plc and scada interface using DDE.

Industrial control systems are always very complex and cumbersome to handle. For electronics engineers, it is becoming very essential to understand the computer control system using a programmable logic controller (PLC) and supervisory control and data acquisition (SCADA) system.

Here is a very efficient and expandable robust control system solution for industrial purposes. For the proposed system, we have used AB Allen Bradley MicroLogix 1400 PLC 1766, Wonderware InTouch 2012R2 SCADA system and KEPServer version 5 software for communicating between the PLC and SCADA. Wonderware InTouch SCADA is becoming increasingly popular due to its powerful user-friendly graphics as shown in Fig. 1.

Fig. 1: A typical Wonderware InTouch graphical user interface

Fig. 2: MicroLogix 1000 PLC (Model 1761-L10BWA features 120V/240V AC power, six 24V DC digital inputs and four relay outputs)

1. The RSLogix family of IEC-1131-compliant Ladder logic programming packages help you maximise performance, save project development time and improve productivity. This family of products has been developed to operate on Microsoft Windows operating systems. Supporting Allen Bradley SLC 500 and MicroLogix families of processors, RSLogix 500 was the first PLC programming software to offer unbeatable productivity with an industry-leading user interface. PLC is programmed using Ladder programming in RSLogix software. Different inputs and outputs have different addresses. Mostly the field devices, which are to be controlled, are connected to the PLC via digital input, analogue input, digital output and RS232, among others. The address of each device, module and register is specific to each PLC manufacturer, including Allen Bradley, Siemens, Schneider, Mitsubishi, Hitachi and Delta.

Fig. 5: Schematic diagram for PLC connections

2. The PLC is connected to the PC via the Ethernet. Since both PLC and SCADA are from different manufacturers, we used an intermediate OPC link (communicator) between these. This job is done by
KEPServer, which is installed on the PC.

3. The PLC IP address is configured using software tool BOOTPServer.

4. When the main SCADA is running, applications like KEPServer should be running in the background and PLC should be in online mode.

5. Using SCADA, a graphical user interface (GUI) is built to monitor and control various parameters like current/voltage and to switch on/off various devices.

Fig. 6: Interfacing the PLC via the Ethernet to a PC to make it available to software for configuration and programming

Fig. 7: Setting BOOTP server properties

Fig. 10: SCL instruction set

For a beginner, it is better to start with the MicroLogix family of PLCs from Allen Bradley. MicroLogix 1000 controller offers control capabilities in an affordable, compact package. MicroLogix 1200 provides features and options to handle an extensive range of applications. The expandable MicroLogix 1500 controller helps you achieve high-level control in a variety of applications. MicroLogix 1100 and MicroLogix 1400 controllers increase application coverage with enhanced network communications at affordable prices. RSLogix 500 programming software and RSLogix Micro programming software provide an instruction set that is common to MicroLogix and SLC 500 controller families.

MicroLogix 1000 PLC system. MicroLogix 1000 PLC (Fig. 2) is available in 10-point, 16-point or 32-point digital input/output (I/O) versions. Analogue versions available with 20 digital I/O points, four analogue inputs (two voltage and two current) and one analogue output (configurable for either voltage or current) provides a compact form factor, with footprints as small as 120mm x 80mm x 40mm (4.72 x 3.15 x 1.57 inches).

It offers fast processing with typical throughput time of 1.5ms for a 500-instructions program and has a pre-configured 1kB program and data memory to ease configuration. It includes built-in EEPROM memory; hence, there is no need for battery back-up or a separate memory module.

The device offers peer-to-peer messaging (up to 32 controllers on a DH-485 network) through a 1761-NET-AIC communication interface. Communication takes place via DeviceNet and EtherNet/IP through 1761-NET-DNI and 1761-NET-ENI communication interfaces.

It also includes a built-in high-speed counter (only on controllers with 24V DC inputs) and lets you customise input response times and noise rejection using adjustable DC input filters.

The device supports simple connectivity through an RS232 communication channel to a PC for program upload, download and monitoring.

