Key differences between PLC and DCS systems

We have so many people with questions as to what are the similarities and differences if any between PLC and DCS systems. We will try to describe the working of these two systems and in the process help you understand the functions of each system.

Initially DCS was focused only on process control with analog signals that were used as main control system in process industries like Refining, Chemicals, and Petrochemicals etc. while PLC was focused on discrete automation with discrete on-off signals, that were used for example in Factory assembly lines and bottle lines but today DCS supports discrete I/O and some logic functions and PLCs support analog I/O with some control functions. In some instances Both PLC and DCS are used in the same plant i.e. PLCs are used on separate units on a plant floor which are then integrated with main plant-wide DCS for Control and Monitoring.

Let us now look at each system separately to help us understand more how they work.

PLC

Programmable Logic Controllers (PLCs) comes in different sizes which means various I/O and program capacities. Smallest sized PLCs are typically referred to as nano PLCs, micro PLCs and mini PLCs. They have fixed I/O and mainly used in stand-alone applications.

Large PLC support redundancy for CPU, power supply and possibly the control network, but typically not for I/O cards though there are large PLCs that support I/O redundancy by using duplicate I/O-subsystems with separate backplanes where the field instruments are wired in parallel to both I/O subsystems. The control network is typically a standard industrial Ethernet application protocol over Ethernet media and IP. The Field cabling comes directly onto the I/O card.

PLC usually support very fast scan times as required in discrete manufacturing but PID loops add to the CPU load, much more than discrete load thus making the scan time slower.

Loops are not handled individually in a PLC. Addition or change to loop requires a download of the entire program which affects other loops in the CPU as well.

PLCs are built around a given native protocol, this maybe: PROFIBUS, Modbus, DeviceNet etc. The PLC comes with its own native interface cards for native protocol supported by the PLC maker but relies on third-party interface cards for other Fieldbus protocols. The engineering software therefore automatically configures the communication interface card for the native protocol.

You can also read: 

• How Distributed Control Systems work
• The Journey from Relay Control Systems, to PLC Systems, how the Transition was done

Key Points to note on PLC

PLCs were designed to eliminate assembly-line relays during model changeovers. PLC is easier to change than relay panels; this has reduced the installation and operational cost of the control system compared with electromechanical relay systems.

BLOCK DIAGRAM OF A PLC SYSTEM

PLC offers the following advantages:

• Ease of programming and reprogramming in the plant
• Programming language is based on relay wiring symbols familiar to most plant electrical and instrumentation personnel
• High Reliability and minimal maintenance
• Small physical size
• Ability to communicate with computer systems in the plant
• Moderate to low initial investment cost
• Available in modular designs

DCS

Distributed Control System (DCS) supports redundancy for controllers, power supply and control network as well as redundant I/O cards including fieldbus interface cards in the same backplane. The control network supports peer-to-peer communication between controllers. The control network is typically a proprietary application protocol over Ethernet media and IP. The field cabling in DCS lands on a Field Terminal Assembly (FTA) where a special system cable with a connector takes the signals to the I/O card.

Loops in a DCS are executed individually. The scan time in a DCS is set individually for each loop. Most loops run at 1000ms although 250ms is common for pressure and flow loops in refining and petrochemicals and even 100ms is also possible. The scan time is constant, and does not change with task loading. This is important for PID control and time-based functions such as integration/totalizing and lead-lag dynamic compensation.

Loops in a DCS are managed individually. A change and download to one loop doesn’t affect the other loops.

A DCS has an integrated development environment where I/O control strategy and operator graphics are created together and stored in a single database. This means once a tag is created in the DCS it automatically becomes available everywhere in the system with the same human readable tag name for use in basic control, advanced control, graphics, faceplates, trending, alarming, and turning etc.  Without mapping data through registers or other tag names makes it easy to do changes or additions.

The Sensor & Actuator level “H1” Fieldbus network supported by DCS is basically FOUNDATION fieldbus for instrumentation and PROFIBUS-DP for motor controls.

The DCS comes with its own native Fieldbus interface cards. The engineering software therefore automatically configures the communication interface cards for the variables used in the control strategy and graphics.

