The Human Response to a Digital Control System

The Human Response to a Digital Control System

BrianNetz, SeniorElectrical Engineer.

Abstract – A brief description of how human interaction is a necessity for the continual operation of digital control systems is presented. This document contains a detailed description of some of the components involved with the digital control system. A strong emphasis is placed on the integration of all of these components.

I. INTRODUCTION

Genesis 1:28a “God blessed them and said to them, ‘Be fruitful and increase in number; fill the earth and subdue it’.”

This passage can easily be taken out of context without giving serious thought to the implications that this means. The responsibility that this passage bears is mind-numbing! Engineers are more directly affected by this command than many other professions and as Christians we are called to learn, understand, and improve on the things that God has given us. This document provides a brief summary of how digital controls systems are incorporated into human control systems to improve response times and to subdue the desired process.

II. THE DIGITAL CONTROL SYSTEM

Digital control systems feature a number of different electrical components that are integrated into one or multiple systems. Digital control systems are used in any number of different applications. They are used to control robotic arms, airplane flaps, robotic cranes, and conveyor systems to name a few. Digital control systems are increasing in complexity and size. They are very versatile ranging from watches to gigantic robots. The main design of a digital design system is to have several different components feeding digital or analog signals to a processor, the processor in turn takes the signals, uses some sort of logic on the input signals, and then the processor turns on or off certain outputs determined by the logic in the processor. This method of always turning on or off different components without feed back and without gain control is known as the “bang bang” approach to control systems.

Since digital control systems are so varied in nature and design, a specific design will be used for this paper. Allen-Bradley is one of the leading manufactures of control system components and they produce several families of PLC’s that will be discussed later in this paper.

A. PLC

The PLC is known as the programmable logic controller and is the “brain” of the entire control system. PLC’s are relatively new to the market but they have already made a substantial impact. They were first made in the late 60’s to eliminate the massive amounts of relays used to make entire systems operate. They were also more reliable as it was a software based thing, not a mechanical device which would wear out over time. Naturally the first ones were not all that fast, and certainly did not have many features. They could also not store very many lines of code. The past 40 years has brought many changes to the PLC. They are now faster, smaller, have more memory, and can now be linked together. The latter has only recently been an achievement as early on, they did not have any standards and hardly anything had the same network protocol [1].

PLC’s work by programming code, ladder logic, visual blocks, or many numerous other types of languagesinto the PLC so that it only triggers the right output under the conditions you set for it. The ladder logic is very comparable to IF…THEN statements. See Fig. 5 for an example of the PLC code. This eliminates the need for a tremendous amount of relays, which also cuts down on cost, labor for wiring, and the chance that it gets wired incorrectly. It can easily be seen how much the PLC has added to the manufacturing market.

The logic that is in most controllers today uses all of the common ones: AND, NOT, OR, NOR, NAND, etc. They also have many advanced logic statements that vary the voltage of the output based proportionally on a separate input that the controller is getting fed. The “digital relays” ,pictured in Fig.3, that are used in the controllers are called “examine if closed” and “examine if open.” These two pieces are commonly used for the logic and eliminate the mess of having hundreds to thousands of relays and the mess of wiring them all up together.

As seen in Fig. 6, the output is highlighted. When using this specific code, if the relay is allowing current through it, the piece of code is highlighted. On the other hand, if no current is being allowed the relay, there is no color around that specific piece of code. This makes it incredibly easy for troubleshooting for engineers. Often digital control systems have PC’s hooked up to the PLC that enables them to “watch” what the system is doing. This enables shorter down times on manufacturing lines, as well as an easier, user-friendly interface.

Allen Bradley created many different families of their manufacturing processors. Their first large sales processor was the PLC-5. The following is the brief history of the Allen Bradley’s PLC-5.

In the early 1970’s Allen Bradley revolutionized the automotive industry. The PLC-5 made its debut in 1985 which lead to the conformation of control engineering around the world. In 1990 the PLC-5’s input/output cards were improved and became standardized. The communication modes for the PLC-5 were greatly improved in 1993 by adding Ethernet capabilities and then broadening it further in 1995. By 2000, Allen Bradley had sold over 400,000 PLC-5’s and 9million input/output cards. The latest change to the PLC-5 was made in 2001 by adding internet capabilities [2]. The PLC-5 was replaced by several of the newer families such as the PLC-500 series and the PLC-5000 series talked about below.

