摘要
It is incredible how many programmable logic controls (PLCs) around us make our modern life possible and pleasant. Machines in our homes heat and cool our air and water, as well as preserve and cook our food. This tutorial explains the importance of PLCs, and describes how to choose component parts using the parametric tools on the Maxim's website.
A similar version of this article was published February 29, 2012 in John Day's Automotive Electronic News.
Introduction
Automation by closed-loop signal-chain control is everywhere. It makes our homes more pleasant. It gives automobiles the ever-widening range of functions that goes beyond the freedom to travel. It is, in fact, quite astounding to look around and realize that so many of the today's products are manufactured with the help a closed-loop signal chain. It is also important to note that in industrial plants and factories, a more complex form of a signal chain is called a programmable logic controller (PLC). In this tutorial, we will see that signal-chain and PLC applications are limited only by our imaginations.
Signal Chains All Around You
Signal chains surround us. They make our modern life possible and pleasant. In fact, an application with a signal chain appears anytime one needs a control loop to monitor, manage, handle, regulate, limit, or organize something.
Machines using signal chains heat and cool air and water in our homes. They chill and cook our food. In industry almost everything that we buy is manufactured utilizing signal chains. Think of your automobile. (Yes, most of us jump into our cars without thinking. We drive our automobiles to work and some people drive using the "bang-bang" approach. This is not the bang-bang servo that we discuss in the Appendix, but actually hitting things.) Automotive antilock brakes, cruise controls, automatic transmissions, and traction controls are examples of signal-chain uses.
A Basic Signal Chain Controls with Feedback
How simple can a signal chain or process control be? Consider a common household oven and an electronic control unit (ECU) in an automobile.
The oven's components are enclosed inside one container, so no long-distance communication is necessary. When the user sets the thermostat to the desired temperature, the oven maintains the internal temperature at the set point (Figure 1).
The smell of cookies in the oven brings back pleasant memories.
Figure 1. A household oven is a simple example of a signal chain or process feedback control.
When the thermostat setting senses that the oven temperature is low, the switch is closed, thus completing the circuit to open the gas valve to the main burner. Once the thermostat detects that the oven has reached the set point, the switch opens, the gas valve closes, and the main burner shuts off. The cycle repeats as needed. The pilot light provides a fail-safe function, while also providing an ignition source for the main burner. If the pilot light were to go out, no voltage would be created by the thermocouple so the main valve would not open.
Modern high-end cars can have up to 80 ECUs. They control engines, antilock brakes, cruise controls, automatic transmissions, and traction. Most ECUs are examples of signal chains because they sense one or more physical parameters, apply a logic or intelligence, and produce an action to benefit the consumer. (See the block diagram of signal-chain feedback in Figure 2). Table 1 outlines the signal-chain functions for various automotive applications.
| Table 1. Signal-Chain Applications in an Automobile | ||||
| Application | Parameters Sensed | Logic or Intelligence | Actions | Benefits |
| Engine control | Temperature, air mass; fuel volume; engine speed and rotation angle; throttle position; engine load | Optimizes multiple sensor inputs for conditions and maximum benefits | Controls air/fuel ratio, ignition timing, idle speed, variable-valve timing | Better fuel economy; reduced engine emissions; longer engine life; best power when needed |
| Antilock brakes | Vehicle speed; wheel speed; brake pressure | Compares wheel speeds during breaking to identify skidding | Controls valves and pumps | Increases driver control; decreases stopping distance under most adverse conditions |
| Cruise control | Vehicle speed; engine speed; brake switch | Sets and maintains vehicle speed through changing conditions | Adjusts throttle position | Better fuel economy; driver comfort |
| Automatic transmissions | Throttle position; engine load; kick-down switch; vehicle speed, wheel speed; torque convertor slip | Optimizes multiple sensor inputs for conditions | Controls shifting; fast under power to minimize band and clutch wear; slow for passenger comfort under lower power loading | Better fuel economy; increased transmission life; better vehicle handling; faster shift speed |
| Traction control | Throttle position; engine load; vehicle speed; wheel speed | Compares throttle vehicle and wheel for varying road-surface conditions | Reduces engine power by removing fuel or spark from cylinder(s) or changing throttle position; applies brakes on one or more wheels | Reduces traction loss on acceleration, enhancing driver control |
Signal chains are also used for other automotive safety and convenience systems: infotainment; parking assist; airbags; seat belts; door locks; electric windows; running, courtesy, head, and tail lights; power steering; heating; air conditioning; seat control; and telephone...to name a few.
The Difference Between a Signal Chain and a PLC
A signal chain system that goes beyond what is needed for such a small, simple system like a household oven is called a PLC. Recalling that "signal chains are all around you," think next of an industrial factory. What controls and configurations are necessary in a factory? In a fully automated bakery, for example, many subsystems are needed such as weigh scales, valves, flow gauges, mixers, yeast-rising warming chambers, ovens, conveyer belts, fans, and packaging equipment. To be fully automated, the bakery needs a process-control system to manage and coordinate all of the time-critical events among these subsystems. To ensure successful coordination and operation for all these operations, each of these subsystems would include one or more PLCs. Even more complex communications are required when the controllers and the controlled elements are separated by a significant distance. In a complex control environment like a factory with operations spread among several buildings or sites, a PLC spends significant time communicating signals and process events to components throughout the system.
