2Robot speed, direction, attitude, position and other sensors
2.1 Wheel/motor sensor
Wheel encoder (rotary encoder, photoelectric encoder) odometer
Function: measure the angle and speed of rotation
Structure: mainly composed of light source, fixed grating, rotating grating and receiver.
Principle: The light source, the fixed grating and the receiver are all installed in the static part connected with the chassis, and the rotating grating is connected with the axis to be measured for rotating motion. When the axis to be measured rotates, the light beam emitted by the light source passes through the rotating grating and the fixed grating to form a narrow beam that passes alternately, and is received by the receiver to generate a measurement signal (sine or square wave pulse).
Features: Proprioceptive sensor , the resolution is measured in cycles per revolution (CPR) , with a typical value of 64-2048.
Application: Used to measure the position or speed of wheels or steering devices , together with the motion parameters of the wheels to estimate the current position of the robot .
2.2 Orientation sensor-Hall effect compass
Hall effect compass (Hall element)
principle: When current passes through a semiconductor material, a voltage corresponding to the magnetic flux density and its direction is generated, the Hall effect. Manufactured by the Hall effect, the element that detects the magnetic field is called the Hall element. A single Hall element provides a one-dimensional measurement of magnetic flux and direction.
Features: low price, low resolution, low bandwidth, high cross sensitivity (susceptible to interference from magnetic fields generated by other magnetic objects and man-made structures) .
Application: In mobile robots, the Hall-effect digital compass is very common; but mobile robots used in indoor environments should avoid using this kind of digital compass.
2.3 Orientation sensor-mechanical gyroscope
Gyroscope: able to maintain the direction relative to a fixed frame of reference (coordinate system), thereby providing an absolute measurement (direction or angular velocity) of the guidance of the mobile system. Usually divided into two categories: mechanical gyroscopes and optical gyroscopes.
Principle: Based on the principle of conservation of angular momentum, the high-speed rotating "wheel" (gyro) in the middle can maintain the direction relative to the fixed reference frame, while the surrounding "steel ring" will change as the robot changes its posture; these "steel rings" The axis where it is located is the corresponding coordinate axis in the gyroscope, and the current state of the device is jointly detected through the three-dimensional space enclosed by these axes.
Features: Generally speaking, the sensitivity, accuracy and precision are much higher than the Hall-effect compass, but the price is much higher.
2.3.1 MEMS gyroscope:
The Coriolis force is used to produce a small capacitance change inside it, and then the capacitance is measured to calculate the angular velocity.
2.3.2 Laser gyroscope:
Using the Sagnac effect, in a closed light path, two beams of light transmitted clockwise and counterclockwise from the same light source interfere with each other. By detecting the phase difference or interference fringe changes, it can be measured The angular velocity of rotation of the closed optical path.
Features: The resolution and bandwidth of laser gyroscopes have far exceeded the navigation information required by general mobile robot applications.
2.4 Accelerometer
2.4.1 Mechanical accelerometer:
The reference mass (Mass) is connected to the shell by a spring and a damper. In the steady state, the relative displacement between it and the shell reflects the magnitude of the acceleration component. This signal is output as a voltage through the potentiometer, and its output is proportional to the acceleration Electrical signal of component size.
2.4.2 MEMS accelerometer (capacitive):
A spring-like structure connects the device to the vibration mass (orange square M) in a capacitive voltage divider (capacitive devider in the picture). The capacitive voltage divider converts the displacement of the vibration mass into an electrical signal. Damping is created by the gas sealed in the device.
MEMS: Micro-electromechanical system with mechanical and electronic characteristics.
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2.4.3 MEMS accelerometer (piezoelectric type):
Using the piezoelectric effect of certain crystals (when a mechanical pressure is applied to certain crystals, a voltage is generated), a small mass is placed on the crystal, and when an external force is applied, the mass will move, thereby inducing a variable Measured voltage.
Features: The accelerometer can only measure acceleration on a single axis. By mounting three accelerometers together vertically, a three-axis accelerometer can be obtained.
Application: Measure the acceleration of gravity or the acceleration of the robot, mainly low-pass accelerometer (bandwidth 0Hz to 500Hz), the most common is capacitive accelerometer; measure the acceleration of robot vibration or collision, mainly high-pass accelerometer (bandwidth a few Hz) To 50kHz), the most common is piezoelectric accelerometer.