ControlLogix processor modules are available in a range of memory. Logix5555 processors (1756-L55Mxx) have 750K, 1.5M, 3.5M, or 7.5M bytes of user memory. You can replace the memory sub-module on a Logix5555 processor to change the memory size.

The ControlLogix556x processor (1756-L61, L62, -L63) has 2, 4, or 8M bytes of fixed memory plus a built-in socket for the addition of CompactFlash nonvolatile memory. This high-performance processor has an advanced CPU with high clock speed and built-in floating-point math coprocessor for faster execution of discrete logic as well as motion control, function block diagram, and real data type math. This improved performance means you can run more motion axes at lower coarse update rates. You can also run more demanding logic in the same processor with motion.
The ControlLogix processor can perform motion planning functions and generate motion commands that are sent to a motion control module. Motion control modules are available with analog or digital SERCOS interfaces. You choose one of the various motion control modules to interface with servo drives or hydraulic servo valves. High-level command execution is performed on the processor [3].

B. Input/Output Cards

Input/Output cards, commonly referred to as I/O cards, are the veins and arteries of the digital control system. These cards are an easy way to deal with all of the inputs and outputs that the PLC requires. The cards can hold up to sixteen different inputs or outputs, depending on the card, and have to be configured correctly in order to talk to the PLC. Once correctly configured, the cards are connected via wires to the desired places. This makes the PLC a very versatile piece of technology as the number of inputs and outputs can vary for many different applications.

C. RELAYS

To understand control systems is to understand relays (see Fig. 1). Relays have been around for a long time and at one point in time they were the primary way for logic control. Robot systems used to be made solely of relays. In order for there to be enough relays to control an advanced robotic system, there would usually have to be several panels filled with relays that were five to six times larger than the robot itself. This meant that control systems would take up a lot of plant floor and were not very practical for small manufacturing firms. All relays must be hardwired which took a lot of time for the electricians or engineers to hook up (see Fig.2). Also this made troubleshooting nearly impossible. If there were eight wires to hook up on each relay, and there are hundreds to thousands of relays, then there are millions of different possibilities to make a mistake. Relays were also expensive, which meant that buying several hundred to several thousand was not a light investment. Just like all mechanical devices, relays break down over time, which made upkeep expensive on systems that utilized many relays.

Fig.1: A relay

Fig.2: Wiring diagram for a relay

Although relays have their downside, they are necessary for the modern control system. Relays come in many different sizes and shapes, and have many different characteristics; however, many of the relays can be modeled using programming software that is put on the PLC (see Fig.3). This cuts down on the initial cost, as well as the cost for replacing them, as there are fewer relays needed.

Fig.3: Example PLC code for relays

I) Relay’s operation:

Relays act a lot like switches. They can either turn on or off a circuit determined by how that relay is hooked up. A relay consists of a coil that switches the current state of other circuits when current goes through the coil. There are two types of states that the circuits connected to the relay can be in; normally open and normally closed. Normally open refers to the state that when the coil is closed by having current go through it, the circuit that is connected to the relay also closes. This can be thought of as a normal light switch; when the light switch is turned on, the circuit is closed and the light goes on. Normally closed state is the compliment of normally open, and it works exactly opposite of normally open. When the coil of the relay is closed, then the normally closed circuit opens. This is like a not gate. When the coil circuit is closed then the normally closed circuit is open, turning it off.

This principle of having one circuit turn on and off other circuits is the underlying notion of the PLC. Instead of using actual relays, the PLC uses virtual relays and any other logic function to turn on and off the desired outputs. This saves the time and money of wiring up many physical relays. Physical relays are still used mainly for turning on circuits that use different voltages than the output voltage that the output cards use for the PLC.

E. Sensors

Sensors are some of the major inputs to the PLC. They are the eyes and the ears of the digital control system. They come in many different kinds, from photoeyes to lasers that calculate distance. The sensors can send many different types of inputs to the PLC as well such as: numbers, fault messages, distances, or simply on or off signals.