This leads us to the most important difference between a PLC and a signal chain: a PLC makes process change easy. We can illustrate this with a short digression into the history of automation in an automotive factory. An automotive company's strategy to manage change resulted in a PLC.
The Invention of a PLC
Factory automation took hold in the automotive industry when Henry Ford¹ expanded mass-production techniques early in the Twentieth Century. He used fixed assembly stations with the cars moving between positions. The employees learned just a few assembly tasks and performed those tasks for days on end. After many years the changeover for a new car model became very costly and time consuming. The production process was controlled by thousands of hard-wired relays, switches, cam and drum timers, and dedicated closed-loop controllers. To retool for the next model required electricians to mechanically rewire all of the thousands of relays. And then the troubleshooting began to ensure that the safety interlocks, control, and sequencing were all correct.
It was this need to manage the seemingly constant change that resulted in the invention of the PLC. In 1968 Dick Morely and his company were designing a new invention, a programmable controller. Meanwhile General Motors®, (GM®) the automobile company, "wanted a solid-state controller as an electronic replacement for hard-wired relay systems."² GM asked Morley for a quotation to solve their issue. Mr. Morely responded. He is now regarded as the father of the PLC and holds a patent on it.³
GM first used a PLC to assemble automobile automatic transmissions. An assembly plant is made of hundreds of machines that need to be coordinated to function smoothly. The PLC allowed the production line to be repurposed easily. Now a reprogrammed basic machine could be reused to produce a new part.
We can illustrate the pivotal importance of this evolution by focusing on one machine, a numerically controlled (NC) milling machine, and by comparing the old control system with a PLC. First, a human loaded a bare piece of metal on the cutting table and locked it down. Several switches verified that the metal was in the proper position. The operator then indicated that his body was out of danger by pushing two widely spaced buttons, one with each hand. A mechanical guard extended to protect the operator from flying metal shavings. Finally, multiple cutters sculpted the metal in a precise sequence. When the part was complete the mill retracted the tools, stopped all movements, and opened the guard so the human could remove the part and replace it with a new metal blank. Before the PLC, each of these small steps in the operation was hard wired with relays, timers, limit and position switches. It worked reliably...until one needed to make even a small change. The worst possible situation was when a new step needed to be added in the middle of the sequence. Someone might need to unwire all the steps after the new step, add the new step (a hardware relay or timer), and rewire everything that followed.
Enter the PLC. With a PLC that same milling machine becomes a general tool that is controlled and quickly changed a software or logic change. Changing a tool and giving it new instructions now allowed the basic mill to make many different parts. By extending this to the many machines in a factory, an automotive assembly line or other factory quickly adapted to change.
Today you find PLCs used in a modern factories, including an automotive assembly line. Similarly, a chemical, food, or cosmetic company may have to mix many different formulas to make their products. Without the PLC and its easy-to-reconfigure logic, manufacturing changes would still be cumbersome and costly. Many of the items that are common in our lives would be unaffordable.
We use PLCs in our daily lives more than each of us realizes. For other family-oriented examples, see the Appendix.
Factory Automation, Control, and Monitoring
There are areas in a factory that are too dangerous for humans. Fortunately, we have the brains and ability to use machines, and we let a PLC (Figure 2) control those machines.
Figure 2. Diagram of a simple, common signal-chain and PLC loop used in many product disciplines.
Table 2 summarizes how PLC and signal-chain basics begin once we sense something, usually a physical parameter. Then PLC and signal-chain applications become so numerous and commonplace that we take many of their functions for granted.
| Table 2. Measured Parameters that Provide PLC Inputs | ||
| Dimensions | Pitch | Position |
| Intensity | Energy | Pressure |
| Impedance | Temperature | Humidity |
| Density | Speed | Frequency |
| Viscosity | Time of flight | Phase |
| Velocity | Distance | Time |
| Acceleration | Pressure | Salinity |
| Water purity | Torque | Volume |
| Weight | State of charge | Gasses |
| Mass | Conductivity | Ph |
| Resistance | Dissolved oxygen | Voltage |
| Capacitance | Ion concentration | Current |
| Inductance | Chemicals | Level |
| Rotation | Charge (electrons) | Color |
On the output side of a signal chain or PLC, we monitor a system or we operate or move something. Table 3 shows things monitored or controlled by a PLC.
| Table 3. Things Monitored/Controlled by a PLC Output | ||
| Valve | Pressure | Switch |
| Motor | Humidity | Solenoid |
| Pressure | Force feedback | Lights |
| Velocity | Room entry | Weight |
| Flow | Sequence | Speed |
| Volume | Authorization | Meters |
| Torque | Attenuation | Display |
| Frequency | Equalization | Calibration |
| Voltage | Communication | Time |
| Current | Gain offset | Tool |
| Distance | Flux density | Pitch |
| Position | Temperature | Filter |
| Power | Galvanometers | Acceleration |
| Brightness | Air-fuel ration | Contrast |
Maxim's portfolio is second to none, especially for portable and green energy devices where low-power consumption is critical. Small size is also necessary in such battery-operated devices. Find more information and recommended parts using Maxim's PLC solutions. With new parts appearing on the website daily, the parametric search tool helps engineers select exactly the right part based on electrical criteria.