Key Points to Note on DCS

DCS is miniaturized version of the multitasking, multivariable, multi-loop controller used for process control. It is functionally and geographically processing distributed system. Equipment making up a DCS is separated by function and is installed in two different work areas of a processing installation. Equipment for operator to monitor process condition and to manipulate the set point of the process operation is located in a central control room; from where the operator can view information transmitted from the process area and displayed on a video display unit and can change control condition from a keyboard. DCS systems are suitable for the following processes:

• Where a single centralized system is not adequate i.e. Power, Steel, Pulp & Paper plants, Fertilizer etc.
• Processes of different level of hierarchy
• Processes which can be divided into different and functionally independent sections, based on functional scope and geographical distribution

DCS offers the following Advantages:

• Compact to contain ON/OFF controllers
• Reduced complexity and easy expandability
• High Speed of the control processing
• Control Algorithms changes do not call for hardware changes
• Continuous trend data is available
• User friendly but higher data security
• Plant data are transparent on the network
• Sequential, batching and feedback control are possible

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Differences between PLC and DCS based Systems

PLC	DCS
Redundancy not possible or limited	Redundancy possible at every level
It is used for low loop count	It is used for any loop count
Performance drops with increasing loop count	No change in performance with increasing loop count
Purely free running mode	Highly efficient multitasking mode
Analog processing simulated through digital computer	Analog processing done in real frequency domain function
No interplant connectivity	Fully Functional Inter-plant connectivity
Individual database for every node	System-wide global database
Typical performance: 100 PID loops/sec	Typical Performance: More than 1000 PID loops/sec.

Do you have comments or questions on PLC or DCS? Feel free to post them in the comments section below. 

Components in a Controlled Automation System

Essential components in any controlled automation system include:

• The actuator (which does the work)
• The controller (which ”tells” the actuator to do the work)
• The sensor (which provides the feedback to the controller so that it knows the actuator is doing work)

An example of a simple controlled automation system is shown below:

A controlled system may either be analog controlled system or digital controlled system. Let's consider the following analog controlled system:

The actuator is a hydraulic servovalve and a fluid motor. The servovalve opens proportionally with the voltage it receives from the controller and the fluid motor rotates faster if it receives more hydraulic fluid. There is a speed sensor connected to the motor shaft, which outputs a voltage signal proportional to the shaft speed. The controller is programmed to move the output shaft at a given speed until a load is at given position. When the program requires the move to take place, the controller outputs an approximately correct voltage to the servovalve, then monitors the sensor’s feedback signal. If the speed sensor’s output is different from expected i.e. indicating wrong motor speed, the controller increases or decreases the voltage supplied to the servovalve until the correct feedback is achieved. The motor speed is controlled until the move finishes. As with any other control system, the program may include a function to notify a human operator if speed control isn’t working.

You can also read: 

• Introduction to Industrial Automation
• The Transition from Relays to PLCs

Having looked at analog controlled system above, let's now consider an example of a digital controlled system:

The above figure represents a simple digital controlled system in which the actuator consists of a pneumatic valve and a pneumatic cylinder that must be either fully extended or retracted. The controller is a PLC that has been programmed to extend the cylinder during some more complicated process and to go on to the next step in the process only after the cylinder extends. When it is time to extend the cylinder, the PLC supplies voltage to the valve, which should open to provide air to the cylinder, which then extend. If all goes well, after a short time the PLC will receive a change in voltage level from the limit switch, allowing it to execute the next step in the process. If the voltage from the switch does not change for any reason ( faulty valve or cylinder or switch, break in a wire, obstruction preventing full cylinder extension etc.), the PLC will not execute the next step. The PLC may even be programmed to turn on a “fault” light when such a delay occurs.

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Key Features and Applications of Remote Terminal Units (RTU)

Remote Terminal Units also called Remote Telemetry Units or Remote Telecontrol Units are Microprocessor controlled devices that interfaces in the physical world to either SCADA (Supervisory Control and Data acquisition) system or DCS (Distributed and Control system).  They transmit data to a master system and uses messages from the master supervisory system to control objects connected to the system.

They are designed for use in applications in remote locations unattended. These locations may have limited to no power, hence RTUs are designed to consume low power than DCS and PLC & this enables operation on solar power and batteries.

