ELEMENTS OF EMBEDDED SYSTEMS
2. Elements of Embedded Systems.
3. Case studies (examples)
Automotive Embedded System (AES)
4. Review questions
5. References & further reading
After learning this chapter you will be able to:
Define and describe the elements of an embedded system
Understand how embedded system works with the help of two case studies:
The previous chapter was an introduction to the world of embedded systems and helped us define what is an embedded system.
This chapter introduces us to the elements of an embedded system and explains how embedded system works with the help of two case studies.
2. ELEMENTS OF EMBEDDED SYSTEMS.
As defined earlier, an embedded system is a combination of 3 things:
And it is supposed to do one specific task only.
Diagrammatically an embedded system can be represented as follows:
Embedded systems are basically designed to regulate a physical variable (such Microwave Oven) or to manipulate the state of some devices by sending some signals to the actuators or devices connected to the output port system (such as temperature in Air Conditioner), in response to the input signal provided by the end users or sensors which are connected to the input ports.
Hence the embedded systems can be viewed as a reactive system.
Examples of common user interface input devices are keyboards, push button, switches, etc.
The memory of the system is responsible for holding the code (control algorithm and other important configuration details).
An embedded system without code (i.e. the control algorithm) implemented memory has all the peripherals but is not capable of making decisions depending on the situational as well as real world changes.
Memory for implementing the code may be present on the processor or may be implemented as a separate chip interfacing the processor In a controller based embedded system, the controller may contain internal memory for storing code
Such controllers are called Micro-controllers with on-chip ROM, eg. Atmel AT89C51.
3. CASE STUDIES (EXAMPLES)
Here are some case studies on some commonly used embedded systems that will help to better understand the concept.
3.1 Washing Machine
Let us see the important parts of the washing machine; this will also help us understand the working of the washing machine:
Water inlet control valve: Near the water inlet point of the washing there is water inlet control valve. When you load the clothes in washing machine, this valve gets opened automatically and it closes automatically depending on the total quantity of the water required. The water control valve is actually the solenoid valve.
Water pump: The water pump circulates water through the washing machine. It works in two directions, re-circulating the water during wash cycle and draining the water during the spin cycle.
Tub: There are two types of tubs in the washing washing machine: inner and outer. The clothes are loaded in the inner tub, where the clothes are washed, rinsed and dried. The inner tub has small holes for draining the water. The external tub covers theinner tub and supports it during various cycles of clothes washing.
Agitator or rotating disc: The agitator is located inside the tub of the washing machine. It is the important part of the washing machine that actually performs the cleaning operation of the clothes. During the wash cycle the agitator rotates continuously and produces strong rotating currents within the water due to which the clothes also rotate inside the tub. The rotation of the clothes within water containing the detergent enables the removal of the dirt particles from the fabric of the clothes. Thus the agitator produces most important function of rubbing the clothes with each other as well as with water.
In some washing machines, instead of the long agitator, there is a disc that contains blades on its upper side. The rotation of the disc and the blades produce strong currents within the water and the rubbing of clothes that helps in removing the dirt from clothes.
Motor of the washing machine: The motor is coupled to the agitator or the disc and produces it rotator motion. These are multispeed motors, whose speed can be changed as per the requirement. In the fully automatic washing machine the speed of the motor i.e. the agitator changes automatically as per the load on the washing machine.
Timer: The timer helps setting the wash time for the clothes manually. In the automatic mode the time is set automatically depending upon the number of clothes inside the washing machine.
Printed circuit board (PCB): The PCB comprises of the various electronic components and circuits, which are programmed to perform in unique ways depending on the load conditions (the condition and the amount of clothes loaded in the washing machine). They are sort of artificial intelligence devices that sense the various external conditions and take the decisions accordingly. These are also called as fuzzy logic systems. Thus the PCB will calculate the total weight of the clothes, and find out the quantity of water and detergent required, and the total time required for washing the clothes. Then they will decide the time required for washing and rinsing. The entire processing is done on a kind of processor which may be a microprocessor or microcontroller.
Drain pipe: The drain pipe enables removing the dirty water from the washing that has been used for the washing purpose.
3.2 Microwave Owen
Let us see the important parts of the microwave oven; this will also help us understand the working of the washing machine:
A microwave oven consists of:
A high voltage transformer, which passes energy to the magnetron
A cavity magnetron,
A Control circuit with a microcontroller,
A waveguide, and
A cooking chamber
A Transformer transfers electrical energy through a circuit by magnetic coupling without using motion between parts. These are used for supplying power to the magnetron.
A Cavity magnetron is a microwave antenna placed in a vacuum tube and oscillated in an electromagnetic field in order to produce high GHz microwaves. Magnetrons are used in microwave ovens and radar systems.
A control circuit with a microcontroller is integrated on a circuit board. The microcontroller controls the waveguide and the entire unit so the microwaves are emitted at a constant rate.
A Waveguide is any linear structure that guides electromagnetic waves for the purpose of transmitting power or signals. Generally constructed of a hollow metal pipe. Placing a waveguide into a vacuum causes radio waves to scatter.
A Cooking Chamber is a microwave safe container the prevents microwaves from escaping. The door has a microwave proof mesh with holes that are just small enough that microwaves can't pass through but lightwaves can. The cooking chamber itself is a Faraday cage enclosure which prevents the microwaves from escaping into the environment. The oven door is usually a glass panel for easy viewing, but has a layer of conductive mesh to maintain the shielding.
3.3 Automotive Embedded System (AES)
The Automotive industry is one of the major application domains of embedded systems.
Automotive embedded systems are the one where electronics take control over the mechanical system. Ex. Simple viper control.
The number of embedded controllers in a normal vehicle varies somewhere between 20 to 40 and can easily be between 75 to 100 for more sophisticated vehicles.
One of the first and very popular use of embedded system in automotive industry was microprocessor based fuel injection.
Some of the other uses of embedded controllers in a vehicle are listed below:
Automatic break system control
Air bag control
AES are normally built around microcontrollers or DSPs or a hybrid of the two and are generally known as Electronic Control Units (ECUs).
Types Of Electronic Control Units(ECU)
High-speed Electronic Control Units (HECUs):
HECUs are deployed in critical control units requiring fast response.
They Include fuel injection systems, antilock brake systems, engine control, electronic throttle, steering controls, transmission control and central control units.
Low Speed Electronic Control Units (LECUs):-
They are deployed in applications where response time is not so critical.
They are built around low cost microprocessors and microcontrollers and digital signal processors.
Audio controller, passenger and driver door locks, door glass control etc.
Automotive Communication Buses
Embedded system used inside an automobile communicate with each other using serial buses. This reduces the wiring required.
Following are the different types of serial Interfaces used in automotive embedded applications:
Controller Area Network (CAN):-
CAN bus was originally proposed by Robert Bosch.
It supports medium speed and high speed data transfer
CAN is an event driven protocol interface with support for error handling in data transmission.
Local Interconnect Network (LIN):-
LIN bus is single master multiple slave communication interface with support for data rates up to 20 Kbps and is used for sensor/actuator interfacing
LIN bus follows the master communication triggering to eliminate the bus arbitration problem
LIN bus applications are mirror controls , fan controls , seat positioning controls
Media-Oriented System Transport(MOST):-
MOST is targeted for automotive audio/video equipment interfacing
A MOST bus is a multimedia fiber optics point–to-point network implemented in a star , ring or daisy chained topology over optical fiber cables.
MOST bus specifications define the physical as well as application layer , network layer and media access control.