Fig. 12: A typical address for AB PLC

MicroLogix 1400 PLC system. The 1766 MicroLogix 1400 PLC system (Fig. 3) is built upon critical MicroLogix 1100 features, including EtherNet/IP, online editing and a built-in LCD panel. These controllers feature a higher I/O count, faster high-speed counters, pulse train output, enhanced network capabilities and a backlight on the LCD panel. Controllers without embedded analogue I/O points provide 32 digital I/O points, while analogue versions offer 32 digital I/O points and six analogue I/O points. You can expand all versions with up to seven 1762 expansion I/O modules.

The Ethernet port provides Web server capability, email capability and protocol support for DNP3 protocol support. The built-in LCD with a backlight lets you view the controller and I/O status. It also provides a simple interface for messages, bit/integer monitoring and manipulation.

Application capabilities can be expanded through support for up to seven 1762 MicroLogix Expansion I/O modules with 256 discrete I/Os, up to six embedded 100kHz high-speed counters (only on controllers with DC inputs), two serial ports with DF1, DH-485, Modbus RTU, DNP3 and ASCII protocol support.

There are 10kB words in the user program memory with 10kB words in the user data memory, and up to 128kB for data logging and 64kB for recipe.

PLC connections
One can buy a compact AB Allen Bradley MicroLogix 1400 PLC 1766, with eight DIO and two built-in serial ports (model 1766) for initial development. We used MicroLogix 1400 controllers, which are suitable for use in an industrial environment. Specifically, this equipment is intended for use in clean, dry environments.

Typical PLC connections for automation of an experimental facility are shown in Fig. 5. Allen Bradley Micro Logix 1400 has various digital input and digital output pins operated by +24V DC. An analogue input module is added separately. Another high-current 1000A power supply device is connected via RS232 protocol, where only three lines, namely, TX, RX and GND, are used for communication via a PLC serial port. Finally, AB PLC is connected to the PC via an Ethernet port.

RS232. RS232 is a serial protocol that converts parallel data to serial bits (pulses) and sends these across three wires, that is, TX, RX and GND. In most cases, the number of wires required is just three, but in special circumstances it may go up to nine wires, say, an RS232 modem.

Allen Bradley 1400 series has two serial ports. The first one is through a PPI cable with a 9-pin D connector at the other end. This needs a NULL modem connector to connect to any PC having a serial port or USB serial adaptor. NULL modem connections are shown in Fig. 4.

Interfacing the PLC to a computer. MAX1400 has three ports. First is COM1 (round connector); second is COM2 (9-pin D connector), which is for RS485/RS232 communications; and the third is Ethernet for Ethernet/IP driver based communications. Therefore it is pretty easy for anyone to configure Ethernet as download port and COM1 as serial port for RS232 communications.

Allen Bradley PLC is connected to the computer via the Ethernet. We need to interface the programmable controller in order to configure and program (Fig. 6).

Adding drivers. Adding a driver is required for RSLOGIX classic software. To do so, you need to click on the middle icon and add RS232 DF1 driver to it.

Software for interfacing. RSLinx Classic is a software tool from Rockwell Automation Networks and Devices. It is a comprehensive factory communication solution, providing Allen Bradley PLC access to a wide variety of Rockwell Software and Allen Bradley applications, ranging from device programming and configuration applications such as RSLogix.

fig. 13: Setting RS232 channel configuration

Fig. 14: Downloading the Ladder program to the PLC

BOOTP server is a suitable server to establish communication between PC and Rockwell PLC.

Communicating to hyper terminal. How does one know that the PLC is transmitting characters out of its serial port? You can view the pulses or protocol produced using a hyper terminal window.

For that, you need to set the hyper terminal settings to 19200, N, 8, 1, which is same as the settings of PLC serial DF1 driver.

Double-click MAC and enter the IP (Fig. 7).

Ladder programming. Once the AB PLC is successfully interfaced to the PC, we need to properly program it using RSLOGIX provided by Rockwell Automation. For programming the PLC, we use Ladder programming. Typical PLC programs use blocks like timers on and off, counters, digital I/Os, RS232 communication blocks and others.

Ladder program is similar to the wiring diagram of the circuit. We redraw the wiring diagram using two vertical lines to represent input power rails and stringing the rest of the circuit between these. Sub-routines are implemented as another ladder.

A typical example of a block used for RS232 programming is AWA (Fig. 9).