In application where supervision is done from distant central location, the SCADA software sits in the central office connected over a backhaul network typically using radio communication to the RTUs located far away and in most cases geographically spread out. The communication may be interrupted for long periods of time therefore RTUs have on-board  data storage continuing local data collection for more than a month if backhaul communication is lost as well as “history backfill’’ uploading this data once the connection is established again. Report by exception communication mechanisms are often used to minimize backhaul communication using Wide Area Networks e.g. Mobile, Microwave, Satellite.

Remote Terminal Units (RTU) Configuration

The RTU configuration software is separate from the HMI (Human Machine Interface) software from a third-party manufacturer i.e. two separate databases. RTU is configured first; next the OPC server is configured. For a native OPC server this happens automatically, but for OPC server from a third-party, manual data mapping is required which can be time-consuming and error prone requiring thorough testing. In most cases native OPC server is preferred. To finalize, the HMI database has to be configured for graphics, alarms, and trends etc.

The 4-20 mA AI and AO cards for a RTU optionally support native HART pass through hence separate HART multiplexer (MUX) hardware and associated work is not required. Native HART pass through AI and AO cards are much easier to integrate and should be specified if 4-20 mA is used. Since RTUs are generally used in very slow monitoring applications that don’t require fast control, some applications do not use the real-time analog 4-20 mA but only the digital HART communication multi-drop topology. This means the field instruments draw less than 4 mA instead of up to 20 mA hence further reducing the overall power consumption.

You can also read:

•  Electrical Power Distribution and Automation System
• The Transition from Relays to PLCs explained

Applications of Remote Terminal Units (RTU)

Remote terminal units are commonly used in the following applications:

• Electrical Power Transmission Networks and Associated Equipment.
• Remote Monitoring of Functions and the whole Instrumentation Network in Oil and Gas (offshore platforms, onshore oil wells, Pump Stations on Pipelines)
• Water and Wastewater collection and supply networks including the Pumping stations.

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How Triacs are used in Industrial Control Applications

The Triac is a three-terminal device that is similar to the SCR except that the Triacs can conduct current in both directions. Its primary use is to control power to AC loads such as turning AC motors on and off or varying the power for lighting and heating systems. The Triac is a solid state device that acts like two SCRs that have been connected in parallel with each other (inversely) so that one SCR will conduct the positive half-cycle and the other will conduct the negative half-cycle. Before the triac was designed as a single component, two SCRs were actually used for this purpose.

The figure 1 below shows the symbol for the triac, and its pn Structure. The terminals of the triac are identified as main terminal 1 (MT1), main terminal 2 (MT2), and gate. The multiple pn structure is actually a combination of two four-layer (pnpn) junctions.

Figure 1

As the name suggests, the load current passes through the main terminals, and the gate controls the flow. Figure 2 below shows the equivalent circuit of the triac, which consists of two back-to-back SCRs with a common gate.

Figure 2

Operation of the Triac

The operation of the triac can be explained by the two-SCR model in Figure 2.  From the figure, you can see the SCRs are connected in an inverse parallel configuration. One of the SCRs will conduct positive voltage and the other will conduct negative voltage.

When MT2 is more positive, the current flows through first SCR; when MT1 is more positive the current flows through Second SCR.

Unlike the two SCRs, the Triac is triggered by a single gate. This prevents problems of one SCR not firing at the correct time and overloading the other.

The operating characteristics of the Triac are best explained using the characteristic curve shown in Figure 3:

Figure 3

In the figure above, you can see that the triac can conduct both positive and negative current. The Voltage is shown along the horizontal x-axis, and current is shown along the vertical y-axis. This diagram also shows a second graph with four quadrants. These quadrants are used to explain the operation of the triac as polarity to its MT1 and MT2 and gate changes.

Notice that the right half of the graph (in quadrant 1) looks just like the SCR curve; no current flows until either the break over voltage is reached or the gate is triggered (indicated by dashed line).

This same pattern is repeated in quadrant 3 (for voltage and current of the opposite direction). Also, like the SCR, once the triac is triggered on, it will remain on by itself until the load current drops below the holding current value (IH)

A Single cycle of AC has a positive and a negative half-cycle. The triac requires a trigger pulse at the gate for each half-cycle and works best if the trigger is positive for the positive half-cycle and negative for the negative half-circle (Although in most cases the triac will also trigger if the gate goes negative in the positive half-cycle and if it goes positive in the negative half-cycle.

