Comparision of different Touchscreen Technologies used in Electronic Devices

With the growing popularity of HMI products I keep hearing about the various technologies implemented in touch screens. How many different types of touch screens are there? What are the differences?

The first ever touchscreen was developed by E.A Johnson at the Royal Radar Establishment, Malvern, UK in the late 1960s. Evidently, the first touchscreen was a capacitive type; the one widely used in smart phones nowadays. In 1971, a milestone to touchscreen technology was developed by Doctor Sam Hurst, an instructor at the University of Kentucky Research Foundation. It was a touch sensor named ‘Elograph’. Later in 1974, Hurst in association with his company Elographics came up with the first real touchscreen featuring a transparent surface. In 1977, Elographics developed and patented a resistive touchscreen technology, one of the most popular touchscreen technologies in use today.

Touch Screens have become very commonplace in our daily lives: cell phones, ATM’s, kiosks, ticket vending machines and more all use touch panels to enable the user to interact with a computer or device without the use of a keyboard or mouse. But did you know there are several uniquely different types of Touch Screens? The five most common types are: 5-Wire Resistive, Surface Capacitive, Projected Capacitive, SAW (Surface Acoustic Wave), and Infrared.

A touch screen has the ability to detect a touch within the given display area. It is made up of 3 basic elements, a sensor, a controller and a software driver. All the variants of touch screen technology carry their own distinctive characteristics, with individual benefits and limitations.

Resistive Touch

5-Wire Resistive Touch is the most widely used touch technology today. A resistive touch screen monitor is composed of a glass panel and a film screen, each covered with a thin metallic layer, separated by a narrow gap. When a user touches the screen, the two metallic layers make contact, resulting in electrical flow. The point of contact is detected by this change in voltage. Resistive touch screen can be divided into 4, 5,6, 7 or 8-wired models, which differentiate between the coordinates of touch.

As one of the most commonly used, resistive touch screen relies on a touch overlay, constructed by a flexible top layer and rigid bottom layer, divided by insulating spacer dots. The inside surface is coated with a transparent material (ITO) that makes electrical contact when pressure is applied. These voltages are then converted to X and Y coordinates, which are sent to the controller.

Resistive touch screen panels are generally more affordable but offer only 75% clarity and the layer can be damaged by sharp objects

Advantages:

•  Can be activated with virtually any object (finger, stylus, gloved hand, pen, etc.)

•  Has tactile feel

•  Lowest cost touch technology

•  Low power consumption

•  Resistant to surface contaminants and liquids (dust, oil, grease, moisture)

Disadvantages:

•  Lower image clarity compared to other touch technologies

•  Outer polyester film is vulnerable to damage from scratching, poking and sharp objects

Surface Capacitive

Surface Capacitive are the second most popular type of touch screens on the market. In a surface capacitive touch screen monitor, a transparent electrode layer is placed on top of a glass panel, and covered by a protective cover. When an exposed finger touches the monitor screen, it reacts to the static electrical capacity of the human body; some of the electrical charge transfers from the screen to the user. This decrease in capacitance is detected by sensors located at the four corners of the screen, allowing the controller to determine the touch point. Capacitive touch screens can only be activated by the touch of human skin or a stylus holding an electrical charge.

Advantages:

•  Better image clarity than Resistive Touch

•  Durable screen

•  Excellent resistance to surface contaminants and liquids (dust, oil, grease, water droplets)

•  High scratch resistance

Disadvantages:

•  Requires bare finger or capacitive stylus for activation

•  Sensitivity to EMI/RFI 

Projected Capacitive

Projected Capacitive is similar to Surface Capacitive, but it offers two primary advantages: in addition to a bare finger, it can also be activated with surgical gloves or thin cotton gloves; and it enables multi-touch activation (simultaneous input from two fingers). 

A projected capacitive is composed of a sheet of glass with embedded transparent electrode films and an IC chip, which creates a three dimensional electrostatic field. When a finger comes into contact with the screen, the ratios of the electrical currents change and the computer is able to detect the touch points.

