PLC Controlled Motor Circuit

Transcription

1 PLC Controlled Motor Circuit William Heater Sinclair Community College Electronics Engineering Technology EET 2278, S16, Spring Term

2 Page 1 Table of Contents Acknowledgements..pg. 2 Abstract.pg. 3 Introduction pg. 4 Principles of Design...pg. 5 Design Details...pg. 6 Construction..pg. 12 Testing...pg. 16 Operation. pg. 17 Conclusions. pg. 19 Recommendations. pg. 20 References.....pg. 21 Addenda pg. 22

3 Page 2 Acknowledgements For the success of this project I would like to thank Warren Kerns and Chad Johnson. These are co workers who assisted in the final layout design idea and many conversations concerning its logical operations. Also to my wife and family who were very patient and understanding about the many hours spent away from them.

4 Page 3 Abstract The PLC (Programmable Logic Controller) Controlled Motor Circuit is designed to be a training tool for anyone interested in industrial PLC use. Motors are used heavily throughout industry and being familiar with some simple concepts of controlling them is important to understand. The trainer utilizes various voltages (120Vac, 5Vdc and 24Vdc), and a PLC to read inputs and send outputs to a number of lamps, push buttons, and switches. All of which are essential to proper control of the motors. The trainer operates three, identical, single phase motors. The ladder logic for the PLC is written as though two of the motors are driving conveyor belts and the third motor is opening and closing a bin gate that the conveyors dump into (there are no actual conveyor belts or bin gates on the trainer, only the motors used as a demonstration of them). The conveyor motors continually run as though they are filling up the imaginary bin. Once the bin is full (as determined by a timing sequence written in the ladder logic), the bin gate motor turns on just long enough to empty the bin. The two conveyor motors also have a safety cover with a switch. If the safety covers are removed, the corresponding motor will stop running and a warning light will illuminate. The trainer motors can be controlled locally by a panel of switches and pushbuttons, or they can be controlled by an HMI (Human Machine Interface) via laptop. If being controlled locally, and the operator fills the imaginary bin to full capacity (again, the timing sequence written in the logic), and the conveyor motor will automatically stop until the operator clears the bin by pressing the dump bin button. Alternatively, an operator can control the process using the HMI. In this case, while the conveyor motors are running, the bin gate will automatically turn on as the imaginary bin reaches full capacity, then turn off when the bin is empty. Automatic mode will continually operate with no input from the operator. The PLC for the trainer contains approximately 60 rungs of ladder logic to make the motors, and lights function as desired. This serves as a good example of how seemingly simple functions such as turning on a motor or blinking a light can require many lines of logic.

5 Page 4 Introduction The PLC controlled motor circuit is not a new concept. It is used throughout industry in a variety of ways for motor control and process monitoring. The idea for this project came about after recognizing a desire to expose fellow students to real life applications for their electronic education. Coupling this idea with my interest in PLCs and ladder logic begat the finished project presented here. The original concept for the project was simple: to program and wire a PLC to start and stop a single phase motor. This simple concept quickly evolved into a more complex idea involving indicator lights, safety switches, and a motor interlock to prevent one motor from running unless a second motor was running. The completed original concept was as follows: Program a PLC that will receive inputs from push buttons to start and stop two motors. Using these inputs, the PLC will output a signal to start/stop the motor. While the motors are running, the PLC should output a signal to illuminate an indicator light, corresponding to the running motor. The PLC will also receive inputs from safety switches, and will not allow the motor to run if the corresponding safety switch input is not present. Lastly, the PLC will not allow motor #2 to start unless motor #1 is already running. As the project progressed, it also evolved. I became aware that the PLC being used was accompanied by a software HMI program which I quickly decided to incorporate. By doing so, the operator could now control the trainer using the push button station or via laptop. I also obtained a third motor and was able to implement it into my programming as a bin gate. Doing this added more complexity to the project and a better example of real world applications. Although, the first design idea went through several changes, the original concept remained the same: a motor circuit controlled by PLC programming.

