Added: Einar Pelaez - Date: 07.01.2022 05:25 - Views: 16536 - Clicks: 6231
So what do you think, is this glass half full or half empty? In fact, why not have the PLC tell us the level, the temperature of the water, the flow rate at which it was filled, the available capacity of the glass and the pressure exerted on the bottom? We need a device that can allow the PLC to take information in from its surroundings and process it.
We also need a device that will allow the PLC to affect its surroundings as needed. Discrete als are als that are either on or off, true or false.
Think of a light switch in your house. Because discrete als exist in one of these two states, they are represented with a square wave as seen below. In my startup days, I did a lot of work with a certain parcel service. Photoeyes, which are devices that emit an infrared light beam and can sense when that beam has been broken, are used extensively to detect and track packages through their sorting process.
The application of your control system will determine the types of discrete devices you choose. These devices can be AC or DC and are available in different voltage ranges. Analog als are als that can vary or change. We live in an analog world and our senses are analog receivers. The dimmer switch will vary the resistance in the line, causing the light to dim or brighten as we choose. Newer dimmer switches have advanced to be more efficient but for this example we are going old school. The voltage supplied to the light will not be a constant level but a changing one set between the upper and lower limits.
This is usually represented by a sine wave. Position, level, temperature, pressure, flow and speed are just some of the measurements that analog devices can provide to a control system. The conversion is done using transducers. A transducer will take a physical quantity like pressure and convert it to an electrical al. A Im good and discrete of transducers use the physical quantity to control the resistance in the electrical circuit. As heat increases so does the resistance in the circuit, altering the supplied voltage or current. Same holds true for pressure transducers that use strain gauges.
As pressure is applied to the strain gauge, the resistance in the circuit goes up and the voltage Im good and discrete current level changes. Some flow detectors will use the flow of a fluid to push a fin that is connected to a rotary potentiometer. Faster flow equals more resistance change. One of the coolest transducers that I ran across in my days working in the offshore oil industry was the sand detector.
This transducer was acoustic and was attached to pipes that were drawing oil out from the sea floor. It would actually listen for sand rubbing against the inside of the pipe. The amount of sound was converted into an analog al used to alert personnel if the drilling was drawing out too much sand, which could collapse the well. The electrical als that transducers provide can be voltage or current based. The PLC supplies the voltage or current and the transducer will return a value in its configured range.
That value will be proportional to the amount of pressure, flow, etc. We now arrive at two important parts of this discussion: scaling and resolution. Scaling is when we take the raw voltage or current value returned and translate it into a meaningful measurement. Im good and discrete would mean that 8mA of fluid is flowing at this time, right? Of course not. We know from the manufacturer that the flow meter is calibrated to read a flow of 0 to gpm.
Therefore, we can scale the raw value mA to equal gpm in our system. Normally, this can be done using the programming software, as seen below, and after our value of 8mA is scaled we see that there is currently a flow of 50 gallons per minute. Resolution and accuracy go hand in hand; the more resolution you have, the more accurate your measurement will be. Analog modules are rated with the resolution they provide. So what does it mean to have a mA module with a bit resolution? A bit binary word can have different combinations.
Therefore, our module with a bit resolution can have with a different measurements within the mA range. In other words, the range can be broken into different pieces. The more pieces, the more accurate. How accurate? Well, our total range is 16mA 20 minus 4 and if we divide our total range by pieces, we see that our module can detect each 0.
One last thing, when using analog als it is important to remember that they are highly susceptible to noise. Noise can cause wrong readings and erratic behavior in a control system.
Take a look at the diagram below. My system will open a pressure relief valve on a tank when the raw pressure al from inside the tank reaches 5. With the noise induced on the right, good luck! If you would like more information on automation or the automating process please download our Automating An Industry Guide to Control System Engineering ebook. Using Transducers Position, level, temperature, pressure, flow and speed are just some of the measurements that analog devices can provide to a control system.
Scaling Scaling is when we take the raw voltage or current value returned and translate it into a meaningful measurement. Noise One last thing, when using analog als it is important to remember that they are highly susceptible to noise. Find out why the PLC, used for the automation of industrial electromechanical processes, is so When choosing the type of input or output module for your PLC system, it is very important to Your source for PLC A variety als from analog devices must be transmitted to a PLC for monitoring and control,Im good and discrete
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What’s The Difference Between “Discreet” vs. “Discrete”?