Advantages:

•  Excellent image clarity

•  More resistant to scratching than Surface Capacitive

•  Resistant to surface contaminants and liquids (dust, oil, grease, moisture)

•  Multi-touch (two-touch)

Disadvantages:

•  Sensitive to EMI/RFI

•  Must be activated via exposed finger, or thin surgical or cotton gloves

SAW Touch

SAW (Surface Acoustic Wave) touch screen monitors utilize a series of piezoelectric transducers and receivers along the sides of the monitor’s glass plate to create an invisible grid of ultrasonic waves on the surface. When the panel is touched, a portion of the wave is absorbed. This allows the receiving transducer to locate the touch point and send this data to the computer. SAW monitors can be activated by a finger, gloved hand, or soft-tip stylus. SAW monitors offer easy use and high visibility.

Advantages:

•  Excellent image clarity

•  Even better scratch resistance than capacitive

•  High “touch-life”

Disadvantages:

•  Will not activate with hard items (pen, credit card, or fingernail)

•  Water droplets may cause false-triggering

•  Solid contaminants on the screen can create non-touch areas until they are removed

Infrared (IR) Touch

Infrared touch screen monitors do not overlay the display with an additional screen or screen sandwich. Instead, a frame surrounding the display consists of LEDs on one side and phototransistor detectors on the other. The phototransistors detect an absence of light and relay a signal that determines the coordinates. The touch is identified and located at the point of interruption of the LED beams.

Infrared monitors use IR emitters and receivers to create an invisible grid of light beams across the screen. This ensures the best possible image quality. When an object interrupts the invisible infrared light beam, the sensors are able to locate the touch point.

Advantages:

•  Highest image clarity and light transmission of all touch technologies

•  Unlimited “touch-life”

•  Impervious to surface scratches

Disadvantages:

•  Accidental activation may occur because the infrared beams are actually above the glass surface

•  Dust, oil, or grease buildup on screen or frame could impede light beam causing malfunction

•  Sensitive to water, snow, rain

•  May be sensitive to ambient light interference

•  Higher cost

source: www.TRU-VuMonitors.com


Below is the comparision of 3 touch screen technologies -Resisitive, Capacitive, Infrafred in reference to mobile:

2020: Future automation

Engineering future: Ambition to change the world is essential as automation spreads into areas beyond industrial applications. See three key technology advances expected by 2020 and a sampling of those advancements today, including mobile robotics.

Control Engineering China (CEC) has witnessed strong growth in the industrial control and automation business in China, and automation will continue to expand beyond the industry in accelerated fashion as we approach 2020. Internet of things (IoT), system integration, and engineering services will accompany a broadening of automation into other markets.

Many enterprises, both local and overseas, have gained business success and rapid growth in the Chinese market. Most importantly, industrial enterprises improved product quality and production efficiency through successfully deploying advanced automation in the production process, generating enormous commercial value. Increasing efficiency is solidifying China's status as a large-scale factory for the world. With fierce market competition, the automation market in China has entered the adjusting phase where growth is slowing. Reliable automation products with long product life cycles are not short-term consumables anymore. Will another golden decade be cultivated in the Chinese automation market after this period of adjustment? Or will development level off? Different people have different answers to these questions.

Trends of 2020

Looking six years into the future for automation engineers, predicting the trends in 2020 will enlighten our next steps.

1. The Internet of things (IoT) now in full swing will enter a high-growth path by 2020. In a time when people and things are all connected by network, automation engineers and end users will care most about the stability and reliability of the industrial network. The intellectualization, collection, and detection of data will mostly be resolved. The harder part is the troubleshooting and reliability of the industrial network. If situations arise in the network, whether in the business network or in the network controlling production, the whole operation of the enterprise will confront difficulties and hinder decision making in real time because control engineers rely on the industrial network for assessment and indicators. The primary concern for engineers will be the stability and reliability of the networked automation.

2. Future automation engineers will no longer be limited to the field of production. Many automation projects, for example, will be implemented within the so-called Smart City. In transportation, buildings, and health care, future automation projects will have closer proximity to people's everyday lives. The engineers' expertise in automation will be applied in many nontraditional fields. These innovations will drastically change people's lives, adding convenience. The automation system will become an important foundation for the future construction and operation of intellectualized and automated societies.

