From a robots point of view, it is more important to distinguish

Local or on-board sensors (sensors mounted on the robot)

Global sensors (sensors mounted outside the robot in its environment and transmitting sensor data back to the robot)

For mobile robot systems it is also important to distinguish

Internal or proprioceptive sensors (sensors monitoring the robots internal state)

External sensors (sensors monitoring the robots environment)

A further distinction is between:

Passive sensors (sensors that monitor the environment without disturbing it, for example digital camera, gyroscope)

Active sensors (sensors that stimulate the environment for their measurement, for example sonar sensor, laser scanner, infrared sensor)

Binary sensors are the simplest type of sensors A typical example is a tactile sensor

Shaft Encoder = required as a fundamental feedback sensor for motor control.

The most widely used ones are either magnetic encoders or optical encoders.

Magnetic encoders use a Hall-effect sensor and a rotating disk on the motor shaft with a number of magnets (for example 16) mounted in a circle. Every revolution of the motor shaft drives the magnets past the Hall sensor and therefore results in 16 pulses or ticks on the encoder line. Standard optical encoders use a sector disk with black and white segments (see Figure 2.3, left) together with an LED and a photo-diode. The photo-diode detects reflected light during a white segment, but not during a black segment. So once again, if this disk has 16 white and 16 black segments, the sensor will receive 16 pulses during one revolution.

Sonar sensors (Polaroid sensors) 50kHz to 250kHz is emitted

The most significant problems of sonar sensors are reflections and interference.

Infrared sensors 40 kHz The wavelength used is typically 880nm

A compass is a very useful sensor in many mobile robot applications, especially self-localization.

Dead reckoning this method starts with a known initial position and orientation, then adds all driving and turning actions to find the robots current position and orientation

The acceleration sensors use measuring a single or two axes at once. we tested were quite sensitive to positional noise.

A particular problem observed with the piezo gyroscope used is drift: even when the sensor is not being moved and its input PWM signal is left unchanged, the sensor output drifts over time This may be due to temperature changes in the sensor and requires compensation.

In order to keep track of the current orientation, one has to integrate the sensor signal over time, for example using the Runge-Kutta integration method. This is in some sense the equivalent approach to dead reckoning for determining the x/y-position of a driving robot.

Inclinometers measure the absolute orientation angle within a specified range, depending on the sensor model.

Inclinometers measure the absolute orientation angle about an axis and not the derivative; they seem to be much better suited for orientation measurement than a gyroscope. However for example balancing robots, gyroscopes have an advantage over inclinometers. With the components tested, the ideal solution was a combination of inclinometer and gyroscope.

Digital cameras are the most complex sensors used in robotics

There are three commonly used capture modes available for receiving data from a digital camera:

Read mode: The application requests a frame from the driver and blocks CPU execution

Continuous capture mode: In this mode, the driver continuously captures a frame from the camera and stores it in one of two buffers.Synchronous continuous capture mode: In this mode, the driver is working in the background. It receives every frame from the camera and stores it in a buffer.

Actuators

H-Bridge - what is needed to enable a motor to run forward/backward.

Stepper motors differ from standard DC motors in such a way that they have two independent coils which can be independently controlled. As a result, stepper motors can be moved by impulses to precede exactly a single step forward or backward, instead of a smooth continuous motion in a standard DC motor.

A typical number of steps per revolution is 200, resulting in a step size of 1.8.

A servo, on the contrary, is a DC motor with encapsulated electronics for PW control and is mainly used for hobbyist purposes, as in model airplanes, cars, or ships.

A servo has three wires: VCC, ground, and the PW input control signal.

It only has a range of about 120 from its middle position.

This delay creates an overshooting or undershooting of the actual motor speed and thereby introduces hysteresis. The on-off controller is the simplest possible method of control.

The on-off controller is the simplest possible method of control.

PID ControlIt comprises a proportional, an integral, and a derivative control part.

Proportional ControllerFor many control applications, the abrupt change between a fixed motor control value and zero does not result in a smooth control behavior. We can improve this by using a linear or proportional term instead.

Integral ControllerUnlike the P controller, the I controller (integral controller) is rarely used alone, but mostly in combination with the P or PD controller. The idea for the I controller is to reduce the steady-state error of the P controller.

Derivative ControllerSimilar to the I controller, the D controller (derivative controller) is rarely used by itself, but mostly in combination with the P or PI controller. The idea for adding a derivative term is to speed up the P controllers response to a change of input.

Communication Model

Polling:One agent (or a base station) is defined as the master node. In a round-robin fashion, the master talks to each of the agents subsequently to let it send one message, while each agent is listening to all messages and can filter out only those messages that apply to it.

Virtual Token Ring:A token (special data word) is sent from one agent to the next agent in a circular list. Whoever has the token may send its messages and then pass the token on to the next agent. Unlike before, any agent can become master, initiating the ring mechanism and also taking over in case a problem occurs

Virtual Token Ring approach.

The master has to create a list of all active robot nodes in the network by a polling process at initialization time.

The master has to monitor the data communication traffic with a timeout watchdog.

If the token is lost several times in a row by the same agent (for example three times), the master decides that this agent is malfunctioning and takes it off the list of active nodes.

The master periodically checks for new agents that have become available (for example just switched on or recovered from an earlier malfunction) by sending a wild card message.