ASCII write append (AWA). The AWA instruction sends a specified number of characters (for example, 50) of the source tag (ST14:0) to a PLC serial port 2 and appends either one or two pre-defined characters. It is used for RS232 communication.

Similarly, an SCL is an instruction used to read and scale an analogue input to a corresponding physical value like temperature or pressure.

In the above example, the address for the analogue input is I: 1.0 and the converted raw data is divided by 10,000 and added to offset to implement a linear transfer function like

y = mx + c

A number of free Ladder programming tutorials are available on the Internet. Inside the PLC, we have various registers like output, input, status and binary. Each bit in these registers specifies a function. Addresses of the devices are built on the basis of names, PLC slot numbers, words and bit numbers. Address representations differ from manufacturer to manufacturer.

The table above shows some of the bit addresses of the PLC and how these are related to field devices.

A typical address of an AB PLC for output coil is built as in Fig. 12.

Once you write your Ladder code for the PLC, download the code into the PLC. Let the PLC be in Run mode after that.

Fig. 13 shows how an RS232 port can be configured for handshaking a power supply device, which has settings 9600, N, 8, 1.

Wonderware InTouch SCADA
Wonderware’s InTouch HMI software for visualisation and industrial process control offers outstanding ease of use and simple-to-configure graphics. For having an interactive GUI to control outputs of PLC, RS232 devices via PLC, we use Wonderware InTouch SCADA. A simple GUI can be made in Wonderware by following simple steps:
1. Open Wonderware Development tool by double-clicking on Desktop icon.
2. Start a new project.
3. From the palette on the right side, choose the appropriate graphical object to be inserted.
4. Click on the screen to insert the object.
5. Double-click on the object to add a suitable tag name and type of object. Type defines usage; it may be analogue or discrete.
6. Double-click on the tag to assign properties to the object. Different operations like visibility or blink can be controlled on the basis of scripting.
7. If the tag is I/O type (meant to read or control a device), specify the address of the device.
8. Address of the device can be obtained from registers of the PLC, which can be viewed in RSLogix.
9. In Modify Access Name window, keep the access name as KEPServerEX_SL. KEPServer acts as a communicator between RSLogix and Wonderware InTouch.
10. In this fashion, develop the entire GUI with specific tag names, addresses of devices and PLC variables used in Ladder logic.

Access name. Each PLC uses a specific protocol to communicate with the I/O server or DA server. InTouch then uses an access name concept to continue this communication for dynamic data exchange (DDE). Typically, because here KEPServer is acting as a communicator between PLC RSLogix and Wonderware InTouch, we specify access name in InTouch as KEPServerEX_SL. Access names are selected in the tag name dictionary for each tag used in the application.

Scripting. In all control systems, it is essential to provide software interlocks to prevent human errors. So the best method would be using Window Scripts in InTouch. Window scripts execute periodically when an InTouch window is opened, or one time when an InTouch window is opened or closed.

Programming here is done using scripts, which is similar to any programming language. A typical scripting example is shown in Fig. 18.

Fig. 16: Wonderware development tool

Fig. 17: Setting tag properties

KEPServer
KEPware’s OPC server KEPServerEX provides an easy and reliable way to connect the Allen Bradley PLC to HMI, SCADA, Historian, MES, ERP and countless custom applications. Here, you can use KEPServer for communicating RSLOGIX to Wonderware InTouch using the Ethernet/IP protocol.

Be careful while specifying tagnames in KEPServer. Tag names and their corresponding addresses should match those mentioned in Wonderware InTouch development.

Running the final application
Once your PLC code is downloaded into the PLC and run from RSLogix, run KEPServer. Open the runtime application of Wonderware InTouch software and open your GUI file. Always make sure that KEPSersver is running in the background while you run Wonderware InTouch application.

Applications
Such a control system can be very useful for industrial automation, in research laboratories and big industries. The graphical interface makes it easy for the operator to operate switching and controlling operations. The PLC provides a very rugged solution to control relays and RS232 devices.

---

Arvind soni is masters in ECE from the BHOPAL RGTU,
and is currently working as engineer-at Arist automation Indore.9111225438

Question : What is a PLC?

Answer: Programmable Logic Controllers are industrially hardened micro computers, designed to replac e electromechanical relays, used to control machines and processes.

Question : What are the main advantages of PLC?