You can also read:

• Basics of Instrumentation and Control Systems
• How Silicon Controlled Rectifiers (SCRs) are used in Industrial Control

Applications of Triacs

The Triac is required in circuits where AC Voltage and Current need to be controlled like the SCR controls dc current. Another difference between the triac and SCR is that the Triac can be turned on either by a positive or negative gate pulse. The gate pulse need only be momentary and the Triac will remain in conduction until the conditions for commutation are satisfied.

A triac can be used as an-off solid-state switch for AC loads or to regulate power to an AC load, such as dimmer switch.

Triacs are available in various packages, some of which can handle currents up to 50 A (which is considerably less than the SCR).

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How Silicon Controlled Rectifiers (SCRs) are used in Industrial Control

Brief Background on Industrial Electronics

Silicon Controlled Rectifiers (SCRs), Triacs and other high –powered transistors are used in many types of circuits to control large voltages and currents. Many of these use 480 VAC 3 Phase circuits and can control over 50 A. These devices offer control circuits for general purpose power supplies, AC and DC Variable speed motor drives, Servo motor controls, Stepper motor controls, high frequency power supplies, welding power supplies etc. In applications like Fs Circuits, where precise voltage and current control is essential, SCRs and thyristors offer reliable performance under demanding conditions. SCRs, Triacs, and any other solid-state devices used for switching larger voltages and currents on and off are commonly called thyristors. Thyristors control switching in an on-off manner similar to a light switch which is different from a transistor that can vary the amount in its emitter-collector circuit by changing the bias on its base. The amount of current that flows through a thyristor must be controlled by adjusting the point in a sine wave where the device is turned on.

Silicon Controlled Rectifiers (SCRs)

The figure 1 below shows a symbol for the SCR and identifies the anode, cathode, and gate terminals. The cathode is identified by the letter C or K. The diagram also shows several types of SCRs.

Figure 1

How Silicon Controlled Rectifiers Work

The SCR acts like a solid-state switch in that the current will pass through its anode-cathode circuit to a load if a signal is received at its gate. The SCR is different from a traditional switch in that the SCR will change ac voltage to dc voltage (rectify) if ac voltage is used as the power supply. The SCR is also different from a traditional switch in that the amount of time the SCR conducts can be varied so that the amount of current provided to the load will be varied from near zero to maximum of the power supply.

Figure 2

The SCR can vary the amount of current that is allowed to flow to the resistive load by varying the point in the positive half-cycle where the gate signal is applied. If the SCR is turned on immediately, it will conduct full voltage and current for the half-cycle (180°). If the turn-on point is delayed to the 90° point in the half-cycle waveform, the SCR will conduct approximately half of the voltage and current to the load. If the turn-on point is delayed to the 175° point in the half cycle, the SCR will conduct less than 10% of the power supply voltage and current to the load, since the half-cycle will automatically turn off the SCR at the 180° point. This implies that the gate of the SCR can be used to control the amount of voltage and current the SCR will conduct from zero to maximum.

You can also read: How Test Diodes are used to Measure loop Currents 

The Operation of SCR explained by the four-layer (Two-Transistor) Model

The SCR is a four-layer thyristor made of PNPN material; in fact the proper name for the SCR is the reverse blocking triode thyristor.

Figure 3

The Figure 3 above shows the PNPN material split apart as two transistors, a PNP and a NPN. The figure (c) shows the SCR as two transistors.

The anode is at the emitter of the PNP Transistor (T2), and the cathode is at the emitter of the NPN transistor (T1). The gate is connected to the base of the NPN Transistor. Since the anode is the emitter of the PNP, it must have a positive voltage to operate and since the cathode is the emitter of the NPN transistor, it must be negative to operate.

When a positive pulse is applied to the gate, it will cause collector current Ic to flow through the NPN transistor (T1). This current will provide bias voltage to the base of the PNP transistor (T2). When the bias voltage is applied to the base of the PNP transistor, it will begin to conduct Ic which will replace the bias voltage on the base that the gate signal originally supplied. This allows the gate signal to be a pulse which is then removed since the current through SCR anode to cathode will flow and replace the base bias on transistor T1.