6 Page 5 Principles of Design There are various electronic components used in the trainer, each requiring its own power. Failure to apply the proper voltages to the proper components can have a devastating effect on the project and will most likely cause irreparable damage to expensive equipment. An example of this would be applying 120Vac to the PLC, which only requires 5Vdc to operate. The motors used are single phase and require 120Vac to function. This voltage can be acquired from any household outlet and is also necessary to power the 5V and 24V adapters. The PLC itself, along with the two output modules require 5Vdc to operate, while the input cards need a 24Vdc signal to recognize the inputs. Applying a 5V signal to the 24V input module will result in the PLC failing to recognize the input signal and cause undesired operation. The second key concept for the project is proper programming of logic. Improper programming will not damage the project, but will cause incorrect operation. Although the trainer is meant to be a learning tool and has plenty of latitude for programming errors, a basic understanding of ladder logic is still necessary. Firstly, the programmer must have a plan for how s/he wants the project to function. This will determine how many input/output bits are necessary. Knowing this is crucial because the PLC is only capable of a fixed number of inputs and outputs, without buying additional equipment. The programmer must be able to recognize that implementing a push button will require the use of 1 of the limited inputs. Accordingly, using a lamp will take away 1 of the available outputs, etc. The PLC used for this project is a Velocio Branch 221. It is capable of 12 digital inputs, 12 digital outputs, and 6 analog inputs (Velocio). Once the plan is established, the programmer is then required to understand the various capabilities of the ladder logic. E.g. the difference between a normally open bit vs. a normally closed bit. A normally open bit means that a logical path is passed when a signal is present. A normally closed bit is opposite to that: a logical path is passed when a signal is not present. The programmer will also be required to make use of output coils, timers, and latch/unlatch functions. Again, the trainer is meant to be a learning tool and the PLC will allow plenty of latitude for the programmer to learn these things as s/he progresses through the programming process.

7 Page 6 Design Details Figure 1 shows a functional block diagram for the PLC Controlled Motor Circuit. Not meant to be a detailed schematic, the diagram shows a power supply box (fed from a 120Vac outlet) that provides power to a terminal block box. The terminal box provides both 24V and 5V power simultaneously to the control panel and PLC. Via push buttons, the control panel sends signals to the input modules, which then relay the signals to the PLC. The PLC uses this information in its logic and outputs accordingly to the output modules. The output modules send a signal to both the control panel (indicator lamps) and the motors via electromechanical relays. As all of these functions are occurring, the PLC and Laptop are continually communicating with each other via a USB cable. Figure 2 shows a detailed schematic of both the power supply box and terminal block box. Beginning with the power supply box, you can see that all power originates from a 120Vac power source (common household outlet). The line side of this circuit is protected with a 3A fuse. The line side and neutral are both daisy chained on the left side of the terminal strip. Terminals 1 & 4 provide the power to the 5V adapter (located in bottom of box), the output of which then runs to the terminal box. Terminals 2 & 5 provide the power to the 24V adapter (located in bottom of box), the output of which then runs to the terminal box. Terminal 3 provides the common line feed for all three motors at the output module E. Terminals 6, 7, & 8 are the return neutral of all 3 motors. Still referring to Figure 2, we can also see a detailed connection diagram for the terminal box. On the left side of this box is the 5V feed from the adapter. Again, the left side of this terminal strip is daisy chained. Terminals 1 & 4 of TB2 provide power to the PLC. Terminals 2 & 5 provide power to output module D, while Terminals 3 & 6 provide power to output Module E. On the right hand side of the terminal box is TB3. The 24V feed for this strip lands at terminals 1 & 15 and is again daisy chained throughout. Terminals 1 through 12 provide the 24V+ to the control panel push buttons. It should be noted that the labels in parentheses are where the wires land first before landing at the Input terminals. Also, the safety flags are not located in the control panel, but rather on the mounting panel. The remaining terminals follow this same format. Figure 3 shows the detailed schematic for the two input modules B & C. The 24V+ is supplied from the terminal box, passes through the corresponding switch/push button and finally lands at the input module. It should be noted that the 24V common lands at the module and a jumper wire is installed on the module to connect the commons. This is true for both input modules. Figure 4 shows the detailed schematic for the two output modules D & E. The 24V+ is supplied from the terminal box and serves as a common feed for the relays on module D. When the relays are switched, the 24V signal is passed to the indicator lamps located on the control panel with the return wires landing at terminal box TB3. Output Module E is a little

8 Page 7 different in that the common is the line feed directly from the power supply box at TB1 3. As the relays on this module are switched, the corresponding motors are turned on/off. The return/neutral of all three motors land at the power supply box TB1. The design details for the ladder logic contained in the PLC will not be described here. The logic design will be different to each individual who is doing the programming depending on the desired functionality of the push buttons, motors, and lamps. Describing each rung of the program is outside the scope of this project, however some pitfalls and potential problems will be addressed in the Conclusion and Recommendations section of this report.