Answer:

1.      Modular design

2.      Increased reliability  

3.      Lower cost

4.      Compact size

5.      Ease of programming

6.      Rugged construction and designed to withstand vibrations, temperature, humidity and noise

7.      Ability to communicate with computer

Question : What are the major areas of application of PLC?

Answer:

1.       Sequence control, timing, counting, and data calculation

2.       Batch or continuous process control

3.       Precise position/motion control

4.       Open loop or feedback control, process data acquisition and display

Question :  What are the advantages of PLC over Relays?

Answer:

1.       They have to be hard-wired to perform a specific function

2.       No hard-wire requirement

3.       When the system requirement change, the relay wiring has to be changed or modified

4.       Can be easily changed or expanded through program

5.       Higher power consumption

6.       Lower power consumption

Question :  Explain the classification of PLC.

Answer:

There are five major classification of PLC based on number of inputs and outputs, cost, functionality and physical size. They are

i.                 Nano : 16 I/O pins

ii.                Micro : 32 I/O pins

iii.              Nano or micro PLC could used on applications such as elevators, car washes or mixing machines.

iv.              Small : 128 I/O pins,2Kbytes memory, capable of simple to advanced level of machine control

v.                Medium: 2048 I/O pins, 32Kbytes memory, used for process control applications

vi.              Large: 8192 I/O pins, 750 Kbytes memory, mostly used to control individual production process or entire plants like paper and pulp, chemical and automotive and power plants.

 Question :  Explain the functions of PLC-input and output modules

Answer:

1. Input interface modules accept signals from the machine or process devices (120Vac) and convert them into signals (5Vdc) that can be used by the controller.

2. Output interface modules convert controller signals (5Vdc) in to external signals (120Vdc) used to control machine or process.

Question :  List out the programming languages in PLC.

Answer:

a. Ladder Logic Diagram

b. Sequential Function Charts

c. Instruction List

d. Function Block

e. Structured Text Diagram

Question: What is Ladder LogicDiagram?

Answer:

Ladder Logic Diagram is a symbolic language used in PLC. This diagram consist of series of symbols interconnected by lines to indicate the flow of current through the various devices. In ladder logic diagram the sides of the ladder forms the power sources and current flows through the various –logic input devices that form the rungs of the ladder.

Question: List the conditions for drawing thw ladder logic.

Answer:

a.      Contacts may be always inserted in the upper left.

b.      Coils must be inserted at the end of the rung.

c.      All contact must run horizontally.

d.      The number of contacts per network are limited

e.      Only one outout may be connected to a group of contacts

f.       Flow must be from left to right.

g.      The contact must be nested.

h.     Contact progression should be straight cross.

Question : Explain On-Delay and Off-Delay Timer instruction in PLC

Answer:

a)      The on delay timer operates such that when the rung containing the timer is true, the timer tiomed out period commences. At the end of the timer time out period an output is made active.

b)     The Off delay timer operation will keep the output energized for  time period after the rung cotaining the timer has gone false.

c)       

Question: What is PLC SCAN ?

Answer:

During each operating cycle the processor reads all the inputs , takes the values and energizes or de-energizes the output according to the user program. This process is known as PLC Scan . A single PLC scan consists of the I/O scan and the program scan.

PAC to a Modern Industrial Application OPC

PAC to a Modern Industrial Application OPC

Look more closely at how a PAC (Programmable Automation Controller) is applied to a modern industrial application using the factory application. The PAC is operating in multiple domains to manage and monitor a production line, a chemical process, a test bench and shipping activities. The PAC must simultaneously manage analog values such as temperatures and pressures, switches, digital on/off states for valves, and indicators; and serial data from inventory tracking and test equipment. At the same time, the PAC is exchanging data with an OLE for Process Control (OPC) server, a Structured Query Language (SQL) database and an operator interface. Handling simultaneously these tasks without need for additional processors, middleware, or gateway is a hallmark of a PAC.

The PAC, office workstation and operator, testing equipment, production line and process actuators and sensors, and barcode reader are connected to a standard 10/100 Mbps Ethernet network installed throughout the facility. In some examples, devices without built in Ethernet connectivity such as temperature sensors are connected to I/O modules on an intermediate Ethernet enabled I/O unit, which is turn communicates with the PAC.