3. In new markets where changes are created continuously by new applications, the automated system and system integration will become a necessary part of the solution. The innovation and development of products and engineering services will become primary work for automation engineers. In some new applications, artificial intelligence will become the spotlight that reflects the core value of automation.

See the future today

The year 2020 is not far away, and the next rapid developing cycle of automation applications might arrive sooner than expected.

As a U.S. business with a global Internet search engine, Google possesses the most abundant advertising business in the world and continues to advance in automation-related technologies. Google Glass, launched in 2012, is undoubtedly a great potential platform for mobile monitoring displays. This year, 2014, the driverless vehicle project announced by Google started with Sebastian Thrun, the director of the artificial intelligence laboratory at Stanford University. In 2005, he led his student group to participate in a successful driverless vehicle project launched by the U.S. Army in the desert. More recently, Google's driverless vehicle received a driver's license in Nevada, and attended the famous NASCAR competition, among the most elite of human-engineering sports.

In addition, the Space Aircraft No. 2 of Virgin Atlantic Airways successfully completed its second trial flight in 2013, and the company expects commercial space aviation as early as in 2014. That means outer space travel and gradual realization of high-speed commercial global flight.

On screen in many science fiction films, the immersion VR platform is a finished product in the innovation lab of Massachusetts Institute of Technology (MIT).

Big data and cloud computing offer the magic power of prediction, making this another future invention with mature mass applications today.

The popularization of each of these landmark applications has created new opportunities for the development of automation. 

Opportunities, automation, integration

At present, China is still in a phase of rapid development, and changes have been taking place everywhere. Some of those changes or variables are cultivated within China and from the global market. Looking to 2020, the automation industry in China has many opportunities. Those involved need to see beyond any challenges and look to future impacts. In this age of rapid technology development with effective integration, Google and Apple have created markets for client applications, realizing the Blue Ocean Strategy with near-miraculous value. Maybe the next great technology opportunity will come from China.

By 2020, however, rapid acceleration will bring such opportunities frequently. Sufficient engineering talent in China will release their huge energy to create marvelous value. In the mean time, we still need the determination to improve ourselves and the ambition to change the world. 

Mobile robotics

On July 16, 2014, Google appointed Alan Mulally, former CEO of Ford Motor Co., as a member of its board of directors, anticipating that he can speed development of Google's self-driving car business with his experience in the automobile industry. In addition to Google, many traditional automobile manufacturers, including Ford, have started investing in research and development for technologies for self-driving cars.

source: http://www.controleng.com

PLC comparison with Control Systems, Control Devices, Relay control and Computer

PLC compared with other control systems:

PLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations. PLC applications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution, and where the non-recurring engineering charges are spread over thousands or millions of units.

For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities.

A microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies, input/output hardware and necessary testing and certification) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit buses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.

Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls. Single-board computers using semi-customized or fully proprietary hardware may be chosen for very demanding control applications where the high development and maintenance cost can be supported. "Soft PLCs" running on desktop-type computers can interface with industrial I/O hardware while executing programs within a version of commercial operating systems adapted for process control needs.

Programmable controllers are widely used in motion control, positioning control and torque control. Some manufacturers produce motion control units to be integrated with PLC so that G-code (involving a CNC machine) can be used to instruct machine movements PLCs may include logic for single-variable feedback analog control loop, a "proportional, integral, derivative" or "PID controller". A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. As PLCs have become more powerful, the boundary between DCS and PLC applications has become less distinct.

PLCs have similar functionality as Remote Terminal Units. An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly becomes more powerful and cheaper, RTUs, PLCs and DCSs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.

In recent years "Safety" PLCs have started to become popular, either as standalone models (Pilz PNOZ Multi, Sick etc.) or as functionality and safety-rated hardware added to existing controller architectures (Allen Bradley Guard logix, Siemens F-series etc.). These differ from conventional PLC types as being suitable for use in safety-critical applications for which PLCs have traditionally been supplemented with hard-wired safety relays. For example, a Safety PLC might be used to control access to a robot cell with trapped-key access, or perhaps to manage the shutdown response to an emergency stop on a conveyor production line. Such PLCs typically have a restricted regular instruction set augmented with safety-specific instructions designed to interface with emergency stops, light screens and so forth. The flexibility that such systems offer has resulted in rapid growth of demand for these controllers.

