in sensor output values? Resolution is often specied in terms of number of bits. If the range of the sensor is [A min, A max ] in terms of the physical signal being measured, then if the resolution is r, the eective number of bits corresponds to n = log 2 ( A max A min r ). Hence, if the number of bits is given to be n, the resolution is r = Amax A min 2 n. An analog sensor output can be considered to have innite number of bits of resolution (since an analog output can change by any arbitrarily small value), but has some nite accuracy. EL6483 EL6483: Sensors and Actuators Spring 2016 10 / 15
Typical sensor/actuator specications Sensors Sensor noise: often specied as a noise density in units of physical quantity per Hz. For example, for an accelerometer, noise density could be in µg/ Hz. The sensor output is usually ltered using a low pass lter to reduce noise. If the eective noise bandwidth of the low pass lter is f n (in Hz), then the sensor noise (root mean square; RMS) after the ltering would be N D f n where N D is the noise density. Eective bandwidth is usually a constant times the lter bandwidth (e.g., 1.6f 3dB where f 3dB is the lter 3dB bandwidth). The 3dB frequency is when the gain is 3dB less than gain of 1 (i.e., gain 0.7079). Actuators Range, accuracy, resolution, noise specications, etc.; analogous to sensors EL6483 EL6483: Sensors and Actuators Spring 2016 11 / 15
db (decibels) and low pass lters db (decibel): if a component has an input V in and an output V out, the gain of the component can be written as a fraction Vout or in db as V in 20 log 10 ( Vout V in ) A low pass lter has higher gain (e.g., gain of 1) at lower frequencies and smaller gain at higher frequencies; hence, a low pass lter passes low frequencies from input to output. A gain of 1 corresponds to 0 db since 20 log 10 (1) = 0. A gain of -3dB (i.e., 3dB less than gain of 1) corresponds to gain G such that 20 log 10 (G) = 3, i.e., G = 10 3 20 = 0.7079. The frequency at which this gain is attained is called the 3dB frequency (f 3dB ) of the lter. EL6483 EL6483: Sensors and Actuators Spring 2016 12 / 15
Example for sensor noise calculation Example: If an accelerometer sensor has noise density 0.1µg/ Hz, then if a low pass lter with f 3dB = 25 Hz is used and if the eective lter bandwidth is f n = 1.6f 3dB, then the RMS noise of the ltered sensor output will be 0.1 1.6 25µg = 0.63µg. RMS of sensor noise: if the sensor data samples are s i = ŝ + n i for i = 1,..., N, where ŝ is the mean (average) value and n i, i = 1..., N, are the noise values, then the RMS of the sensor noise is N i=1 n2 i N = N i=1 (s i ŝ) 2 N. EL6483 EL6483: Sensors and Actuators Spring 2016 13 / 15
Other sensor and actuator specications Oset/bias - e.g., a constant shift of the sensor readings. For example, a temperature sensor with oset/bias b would measure T + b where T is the actual temperature. b is the oset (or bias). Linearity / nonlinearity; e.g., how close to a line is the plot of the sensor output vs. the physical quantity being measured? Hysteresis or memory eects, e.g., when the output of sensor depends not only on the current value of the physical quantity being measured but also on its previous values. Eects of ambient environment; most importantly, temperature; temperature variations often change oset and eective scaling factor (gain); other ambient environment conditions such as humidity are also important for some types of sensors. EL6483 EL6483: Sensors and Actuators Spring 2016 14 / 15
Other sensor and actuator specications Supply voltage range, current requirements, power requirements; power requirement could vary between a quiescent and an active level (e.g., an ultrasonic distance sensor utilizes more power in the brief time interval that it is transmitting a pulse). Max ratings of the device operating temperature range, storage temperature range, acceleration range, etc. EL6483 EL6483: Sensors and Actuators Spring 2016 15 / 15