Methods of turning on an SCR

Normally the SCR is turned on by a pulse to its gate but we have 3 other methods you can also use to turn it on.

These methods include:

• Exceeding the forward break over voltage
• By Excessive heat that allows leakage current
• Exceeding the dv/dt level (allowable voltage change per time change) across the junction.

Methods of turning off/Commutating SCRs

Once an SCR is turned on, it will continue to conduct until it is turned off (commuted). Commutation will occur in an SCR only if the overall current gain drops below unity (1). This means that the current in the anode-cathode circuit must drop below the minimum (near zero) or a current of reverse polarity must be applied to the anode-cathode. Since the ac sine wave provides both of these conditions near the 180° point in the wave, the main method to commutate an SCR is to use ac voltage as the supply voltage. In an ac circuit, the voltage will drop to zero and across over to the reverse direction at the 180° point during each sine wave. This means that if the supply voltage is 60 Hz, this will happen every 16 msec. Each time the SCR is commutated, it can be triggered at a different point along the firing angle, which will provide the ability of the SCR to control the ac power between 0° to 180°. The main drawback with using ac voltage to commutate the SCR arises when higher-frequency voltages are used as the supply voltage. Note that, the SCR requires approximately 3-4 msec. to turn off; therefore the maximum frequency is dependent on the turn-off time.

Figure 4

Figure 4 (a) A switch is used to commutate the SCR in a dc circuit by interrupting current flow.  This type of circuit is used to provide control in alarms or emergency dc voltage lighting circuits, (b)  A series of RL resonant circuit circuit used to commutate an SCR and (c) A parallel RL resonant circuit used to commutate an SCR. 

SCRs are also used in the inverter section where the dc voltage is turned back into ac voltage. Since the devices must provide both the positive and the negative half-cycles, a diode is connected in inverse parallel to provide the hybrid ac switch. This combination of devices is not frequently used currently as larger Triacs and Power transistors can do better job in this kind of applications.

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Differential Pressure Transmitters (DP)

One of the most common and useful pressure measuring instrument used in most industrial measurement applications is the differential Pressure transmitter. This device senses the difference in pressure between two ports and outputs a signal representing that pressure in relation to a calibrated range.

Differential Pressure transmitters constructed for industrial measurement applications typically consists of a strong (forged metal) body housing the sensing element(s), topped by a compartment housing the mechanical and/or electronic components necessary to translate the sensed pressure to a standard instrumentation signal e.g. 3-15 PSI, 4-20 mA, Digital Fieldbus codes as shown in the below Diagrams:

In the below example of Rosemount differential pressure transmitter, the pressure-sensing element is housed in the bottom half of the device (forged-steel structure) while the electronics are housed in the top half (the coloured, round, cast-aluminium structure)

Every differential Pressure (DP, d/p or ∆P) transmitter has two pressure ports to sense different process fluid pressures. These ports typically have ¼ inch female NPT threads to readily accept connections to the process. One of these ports is labelled “high” and the other is labelled “low’’. This labeling does not necessarily mean that the “high” port must always be at a greater pressure than the “low’’ port. What these labels represent is the effect any increasing fluid Pressure applied to that port will have on the direction of the output signal’s change. Note that, a differential pressure instrument responds only to differential pressure while ignoring the common-mode pressure (gauge pressure common to both ports).

The most common sensing element used by modern DP transmitters is the diaphragm. One side of this diaphragm receives process fluid pressure from the “high’’ port while the other receives process fluid pressure from the “low’’ port. Any difference of pressure between the two ports causes the diaphragm to flex from its normal resting (center) position. This flexing is then translated into an output signal by any number of different technologies depending on the manufacturer and the transmitter model.

Differential Pressure (DP) Transmitter Applications

The combination of two differential pressure ports makes the DP transmitter very versatile as a pressure-measuring device. This one instrument can be used to measure pressure differences, positive (gauge) pressures, negative (vacuum), and even absolute pressures, just by connecting the “high” and “low” sensing ports differently.