Reference:
Above topic is referenced from http://en.wikipedia.org/wiki/Programmable_logic_controller.

PLCS VERSUS RELAY CONTROL:

When deciding whether to use a PLC-based system or a hard-wired relay based system, one must ask several questions. Some of these questions are:

Is there a need for flexibile changes in control logic?  Will there be a need for rapid modification? If yes, a PLC is more suited as it can be re-programmed on the spot. But if changes are not likely to be required, a relay system may be better, subject to other conditions listed below.

Is there a need for high reliability? In general, it is easy to maintain PLCs as they do not have mechanical parts that a electromagnetic relay based system has. So they have longer life. Secondly, PLCs have diagnostics and it is easy to replace the faulty unit as a whole. But a relay system for a complex logic is usually very difficult to troubleshoot. This is because it may not be obvious as to which part is malfunctioning.

Are space requirements important? PLCs save space and this is more true with more complex logic.

Are advanced, intelligent controls required? If so, PLCs are better.

Must similar control logic be used on different machines? If the volume of machines is very large, it may be cheaper to develop dedicated control systems like embedded microcontroller based systems.

Will the additional functionality provided by PLCs like communication, displays, memory be useful?

The merits of PLC systems make them especially suitable for applications in which the above requirements are particularly important. If the system was implemented using electromechanical relays (standard and timing), it would have made the control panel a maze of large bundles of wires and interconnections! If the system requirements need flexibility or future growth, a programmable controller's advantages outweighs any initial cost advantage of a relay control system. Even in a case where no flexibility or future expansion is required, a large system can benefit tremendously from the troubleshooting and maintenance aids provided by a PLC. The extremely short cycle (scan) time of a PLC allows the productivity of machines that were previously under electromechanical control to increase considerably. Also, although relay control may cost less initially, this advantage is lost if production downtime due to failures is high.

PLCs VERSUS COMPUTERS:

The architecture of a PLC’s CPU is basically the same as that of a general purpose computer; however, some important characteristics set them apart. First, unlike computers, PLCs are specifically designed to survive the harsh conditions of the industrial environment. A well-designed PLC can be placed in an area with substantial amounts of electrical noise, electromagnetic interference, mechanical vibration, higher temperatures and noncondensing humidity.

A second distinction of PLCs is that their hardware and software are designed for easy use by plant electricians and technicians. The hardware interfaces for connecting field devices are actually part of the PLC itself and are easily connected. The modular and self-diagnosing interface circuits are able to pinpoint malfunctions and, moreover, are easily removed and replaced. Also, the software programming uses conventional relay ladder symbols, or other easily learned languages, which are familiar to plant personnel.

A PLC does not have a boot time, like a PC. It turns on and is ready for action in a few seconds after applying power. But a PC will take a lot more seconds to turn on. Also, a simple PC has to be shutdown properly, which is not requried for a PLC.

Whereas computers are complex computing machines capable of executing several programs or tasks simultaneously and in any order, the standard PLC executes a single program in an orderly, sequential fashion from first to last instruction. PLCs as a system continue to become more intelligent. Complex PLC systems now provide multiprocessor and multitasking capabilities, where one PLC may control  several programs in a single CPU enclosure with several processors. Latest PLCs now are becomming smaller, faster, offer more features, support USB, Ethernet etc.

Comparison with other control devices:

The main difference from other computers is that PLCs are armored for severe condition (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.

The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.

Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.

PLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations in ladder logic (or function chart) notation. PLC applications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a “generic” solution, and where the non-recurring engineering charges are spread over thousands of sales.

For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities.

A microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies and input/output hardware) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.

Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls.

PLCs may include logic for single-variable feedback analog control loop, a “proportional, integral, derivative” or “PID controller.” A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut.

Features of PLC including the ladder example

Features:

The main difference from other computers is that PLCs are armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.

How Does A PLC Operate?

There are four basic steps in the operation of all PLCs; Input Scan, Program Scan, Output Scan, and Housekeeping. These steps continually take place in a repeating loop.

