Automation and Control technologies part-2

Sensors and actuators used for automation

Motion (orientation/inclination) sensors

• Accelerometer

Measure acceleration in one or many directions. Position can be deduced by integration. Mostly used methods are mass spring method and gas pocket method.

                           Applications are,

ü  Self-balance robot

ü  Mobile phones(navigation and gaming)

ü  Smart hand bands

• Gyroscope(Rate)

Measures rate of rotation along 3 axes. Integration is required to get rotation angles.    

                           Applications are,

ü  GPS navigation systems

ü  Image stabilization

ü  Inertial measurement units

ü  Platform stabilization

• Compass
• Tilt sensor

Detect tilting angle (35~65 degrees in right and left inclination).

Force/Pressure/Strain sensors

•  Force sensitive resistor: Changes resistance with the force applied. Used in walking robot.      
• Strain gauge: Produces voltage that is proportional to the force applied.
• Flexion bend sensors: Resistance changes with the bend angle.10kΩ [0], 40kΩ [90].Used to measure bend of a joint, weight sensors, Availability checking.

• Air pressure sensor: Can measure air pressure up to 150 PSI. This sensor is used in TPMS-tyre pressure management system, Flow sensing devices, Helicopters, Submarines, etc.

Position sensors

• IR range sensor

Range/distance is determined by the angle between horizontal plane and IR beam. Applications are,

• Parking sensors(systems mounted on car bumpers that sense distance to nearby cars for parking)
• Ground proximity warning system for aviation safety
• Vibration measurements of rotating shafts in machinery
• Top dead centre (TDC)/camshaft sensor in reciprocating engines.
• Sheet break sensing in paper machine.
• Anti-aircraft warfare
• Roller coasters
• Conveyor systems

• Rotary encoders

Rotary encoders are used to measure rate of rotation, direction of rotation and shaft position. There are three types of Rotary encoders.

o   Magnetic

o   Optical

o   Resistive

• Linear encoders

o   LVDT

LVDT applications are Crank shaft balancer, Pill making machine, Portable friction welder, etc.

Temperature and Humidity sensors

Light sensors

• CDS (cadmium sulfoselenide photo conducive photocells)
• Light dependent resistor (LDR)

Hall Effect sensors

Sense magnetic flux. Applications are,

ü  Bullet train

ü  DC motor controlling

ü  DC Current sensors

Novel sensors

•   Kinect sensor
  
  Motion sensing input device which can identify gestures and voice commands. This sensor is used in Xbox360 game player.

Actuators

Actuators are hardware devices that convert a controller command signal into a change in a physical parameter

• The change is usually mechanical (e.g., position or velocity)
• An actuator is also a transducer because it changes one type of physical quantity into some alternative form
• An actuator is usually activated by a low-level command signal, so an amplifier may be required to provide sufficient power to drive the actuator
• Electrical actuators
• Electric motors
• DC servomotors
• AC motors
• Stepper motors
• Solenoids
• Hydraulic actuators
• Use hydraulic fluid to amplify the controller command signal
• Pneumatic actuators
• Use compressed air as the driving force

                           Programmable logic controllers

A programmable logic controller (PLC), or programmable controller is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, or robotic devices, or any activity that requires high reliability control and ease of programming and process fault diagnosis.

                           Internal Architecture of a PLC

The basic architecture of a PLC consists of main components-the processor module, the power supply, and the I/O modules. The processor module consists of the central processing unit (CPU) and memory. In addition to a microprocessor, the CPU also contains at least an interface to a programming device and may contain interfaces to remote I/O and other communication networks. The power supply is usually a separate module, and the I/O modules are separate from the processor. The types of I/O modules include discrete (on/off), Analog (continuous variable), and special modules like motion control or high-speed counters. The field devices are connected to the I/O modules.

Ladder programming

Ladder logic is widely used to program PLCs, where sequential control of a process or manufacturing operation is required. Ladder logic is useful for simple but critical control systems or for reworking old hardwired relay circuits. As programmable logic controllers became more sophisticated it has also been used in very complex automation systems. Often the ladder logic program is used in conjunction with an HMI program operating on a computer workstation.

Industrial communication in Distributed control system (DCS).