2.5 Inertial Measurement Unit (IMU)
Inertial measurement unit (inertial navigation system): Use gyroscope and accelerometer to estimate the relative position, velocity and acceleration of the robot.
The inertial navigation system composed of 3 orthogonal accelerometers and 3 orthogonal gyroscopes can estimate the attitude of the robot with 6 degrees of freedom (x, y, z position and the direction of roll, tilt, and deflection) and corresponding Speed and acceleration.
Inertial navigation systems are extremely sensitive to the errors of gyroscopes and accelerometers. Almost all inertial navigation systems will drift after a long period of operation. In many robot applications, camera or GPS has been used to correct attitude and eliminate drift.
2.6 Global Satellite Navigation System
The Global Navigation Satellite System (the Global Navigation Satellite System), also known as the Global Navigation Satellite System, is a space-based radio navigation system that can provide users with all-weather 3D coordinates and speed and time information on the surface of the earth or anywhere in near-Earth space. GPS.
Common systems include GPS, BDS, GLONASS and GALILEO four satellite navigation systems. The GPS (Global Positioning System) in the United States was the first to appear, and the most technologically complete GPS system at this stage. With the opening of comprehensive services of BDS and GLONASS systems in the Asia-Pacific region in recent years, the development of BDS systems in the civilian field in particular has become faster and faster.
2.6.1 Basic principles of GPS and BDS positioning (single point satellite positioning):
Measure the distance between a satellite with a known location and the user's receiver, and then synthesize the data of multiple satellites to know the specific location of the receiver. ( Satellite sending orbit position (ephemeris) plus time ; the receiver calculates its position through trilateration and time correction).
GPS and BDS positioning actually use four satellites with known positions to determine the location of the receiver.
- Position1-4 are the current positions (space coordinates) of the four satellites, which are known
- d1-4 is the distance from the four satellites to the receiver to be positioned, which can be requested
- Location is the location of the satellite receiver to be located, to be requested
2.6.1.1 Why do we need four satellites?
(Note: d1-4 is the distance between the four satellites and the receiver to be positioned, calculated)
➢ Each satellite is broadcasting its own position, and when sending the position information, it will also attach the data packet sent out. Timestamp.
➢ After the receiver receives the data packet from the satellite broadcast, the time when the data packet is received on the receiver is subtracted from the time on the time stamp, which is the time it takes for the data packet to be transmitted in the air.
➢ The time of data packet transmission in the air multiplied by the transmission speed (speed of light) is the distance of data packet transmission in the air, which is the distance from the satellite to the receiver.
The transmission speed of the data packet is very high (the ideal speed is the speed of light), then a very small error for the timestamp will be amplified many times and the whole result will be invalid.
(Although three satellites can ideally provide the position of 3 axes, in order to correct the time, four satellites are still needed, and the additional information is used to solve for 4 variables: 3 position axes and 1 time correction.)
2.6.1.2 Error sources of single-point satellite positioning
- Ephemeris data error : 1 meter
- Tropospheric delay : The 1 meter troposphere is the lower part of the atmosphere (from 8 to 13 kilometers on the ground), which experiences changes in temperature, pressure and humidity associated with changes in weather. Complex models of tropospheric delay need to estimate or measure these parameters.
- Unmodeled ionospheric delay : 10 meters. The ionosphere is 50 to 500 kilometers of the atmosphere, composed of ionized air. The transmitted model
can only eliminate about half of the possible 70 ns delay, leaving an unmodeled residual of 10 meters (30 ns). - Multipath : 0.5-100m multipath is caused by reflected signals from the surface near the receiver. These signals may interfere with or be mistaken for signals following the straight path of the satellite. Multipath is difficult to detect and sometimes difficult to avoid.
2.6.1.3 Differential satellite positioning
Place a receiver on the reference station for observation. According to the known precise coordinates of the base station, the distance correction number from the base station to the satellite is calculated, and the base station sends this data out in real time. While observing, the user receiver also receives the correction number sent by the base station and corrects its positioning result, thereby improving the positioning accuracy.
According to the information sent by the differential reference station, the differential positioning can be divided into three categories: position differential, pseudorange differential and phase differential .
The working principles of these three types of differential methods are the same, but the difference is that the specific content of the correction number is different, and the differential positioning accuracy is also different.
