Sensor Systems

Sidebar

theory:sensor_technology:st23_bus_interfaces

Bus Interfaces

Sensors have to be at specific locations to pick up the required physical quantity correctly. As a result, a sensor system may result into many, long wires to the sensors or sensor modules. While many sensors are still connected by analog wires (either with or without a pre-amplifier), the overall trend is to move to digital, standardised bus systems.

The advantages of bus interfaces and networks

The reasons to use a bus or bus system are:

• Reduce the number of wires and simplify the wiring plan by using star-based or chain networks
• Reduce the influence of electronic noise by going digital
• Implementig channel coding
• Standardization of physical and electrical interfaces for cost reduction and easy serviceability

There are three examples where digital buses were introduced after electronic systems evolved for decades:

I2C, SPI and electronic appliances

In the 1970's, TV sets had become complex and in the mean time, digital controllers were introduced. The wiring architectures had become very sophisticated and there became clear different wires for control- and setting data opposite to data channels for (digital) video information. Therefore, Philips introduced in 1979 the Inter IC Bus, short I2C bus. This is a serial, synchronous bus which was originally intended for rates up to $100 kb/s$ over a maximum range of $1m$. This has changed the architecture of TV sets, and also other equipment, dramatically. Sub-modules within a TV architecture got their own status, linked only by the I2C bus, and remote units, like the control panel, were easier to wire.
Although the I2C bus is not intended for sensors, it is used for sensor chips. An alternative, which is similar in the sense that it comprises a clock-line and a serial data line, is the Serial Peripheral Interface (SPI) as introduced by Motorola in the late 1980's. Also this bus protocol is implemented in many sensor chips.

CAN bus in car industry

Similar to what happened in the architecture of appliances due to I2C, the automotive industry experienced a shift towards digital buses in the late 1980's. The Controller Area Network bus (CAN bus) is a vehicle bus standard developed at Robert Bosch GmbH. It has changed the car's architecture of the control system into central microcontrollers, communicating to remote units over a serial bus.
Later on, more dedicated buses for safety critical subsystems were developed like FlexRay and the AUTOSAR (AUTomotive Open System ARchitecture) has formalized the software architecture. The CAN bus is also used as industrial fieldbus in general automation environments, primarily due to the low cost of some CAN controllers and processors.

Industrial Ethernet

In industrial machines for manufacturing, logistics (conveyer belts) and packaging, we see the integration of business and technical processes. Manufacturers want to control and monitor their production in a central place. As a result, we see the introduction of IT technologies and methods on the manufacturing floor. To be more specific, the technical consequences are:

• Standard Operating Systems on PC automation systems (like Windows, Windows Embedded, Linux and Linux Embedded)
• Use of Ethernet
• Standard internet protocols like http (remote control, etc.)
• Technologies like OPC, XML or TCP/IP

Current sensor modules for industry, are available with one of the Ethernet based industry buses.

Buses optimised for sensor networks

The most common protocols to implement communication from sensors to the main controller in electronic microsystems are I2C and SPI. A good overview is given in the paper by Zhou1). Also the webpage of Byte Paradigm2) gives a good comparison of I2C and SPI.

Industrial buses

In fact, an industrial machine is normally a distributed computer. This means there is a central controller (either PLC or Industrial PC as a master) which is connected to distant slave couplers by means of a field bus3), 4). From the coupler slices may address sensors and actuators by means of low-end field buses in a slave-slave or master-slave relation.

The current topics of interest are:

• Shift towards Ethernet based protocols (PROFINET, EtherCAT, EtherNet/IP)
• Integration of ICT services on the same bus
• Ring redundancy
• Power over Ethernet
• Real-time features (because they are part of a distributed computer)
• Integrated cyber security systems (detect autenticity)
• Integrated safety systems (guaranteed and predictable stop conditions)
• Extension to lower-end interconnect like point-to-point IO-Link

• Industrial EtherNet Facts, System comparison - the 5 major technologies5)
• EtherCAT Communication6) and Industrial Ethernet Technologies7) by the EtherCAT Technology Group

• CAN
• FlexRay

Sensor Technology TOC

These are the chapters for the Sensor Technology course:

1)
Zhou, J., & Mason, A. (2002), Communication buses and protocols for sensor networks, Sensors, 2(7), 244-257, www.mdpi.com/1424-8220/2/7/244/pdf
2)