In every DP transmitter application, there must be some means of connecting the transmitter’s pressure-sensing ports to the points in a process. Metal or plastic tubes (or pipes) work well for this purpose, and are commonly called impulse lines or gauge lines or sensing lines. Typically these tubes are connected to the transmitter and to the process by means of compression fittings which allow for relatively easy disconnection and re-connection of tubes.

Key applications of DP transmitters include:

Measuring Process Vessel Clogging – We may use the DP transmitter to measure an actual difference pressure across a process vessel such as a filter, a heat exchanger, or a chemical reactor. The diagram below shows the use of a DP transmitter to measure clogging of a water filter:

From the diagram above, you can see the high side of the DP transmitter connects to the upstream side of the filter and the low side of the transmitter to the down side of the filter. This way, increased filter clogging will result in an increased transmitter output. Since the transmitter’s internal pressure-sensing diaphragm only responds to differences in pressure between “high” and “low” ports, the pressure in the filter and pipe relative to the atmosphere is completely irrelevant to the transmitter’s output signal. The filter could be operating at a line pressure of 15 PSI or 15000 PSI – the only variable the DP transmitter measures is the pressure drop across the filter. If the upstream side is 15 PSI and the downstream side is 14 PSI, the differential pressure will be 1 PSI sometimes labelled PSID, where “D” is differential. If the upstream pressure is 15000 PSI and the downstream pressure is 14,999 PSI, the DP transmitter will still see a differential pressure of just 1 PSID.

Measuring positive gauge pressure – DP instruments can also serve as gauge pressure instruments. If we simply connect the “high” side of a DP instrument to a process vessel using an impulse tube, while leaving the “low” side vented to atmosphere, the instrument will interpret any positive pressure in the vessel as a positive difference between the vessel and the atmosphere. 

Most DP instrument manufacturers offer gauge pressure versions of their differential instruments with “high” side port open for connection to an impulse line and the “low’’ side of the sensing element capped off with a special vented flange, effectively performing the same function as in the above figure.

Measuring absolute Pressure – Absolute pressure is defined as the difference between a given fluid pressure and a perfect vacuum. We may build an absolute pressure sensing instrument by taking a DP transmitter and sealing the “low” side of its pressure-sensing element in connection to a vacuum chamber as shown below. This way, any pressure greater than a perfect vacuum will register as a positive difference.

Measuring Vacuum – The same principle of connecting one port of a DP device to a process and venting the other works as well as a means of measuring vacuum (Pressure below that of atmosphere). All we need to do is connect the “low” side to the vacuum process and vent the ‘’high” side to the atmosphere as shown below: 

Any pressure in the process less than atmospheric will register to the DP transmitter as a positive difference (with P-high   greater than P-Low _). Thus the stronger the vacuum in the process vessel, the greater the signal output by the transmitter.

Inferring liquid level – Liquids generate pressure proportional to height (depth) due to their weight. The pressure generated by a vertical column of liquid is proportional to the column height (h), and liquid’s mass density (ρ), and the acceleration of gravity (ɡ):        P=ρɡh

As the liquid in the vessel increases, the amount of hydrostatic pressure applied to the transmitter’s ‘’high’’ port increases in direct proportion. The width of the vessel is irrelevant to the amount of pressure produced only the liquid height (h), density (ρ), and gravity (ɡ) are significant. Thus the transmitter’s increasing signal represents the height of liquid inside the vessel no matter the size or shape of the vessel.

h = P/ρɡ   

You can also read: Advantages and Disadvantages of Pneumatic Instruments

Inferring gas and Liquid Flow – DP transmitters are widely used in measurement of fluid flow. Pressure dropped across a constriction in the pipe varies in relation to flow rate (Q) and fluid density (ρ). So long as fluid density remains fairly constant, we may measure pressure drop across a piping constriction and use that measurement to infer flow rate. The most common form of constriction is the orifice plate. This is a metal plate with a precisely machined hole in the center. As fluid passes this hole, its velocity changes, causing a pressure drop to form.

Since both ports of the transmitter connect to the same process line, static fluid pressure within that line has no effect on the measurement. Only differences of pressure between the upstream and downstream sides of the constriction (orifice plate) cause the transmitter to register flow.

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