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.

Automation and Control technologies

ü  Automation

• What is Automation? 

• Benefits and basic architecture.   

• Analog control and Digital control   

• Advantages and benefits of digital control.       

• Controllers for digital control 

• Control methods. 

• Sensors and actuators used for automation         

• Programmable logic controllers 

• Internal architecture of a plc.   

•  Ladder programming.    

• Industrial communication. 

ü  Control technologies

• PID control.

• Use Matlab for PID controlling.

                                                    Automation

Automation is make a process in a factory to operate by machines in order to minimize or diminish human involvement in production environment. There are many advantages and benefits of industrial automation.

•  Productivity improvement     

• Accuracy

• Cost saving

• Safety of the employees

                                           Analog control vs. Digital control.

     Digital control is established by re-programmable or hard coded microprocessor which is made by millions of transistors. Analog control circuits are made by resistors, inductors, capacitors and Operational amplifiers. Each method has its cons and pros.

1	Digital control
Advantages		Disadvantages
Flexibility		Easy to break
Speed		Hard to do fast loops
Cost		Complex  programming
Accuracy
Less implementation errors.

2	Analog Control
Advantages		Disadvantages
Robust		Slow development.
Dynamic range		Hard to do complicated control.
Continuous processing		Electro-magnetic and RF interference.
Hard to modify		Hard to modify
		Hard to build in comparative logic
		Hard to build in intelligence
		Hard to do MIMO

Controllers for digital control hardware

• Digital computer

Huge processing power.Can be used for large automation systems.Can implement any industrial communication method/protocol

• Microcontrollers.

Small computer in single IC.IC contains processor, Memory and input/output interfaces.Used in automatically controlled products, embedded systems, toys, automobile engine control systems, industrial automation etc.

• Digital signal processors

Optimized for fast operational needs of digital signal processing (audio/video data).

• Programming logic devices

Used to build reconfigurable logic circuits.Combination of a logic device and a memory device.

• Programming logic controllers

Digital computer used for industrial applications like automation.Designed for industrial conditions, e.g., temp ranges, immunity toElectrical noise, resistance to vibrations, etc.

Starting Methods of Three Phase Induction Motors-Soft starter,VFD

Soft Starter

Principle of operation 

These power electronics starters or soft starters are made using thyristors; an electronic semiconductor which will conduct a continuous current when an input signal pulse is applied to its gate, and they help to regulate the energy consumed by the induction motor.  When the thyristor is turned on initially and if there is an input sinusoidal signal, a voltage will be applied in to the motor where as if the thyristor is turned off the voltage goes to the motor will become zero. The unit is solid state, using a microprocessor to control inverse parallel (back to back) pairs of SCR's.An SCR/thyristor is a semiconductor device that latches when triggered. Once triggered it allows current to flow in one direction only and turns off at zero current. The firing angle of the SCR's are controlled to achieve the desired acceleration of the motor. 
The soft starter incorporates a closed loop torque control system to provide better control over starting & stopping 3 phase induction motors. Conventional voltage ramp control systems typically produce low torque at low speeds. When starting & stopping variable torque loads (e.g Pumps) this can results in a very rapid acceleration or deceleration with a nonlinear change in motor speed 

Circuit/Connection diagrams 

Characteristic curves during starting

Commercial products 

Applications

• Pump applications where pressure surges should be avoided.
•  Fans and other systems with belt drives to avoid belt slipping.
• Conveyor belt systems can be started very smoothly.

  

AC variable speed drive 

Principle of operation 

A variable speed drive is a piece of equipment that regulates the speed and rotational force, or torque output, of an electric motor. Control can mean either manually adjustable - by means of a potentiometer or linear hall effect device, (which is more resistant to dust and grease) or it can also be automatically controlled for example by using a rotational detector such as a Gray code optical encoder. 

In general, a VFD(Variable Frequency Drive) in its most basic configuration controls the speed of an induction or synchronous motor by adjusting the frequency of the power supplied to the motor. 

When changing VFD frequency in standard low-performance variable-torque applications using Volt-per-Hertz (V/Hz) control, the AC motor's voltage-to-frequency ratio can be maintained constant, and its power can be varied, between the minimum and maximum operating frequencies up to a base frequency. Constant voltage operation above base frequency, and therefore with reduced V/Hz ratio, provides reduced torque and constant power capability. 

Regenerative AC drives are a type of AC drive which have the capacity to recover the braking energy of a load moving faster than the motor speed (an overhauling load) and return it to the power system. 