For example, on the Data Converters Product Page you see a selection of resolutions. Choosing a 12-bit DAC causes the parameter table to be displayed for over 250 parts with 20 parameters (10 less-used parameters are hidden on the left). To use the tables, first click on the "Hidden Columns;" select ICC (mA); click "Hidden Columns;" select ICC (mA); click "Smallest Available Package (mm²)." Now move the slider under ICC (mA) to < 1mA and move the slider under Smallest Available Package (mm²). This shortens the list to over 70 parts. Continue selecting the number of channels, power-on reset (POR) state, output type, and interface to reduce the selection to a manageable numbers of parts and data sheets.
There is another example of the easy-to-use web functions for parts selection. Return to the home page (click the Maxim logo at the upper left of most pages). Mouse over "Power Management" in the left column and select "Voltage References." Now you can select a voltage, or "low power" or "small package" to see featured products.
Need more help? We have field support with Maxim and distribution sales and field application engineers worldwide.
Conclusion
Control-loop signal chains or PLCs, in relatively simple or complex applications, are found all around us. They heat and cool, and keep a temperature constant in any structure, regardless of size. Signal chains monitor and control antilock brakes, cruise controls, automatic transmissions, and traction controls in automobiles. Many household appliances today contain a signal chain. We spoke about an oven, but the list is seemingly endless: the microwave; washers and dryers; and even the lawn sprinkler that can sense ground moisture and regulate water use. Finally, it is important to remember that all these goods are produced in a factory where a PLC monitors and controls most every electronic system that operates or moves.
References
- For more information on Henry Ford and factory automation of automobiles, you can start here (http://en.wikipedia.org/wiki/Assembly_line).
- Dunn, Allison, "The father of invention: Dick Morley looks back on the 40th anniversary of the PLC," Manufacturing Automation Magazine, September 12, 2008, www.automationmag.com/features/the-father-of-invention-dick-morley-looks-back-on-the-40th-anniversary-of-the-plc.html.
- Dick Morley is known as the "father" of the PLC. His US Patent 3,761,893 is the basis of many PLCs today. http://patft.uspto.gov/netahtml/PTO/srchnum.htm.
Appendix: More PLCs in the Home
In a home, my home, we use a PLC in the crudest possible way. We have a "bang-bang servo," which is basically an old-fashioned human intervention for our heating, ventilating, and air-conditioning (HVAC) system. Bang-bang servos work because the system has a bandwidth so incredibly low that things change over minutes and hours. The thermal inertia of the walls, floor, and ceiling in our homes is very high, so a simple on/off thermostat controls heating and the air conditioning. Meanwhile, ventilation uses the most expensive and complex servo loop possible: a whole house fan pulls in air through open doors and windows and then exhausts it through the attic. The servo is very complex because it operates on human intervention. During the day we choose to use the fan instead of air conditioning to save energy. A timer turns the fan off at which time we have to shut the doors and windows.
Operation of the heating and air-conditioning control system is, theoretically, simpler than the bang-bang method above, but humans can complicate anything. The system is controlled by a bimetallic thermostat; the heater runs until the set point is exceeded. The heater turns off until the bimetallic strip cools and then the heater goes back on and repeats the cycle. The opposite happens for cooling. Depending on the weather, the heat loss or gain in the house changes the duty cycle of the on- and off-times. It is simple and reliable.
So much for theory of operation...humans have a propensity to complicate even the simplest systems with unexpected and often comical consequences. We replaced the old natural gas-fired forced-air heater with a modern high-efficiency furnace. This did two great things: it got me a lot of points with my wife because the new furnace fits entirely in the attic crawl space; and we have the prospect of saving money over time. The old furnace used a metal pipe to exhaust the very hot combustion products. The new one is a condensing furnace with the exhaust through a plastic pipe, and the air is just warm to the touch. At the same time we installed a more efficient air conditioner and a new thermostat. The new thermostat works well, but it is our human sensibilities that are skewed. First, it is digital, has a microprocessor and a real-time clock (RTC). It has many modes, more than we ever use. Now I have to learn how to program it. (My wife does not program it because she put it on my task list.) Second, placebos have powerful effects on humans. The old thermostat had hysteresis. It might have been as large as three or four degrees but we did not know and did not care. Now we both peek at the display as we walk past and get a silly, knowing smile.
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