I) Photoeye:

Photoeyes are a common, easy way to solve detection problems. They are not as precise as some means of measuring, but they are cheaper and effective for general use applications. The photoeye has three main parts to it, the emitter, collector, and relay. The emitter produces a beam of light which is then reflected off of something and interpreted by the photo-collector. Depending on the type of photoeye, the relay is either tripped when the collector does not see the beam or when it sees the light. When the relay is tripped, the photoeye sends a signal to be interpreted by the PLC. There are two main kinds of photo eyes as described below, but the principle remains the same. The photo detects whether there is something right in front of it.

a: Reflective photoeye:

Reflective photoeyes work by emitting a beam that is reflected back to it. The choice of reflective material greatly influences the distance the photoeye will work. There are three different types of reflective material to use which consists of: a mirror, reflector, or anything that really does not reflect enough. In Fig. 4 the three different types of reflective material are shown with the general results produced by using each type of material. The mirror can reflect a greater distance, but since it only reflects one small beam back, it is not practical as it would be too easy to miss the reflection. The non-reflective material is also a bad decision as the beam will not make it back to the photoeye. A reflector of some sort would be the best decision as it emits a nearly as strong as the mirror signal back to the photoeye, but disperses the signal at various angles to give a better signal back to photoeye. This is the most practical method as most of the environments that the reflector will be in will be experiencing many different vibrations and collisions in a manufacturing setting. When the signal is bounced back and the photoeye recognizes it, the relay inside is tripped. This means that the photoeye is sending a signal to the PLC that is saying that the photoeye is clear and nothing is blocking its field of vision. The general range of a reflective photoeye is in the magnitude of six to eight feet, depending on how good of sensor and reflector is being used.

Fig.4: Different reflective qualities

b: Proximity photoeye:

The proximity photoeye works a lot like a reflective photoeye except it does not require a reflective surface to register if there is something in front of it. The photoeye emits its beam and if there is a change in intensity it figures there is something in front of it. The range of the proximity photoeye is a lot shorter than the reflective photoeye, but they work out really well in situations where mounting reflectors is nearly impossible.

II) Light curtain:

Light curtains act a lot like photoeyes (see Fig. 5). An easy way to think about light curtains is to envision an array of photoeyes very close together in a straight line. Light curtains act the same way as photoeyes as they emit a small beam of light and register the reflection back to it. If any one the little beams of light is broken, the curtain is tripped and it sends a signal to the PLC. Light curtains are often used as safety instruments as they are placed in front of an area that should not be entered during normal operations of the machine.

Fig.5: Graphical representation of the operation of a light curtain

III) Proximity Sensor:

Some proximity sensors actually use ultrasonic waves to register if anything is in front of the sensor. Generally proximity sensors have a very short range, magnitude of several millimeters. These are generally used for more precise shorter detection. For example, a photoeye might be used to see if there is a fridge sitting on a bed of conveyors, while a proximity sensor might be used to see if there is a part in the welding grips, ready to be welded.

F. Dislay

A great tool for digital control systems is their ability to create unique and easy-to-understand displays. Two of most common software packages for monitoring a digital control system are RSView and PanelView. These two programs hook directly to the PLC and can output anything you want it to. Although lots of time is often required to get everything communicating properly and the designs made for the screens, it can show a very accurate portrayal of the system as well as have capabilities for human interfacing.

A distinct advantage of the digital control system is its ability to output to a screen or a series of screens. This often ups the initial cost of buying the displays, but looking further into it, it saves a lot of time for the mechanics. If the system faults out, display screens can (if they are programmed correctly) show the mechanic what is wrong with the system, saving time and labor cost for the mechanic to investigate the problem.

G. An example, tying it altogether

The following section gives examples of code that Allen-Bradley uses for their ladder logic, some block diagrams, and simulink models to give a more in-depth look at how the PLC interprets signals and functions as a whole.

The control system used in this example consists of a laser distance input (a distance to an object is calculated by using a laser that is reflected back to the receiver to calculate the distance) and an output to turn a motor on. The goal of this design is when the distance to a certain object is less than five feet, a different motor will turn on. The ladder logic code is show in Fig. 6. A Simulink block diagram model is shown in Fig. 7 of the exact same control scheme.

Fig. 6: Ladder logic code for example.

Fig. 7: Simulink block diagram.

In Fig. 7 a ramp input is used to simulate the input distance of the laser. The ramp input starts at ten and linearly decreases to zero at a rate of 1 foot per second. The IF statement is just like the one seen in the ladder logic and operates the same way. For the Simulink model ties the output to ground if the input signal is greater than five and ties the output to high if the signal goes below five. This is how PLC’s work. It is a “bang bang” approach; the output is either tied high or tied low depending on the logic of the controller.