The VFD article provides additional information on electronic speed controls used with various types of AC motors. 

Circuit/Connection diagrams

 Characteristic curves during starting

Commercial products

Applications

• Sewage lift station 

Sewage usually flows through sewer pipes under the force of gravity to a wet well location. From there it is pumped up to a treatment process. When fixed speed pumps are used, the pumps are set to start when the level of the liquid in the wet well reaches some high point and stop when the level has been reduced to a low point. Cycling the pumps on and off results in frequent high surges of electric current to start the motors that results in electromagnetic and thermal stresses in the motors and power control equipment, the pumps and pipes are subjected to mechanical and hydraulic stresses, and the sewage treatment process is forced to accommodate surges in the flow of sewage through the process. When adjustable speed drives are used, the pumps operate continuously at a speed that increases as the wet well level increases. This matches the outflow to the average inflow and provides a much smoother operation of the process 

• Cooling tower fans 
• Dusting fan in steel and iron industry 
• Compressors in oil and gas industry 

Starting Methods of 3 phase IM- Star-Delta,Auto transformer

 Star Delta Starter 

Principle of operation

 Star Delta Starters can be seen in 3 states. 
                 1) Star connected state.
                 2) Open state.
                 3) Delta connected state.
Main and star contactors are normally closed and complete circuit at the starting time. Voltage is reduced to 1/√3 of the Line Voltage through every winding in the star connected state. When motor reaches a better rotational speed, like 90% of full r.p.m ,the starters timer connects the Delta connector after disconnecting Star connector. During the state transition the circuit becomes open and at that time the motor doesn’t stay in any of those two states. This is mentioned as open transition switching because of that. Winding voltage becomes equal to the line voltage in Delta connected state.
At the beginning if the application is light loaded, this starting method is used If the motor is heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position. 

Circuit/Connection diagrams 

 Switching "Start push button" completes Star circuit by switching ON main contactor "K1M" and Star contactor "K2M" through a timer. Now motor keeps on running in Star configuration.   Once circuit is completed and main contactor "K1M" is activated, one of its "NO" contact point, connected parallel to "Start push button", becomes close and keeps circuit complete by providing a continuous holding even after Push button is released
  Motor remains in Star state configuration till "timer" remains close. After a time delay is reached, timer becomes open, thereby de-activating Star contactor. Immediately after that another switch becomes close, thereby activating Delta contactor "K3" to complete Delta circuit. 

Characteristic curves during starting 

                                    Torque Vs Speed 

                                       Current Vs Speed

Commercial products 

The MCW 37kW Open Chassis Star-Delta Starter is manufactured from three 415V coil contactors, a timer, and a thermal overload relay (Trip Class 10). 

Applications 

• Compressors need low torque to accelerate to the full speed 
• Pumps for hydraulic elevators. 
•  Motor applications which need low acceleration

Auto transformer starters 

Principle of operation

The principle of auto-transformer leads to the concept of Auto transformer starter which consists of additional flexibility compared to the other starters. Motor directs line voltage using some relay arrangement method in DOL starter. Starting voltage is a fixed reduced value in star delta starter. The specialty of the auto transformer is the starting voltage is reduced and also can vary in a range which is done by studs connected to different tapings. The versatility to the starter is provided by this method which enables connecting motors with different ratings by controlling the output voltage. This is a result of the adjustable starting voltage at the beginning. 

 

There are some main components of auto transformer starters. 

1) Three phase auto-transformers.

2) Contactor with movable rod. 

3) Handle arrangement. 

 

The line voltage at the primary windings of auto transformers will be connected by moving the operating handle because of the arrangement of movable rod. In addition the secondary windings of the transformer and the motor terminals are connected using that way. Due to the requirements of starting torque and starting current the tapings can be seen varied because the square of the transformation ratio decides this ratio. 

 

Movable handle H controls assembly of all switches and the double throwing switch is connected to every transformer each. All the switches connected with respective contactors and the motor begin running with a low voltage when the handle is moved.  

 

For the maximum performance the motor should fed with full load current after the safe starting of the starter. The handle H should be put into the run position immediately when motor accelerates and get a speed of 80% of its rated value. Full rated voltage is applied at motor terminals and auto-transformer is detached from the circuitry in that way.  

 Circuit/Connection diagrams

Characteristic curves during starting 

Commercial products

Applications 

• For Induction motors which need a smoother startup. 
• Power transmission and Distribution systems
• Railways
• Audio systems