In recent years, the use of smart devices and field buses makes distributed control system (DCS) to be prominent in large and complex industrial processes as compared to the former centralized control system. This distribution of control system architecture around the plant has led to produce more efficient ways to improve reliability of control, process quality and plant efficiency.

Nowadays, distributed control system has been found in many industrial fields such as chemical plants, oil and gas industries, food processing units, nuclear power plants, water management systems, automobile industries, etc.

In the supervisory level the operation is monitored and analysed. Huge amount of data transfer occurs in this level. Mostly used communication methods is Industrial Ethernet.

The local control is done by Control level and the field devices are connected to field level PLCs. For both control level and field level, Field bus, ASI, Device Net communication methods are used

There are some main networks for industrial communication.They are,

• Insdustrial Ethernet

• Sercos111

• CANopen

Industrial Ethernet

Industrial Ethernet is the term used to refer to industrial communication protocols using Ethernet standard physical layers such as: 

 EtherNet/IP 

 Modbus TCP 

 TCP and UDP 

On an Ethernet Industrial network, it is possible to connect: 

Industrial products (industrial communication protocols) such as controllers, variable speed drives, robots, etc. 

• The EtherNet/IP protocol 

EtherNet/IP is an industrial communication protocol based on CIP (Common Industrial Protocol), owned and managed by the ODVA, an independent standards organization (www.odva.org). 

EtherNet/IP is the result of implementing CIP protocol on standard Ethernet. EtherNet/IP operates on the same device and the same infrastructure as Modbus TCP, and both protocols can be activated simultaneously on the network at any time. 

EtherNet/IP is a robust protocol for use with sophisticated devices such as cameras, robots, etc. 

• The Modbus TCP protocol 

Modbus has been the industry communication standard since 1979. During the internet revolution, Modbus was combined with Ethernet to form Modbus TCP, a completely open Ethernet protocol. 

The Modbus application layer is simple and universally familiar with its 9 million installed connections. 

Thousands of manufacturers have already implemented this protocol. Many have already developed a Modbus TCP connection and numerous products are currently available. 

The simplicity of Modbus TCP enables any fieldbus device, such as an I/O module, to communicate on Ethernet without the need for a powerful microprocessor or a lot of internal memory. 

The application protocol is identical on Modbus serial link and Modbus TCP; messages can be routed from one network to the other without converting the protocol. 

Since Modbus operates on the TCP/IP higher layer, users benefit from IP routing, thus enabling devices located anywhere in the world to communicate without worrying about the distance between them. Modbus and Modbus TCP are recognized as a fieldbus by the international standard IEC/EN 61158. They also comply with the “national Chinese standard” managed by ITEI. 

The Modbus TCP protocol uses a client/server architecture for data exchanges.

Sercos111

Fully Ethernet based communication solution enabling communication for Motion-centric machines with both drives and field devices. SERCOS III can also streamline the integration of safety automation, and is a true standard. It does not rely upon a specific manufacturer, and it’s one of the most powerful Ethernet-based communication solutions currently on the market.

• Standardized, Ethernet-based fieldbus-communication

With IEC 61491, Sercos (serial real-time communication system) established itself as the worldwide communication standard for automation. Sercos III uses industrial Ethernet for its transmission physics. The bidirectional full-duplex Ethernet communication provides full media redundancy.

• Universal automation bus

The use of standardized profiles for drive technology, I/O, and communication between controllers (C2C) is increasing the range of possible applications for sercos III: For the first time, sercos can be used as a fully integrated Ethernet-based solution for drive and field bus communications, including communications in safety automation. Sercos III also permits parallel transmission of standardized or proprietary IP protocols over a non-real-time channel.

CANopen

Performance and reliability

The CANopen bus is a vendor-independent, multi-master bus which ensures reliable deterministic access to real-time data in control system devices. The CSMA/CA protocol is based on broadcast exchanges, sent cyclically or on an event, which ensures optimum use of the pass band. A message handling channel can also be used to define slave parameters. The bus uses a double-shielded twisted pair, on which 127 devices maximum are connected by simple tap junctions. The variable data rate between 10 Kbps and 1 Mbps depends on the length of the bus (between 30 and 5000 m).

For field level Modbus,Profibus,HART,AS-Interface and Profibus DP interbus field bus protocols are supported.