9 Figure 1: Functional Block Diagram Page 8

10 Figure 2: Power Supply and Terminal Box Connections Page 9

11 Figure 3: Input Module Connections Page 10

12 Figure 4: Output Module Connections Page 11

13 Page 12 Construction Construction of the PLC Controlled Motor Circuit required quite a bit of fabrication: Step 1: Construct the base. A thin piece of metal was used (approx. 27 x20 ), with 1 x1 aluminum angle attached to the underside to elevate and provide room for wire runs underneath the base. See Picture 1. Picture 1: Base Panel Construction Step 2: Cut holes in Control Panel. The control panel is home to many push buttons, switches, and lamps. Determine the layout and cut the holes accordingly. See Picture 2. Picture 2: Control Panel Cutout

14 Page 13 Step 3: Install push buttons, lamps, and switches in panel. See Pictures 3 & 4. Picture 3: Components installed Picture 4: Underside of Control Panel Step 4: Mount Boxes, Control Panel, Motors, PLC and Modules. Determine the desired layout, drill mounting holes in the base and mount. This is also a good time to mount the terminal blocks inside the boxes and create the safety covers to protect the motors. See Pictures 5 & 6. Note that unseen in Picture 6, the safety covers each have 4 posts that fit into pre drilled holes in the base. This prevents the covers from moving around and inadvertently opening the safety switch. Picture 5: Mounting before Wiring Picture 6: Safety Covers

15 Page 14 Step 5: Solder connections inside Control Panel and run the wiring. Note that the push buttons used here required soldering a 500 ohm resistor to each push button for proper illumination of the LEDs. If illuminated push buttons are not used, this additional step is not necessary. See picture 7. Picture 7: Connections and Wiring of Control Panel Step 6: Run and terminate all wiring. Here it is vital that you follow the schematics closely. Failure to do so will most likely result in a messy bundle of wire and much confusion. See Pictures 8 through 11. Picture 8: Wire Pulls Picture 9: Power Supply Box

16 Page 15 Picture 10: Terminal Box Picture 11: Module Wiring

17 Page 16 Testing and Validation Before I applied power to my project, I took a few precautionary measures. The first thing I did was to verify all wiring by using an ohmmeter and the schematics. I also tested all push buttons for proper operation. Once satisfied that the wiring was correct, I then opened all knife switches in the power supply box and terminal box. After these steps were complete, I plugged in the 120Vac power to the power supply box. Using a multimeter, I verified 120V at TB1 1 &4 (Figure 2). I then closed all knife switches on TB1 and verified 5 volts across TB2 1 & 4. I also verified 24 volts across TB3 1 & 15. Satisfied that the proper voltages were present in the proper places, I then closed all of the remaining knife switches. When this was complete, I noted that the PLC had power and lights, the modules had power indication, and the control panel buttons were illuminated. All of which were positive and expected signs of proper wiring. My next step was to test the control panel. I pressed several buttons, but had no reaction from the motors or lamps. Using a multimeter, I verified voltages at the PLC input modules. I hooked up a laptop and verified that a program was loaded into the PLC but still could not operate the motors properly. Using the software included with the PLC, I began debugging the program. I found that the program was waiting for inputs from the HMI program. After initializing the HMI program and placing in Auto mode, the project began operating as programmed. Further testing revealed that when the HMI program is closed out, the bits for auto/manual control default to manual. This prevents the program from proper operation unless the HMI is up and running. This is not a major concern, since the project is designed to run with an HMI. Although operating as designed, I found a few minor issues with the program I had written. For example, the E stop did not stop the bin motor from running when pressed. The remedy for this was to simply place the E Stop input bit in series with the motor output logic rung. Other minor program tweaks were necessary and were corrected in much the same way.

18 Page 17 Operation To properly operate the PLC Controlled Motor Circuit, you must first understand that it is designed to be part of an imaginary process. This process is one that moves material along two conveyor belts, both of which dump into the same bin (Figure 5). M M M Figure 5: Overview of Process There are two operator stations, both of which can control the process. The first station is the HMI. For either one of the conveyor motors to operate, all permissives have to be met. These permissives include: Safety Circuit OK (E stop and safety cover switch), HMI in Auto, and control panel in Auto. Once all permissives have been met, the operator must press the Start button on the HMI and the motor will begin to run. As the motor runs, the HMI displays a level indication of the bin. When the bin reaches 90% full, the bin motor will automatically turn on until the bin is empty. This process repeats indefinitely until one of the permissives are no longer met or the operator presses the Stop button on the HMI. Control of the second conveyor motor is identical to the first. However, when both conveyor motors are running simultaneously, the bin will fill up twice as fast and dump half as fast. An option that the HMI station has that the control panel does not have is that of safety override. If for some reason, the operator wished to bypass the safety circuit (not recommended in real world applications), s/he simply has to toggle the safety override radio button on the HMI. Doing so results in the E stop and safety switch being removed from the circuit that makes it necessary for the motor to run. The control panel operation is nearly identical to that of the HMI. The control panel and HMI must both be in Auto and the safety circuit must be intact before the motors will run. Here, the operator utilizes actual pushbuttons to start and stop the motors. When a motor is running, the corresponding lamp will be illuminated. A key difference, however, is the jog button found on the control panel. When the operator selects control to manual, he can