The PAC communicates with remote racks of I/O modules to read or write analog, digital and serial signals using this Ethernet network. The network also links the PAC with as OPC server, a SQL database, and an operator interface. A wireless segment is part of the network, so the PAC can also communicate with the mobile assets like the temporary operator workstations and forklift.

The PAC can control, exchange data and monitor with this wide variety of systems and devices because it uses the same standard network technologies and protocol that they use. This example includes wireless and wired Ethernet networks, IP (Internet protocol), network transport, SQL and OPC. In other control situation, common application level protocols such as Modbus, Simple Network management Protocol (SNMP), Potin to point Protocol (PPP) over a modem could be required. The PAC has the ability to meet these requirements of diverse communication.

S5/S7 OPC Server

The S5/S7 OPC server gives you fast and convenient access to process data in S7-200, S7-311, S7-411 and WinAC controllers, in C7 and M7 units as well as in Pilz, S5, Saia-Burgess or VIPA controllers. The variables are addressed consistently using STEP7 resp. STEP5 semantics. You can use any OPC compliant client application to write and read all input and output data, markers, data blocks, timers and counters in the controllers. Up to 256 controllers can be simultaneously accessed. The program PLC does not need to be modified for communication with the S5/S7 OPC Server.

Access the controller is available via Ethernet of Profibus, USB or via serial communication without or with a modem. CPs from various manufacturers, Netlink PRO and gateways from Softing are supported for Ethernet communication. Appreciation need to be given to the intelligent grouping of write and read requests, the data throughput is optimized so efficiently that Softing’s S5/S7 OPC Server won the best in class award in a practice related performance comparison of seven S7 OPC servers from renowned manufacturers.

The name of OPC space is extremely quick to configure by importing symbolic names from STEP7 projects or Excel files. Existing projects can be exported from software applications like process visualization systems or process control systems to Excel, and are then loaded conveniently into the S5/S7 OPC server.

The OPC server provides a web server for diagnostics and process data visualization. This allows you to display the PLC states and data very quickly on remote or local PCs by simply using any standard web browser. Powerful remote access without DCOM, the Softing OPC Tunnel has already been integrated in the S5/S7 OPC server for safe. Using the OPC Tunnel eliminates the problems associated with DCOM configuration security settings and saves significant time and costs. It only takes few minutes to setup communication for network applications.

OPC to Provide a Standard Infrastructure of Process Control

OPC or OLE for Process Control is an industry standard for system inter-connectivity. OPC uses COM and DCOM technologies Microsoft to allow a data transfer application with one or more computers through the architecture of TCP / IP.

OPC defines a set of common interface, thus the application will retrieve the data with the same format, regardless of the data from the PLC, DCS, applications or other software. OPC is a communication solution that lives taken, installed and running.

OPC goal is to provide a standard infrastructure for data exchange control process. For example a company has a variety of data such as PLC, DCS, databases, RTU, etc. These data are available through a variety of connections, such as, serial, Ethernet or radio transmitter. While the process control applications can use a variety of operating systems such as UNIX, DOS or VMS.

OPC in this case serves to:
1. OPC standardizes process control data communication.
2. OPC standardizes the technology, not a product.
3. OPC provides true interoperability and scalability.
4. OPC can reduce implementation time and costs.

With OPC allows you to visualize, analyze, report through the application of any part with one or more OPC specifications.

By choosing the standard OPC technology, you activate the real interoperability, reduce implementation costs, and make full scalability system for the future.

plc wiring blog

S0 = ‘OFF’ Push button

S1 = ‘ON’ Push button
K1 = Line contactor
K2 = Star contactor
K3 = Delta contactor
K4 = Star delta timer (7PU60 20)
F2 = Overload relay
F1 = Backup fuse
F3 = Control circuit fuse

This is a starting method that reduces the starting current and starting torque. The device normally consists of three contactors, an overload relay and a timer for setting the time in the star-position (starting position). The motor must be delta connected during a normal run, in order to be able to use this starting method. The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start. This starting method only works when the application is light loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position. When starting up pumps and fans for example, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approx. 80-85 % of the motor rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks. In some cases the current peak can reach a value that is even bigger than for a D.O.L start. Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.

Wiring of Star Delta Starter with Timer Control Panel

PLC Program for Star Delta Starter