Four Steps In The PLC Operations 1.) Input Scan Detects the state of all input devices that are connected to the PLC 2.) Program Scan Executes the user created program logic 3.) Output Scan Energizes or de-energize all output devices that are connected to the PLC. 4.) Housekeeping This step includes communications with programming terminals, internal diagnostics, etc...

These steps are continually processed in a loop.

Scan time:

A PLC program is generally executed repeatedly as long as the controlled system is running. The status of physical input points is copied to an area of memory accessible to the processor, sometimes called the "I/O Image Table". The program is then run from its first instruction rung down to the last rung. It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs. This scan time may be a few milliseconds for a small program or on a fast processor, but older PLCs running very large programs could take much longer (say, up to 100 ms) to execute the program. If the scan time was too long, the response of the PLC to process conditions would be too slow to be useful.

As PLCs became more advanced, methods were developed to change the sequence of ladder execution, and subroutines were implemented. This simplified programming and could also be used to save scan time for high-speed processes; for example, parts of the program used only for setting up the machine could be segregated from those parts required to operate at higher speed.

Special-purpose I/O modules, such as timer modules or counter modules, could be used where the scan time of the processor was too long to reliably pick up, for example, counting pulses from a shaft encoder. The relatively slow PLC could still interpret the counted values to control a machine, but the accumulation of pulses was done by a dedicated module that was unaffected by the speed of the program execution.

System scale:

A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O.

Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules are customized for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.

User interface:

PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A human-machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interface (GUIs). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the PLC connected via a communication interface.

Communications:

PLCs have built in communications ports, usually 9-pin RS-232, but optionally EIA-485 or Ethernet. Modbus, BACnet or DF1 is usually included as one of the communications protocols. Other options include various fieldbuses such as DeviceNet or Profibus. Other communications protocols that may be used are listed in the List of automation protocols.

Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser.

PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.

Programming:

PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Often, a single PLC can be programmed to replace thousands of relays.

Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. Initially most PLCs utilized Ladder Logic Diagram Programming, a model which emulated electromechanical control panel devices (such as the contact and coils of relays) which PLCs replaced. This model remains common today.

IEC 61131-3 currently defines five programming languages for programmable control systems: function block diagram (FBD), ladder diagram (LD), structured text (ST; similar to the Pascal programming language), instruction list (IL; similar to assembly language) and sequential function chart (SFC). These techniques emphasize logical organization of operations.

While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.

Digital and Control Signals:

Digital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below 2VDC representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O.

Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. As PLCs typically use 16-bit signed binary processors, the integer values are limited between -32,768 and +32,767. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 0 - 10 V input or 4-20 mA would be converted into an integer value of 0 - 32767.

Current inputs are less sensitive to electrical noise (i.e. from welders or electric motor starts) than voltage inputs.

Example:

As an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank.

Using only digital signals, the PLC has two digital inputs from float switches (Low Level and High Level). When the water level is above the switch it closes a contact and passes a signal to an input. The PLC uses a digital output to open and close the inlet valve into the tank.

When the water level drops enough so that the Low Level float switch is off (down), the PLC will open the valve to let more water in. Once the water level rises enough so that the High Level switch is on (up), the PLC will shut the inlet to stop the water from overflowing. This rung is an example of seal-in (latching) logic. The output is sealed in until some condition breaks the circuit.

|                                                                                  |

|   Low Level      High Level                 Fill Valve    |

|------[/]------|------[/]----------------------(OUT)--------|

|               |                                                                  |

|               |                                                                  |

|               |                                                                  |

|   Fill Valve  |                                                            |

|------[ ]------|                                                              |

|                                                                                   |

|                                                                                   |

An analog system might use a water pressure sensor or a load cell, and an adjustable (throttled) control (e.g. by a valve) of the fill of the tank.

In this system, to avoid 'flutter' adjustments that can wear out the valve, many PLCs incorporate "hysteresis" which essentially creates a "deadband" of activity. A technician adjusts this dead band so the valve moves only for a significant change in rate. This will in turn minimize the motion of the valve, and reduce its wear.

A real system might combine both approaches, using float switches and simple valves to prevent spills, and a rate sensor and rate valve to optimize refill rates and prevent water hammer. Backup and maintenance methods can make a real system very complicated.