19 Page 18 then use the jog button to turn on the motors. However, s/he must continue to hold the button in. Once the jog button is released, the motor will stop. With both conveyors in manual mode, if the bin level reaches capacity, neither conveyor motor will function until the operator presses the dump bin button to empty the bin. A final note on operation. While either of the conveyor motors are running, if the corresponding safety cover is removed, the safety circuit permissive is no longer met and that motor will cease to run. This is provided that the safety circuit is not overridden by the HMI.

20 Page 19 Conclusions The PLC Controlled Motor Circuit project has been quite a learning experience. The time involved in planning, fabricating, wiring, and testing was much more than originally anticipated. However, the project is functioning as designed and has a clean, professional look. The HMI was a pleasant discovery. The company that produces the PLC also provides the software for this very simple HMI. This was my first experience with such software and it was a learning experience to program the lights, gauges and buttons necessary to make it operate the motors via laptop. However, I would like to have seen more functionality and graphics made available on this software. There are some disappointments in my project. One is related to the programming. A few of the tweaks I made while troubleshooting were not accompanied by proper rung comments. Although the program works fine, I suspect I will have difficulty figuring out why some of the logic is present because I did not take the time to describe the reasoning in rung comments. Another disappointment was the lack of wire labeling. This was due solely to the small gauge wire. I could not find appropriate wire labels for such small wire. I opted to use color coding, but found that I needed many more colors than what I could find in the stores (keeping budget in mind). In the end, I relied heavily upon my detailed schematics and an ohmmeter to wire my project. Although the wiring has a clean look, I would still like to see the labels. Overall, the final result of this project met or exceeded all of the criteria of its original design. Rather than operating two motors, the project operates three motors and also utilizes an HMI interface that was not conceived of in the original design. The end result is more complex than the original idea.

21 Page 20 Recommendations There are a few things that I would do differently if building this project again. One of which is the use of resistors on the push buttons. After I had soldered several of these resistors, I realized that I could have accomplished the same voltage drop across all the push buttons by placing a single resistor in series with the 24V return line. This would have saved a lot of time and improved the appearance of the project. Another item that could have been approached differently is my programming. When I began programming the ladder logic, it was my thought that I would only need a few rungs to make the project operate appropriately. As the programming progressed, I realized that I should have programmed an input list and output list. The input/output list could have tied a single input/output to a corresponding internal (soft) bit. I could have then used the soft bits throughout the rest of the programming. Doing this would have enabled me to change the state of all soft bits at once simply by changing a single hard input/output. This would have made troubleshooting and testing much simpler. Another programming item that I could have done better is the rung comments. Although I did make notes throughout my program using rung comments, I have learned that you can never have too many of these comments. The more comments and more descriptions in the program, the easier it is to troubleshoot later. Along these same lines is descriptive bit labels. Here, I am satisfied with my work but it is a good practice for all programmers to use as much description as possible when labeling bits in their programs. Again, doing so will facilitate troubleshooting in the future.

22 Page 21 References Velocio. n.p, n.d Web. 24 Apr < content/uploads/2014/07/branchds1.pdf>

23 Page 22 Addenda Addendum 1 Specifications General Information The PLC Controlled Motor Circuit is intended to provide the user with a training example of how different voltages are used in a control circuit to start and stop a motor. The user must provide a laptop or home computer for proper function. This unit is not designed to be used in a production environment, nor is the motor intended to provide any external work. Proper use of this product will provide the user with an opportunity to start/stop a motor via pushbuttons, switches, and forcing outputs in the PLC. The user will also be able to monitor and manipulate the PLC logic to observe results of said changes. Maintenance and Troubleshooting The PLC Controlled Motor Circuit is accompanied by an electrical schematic to aid in troubleshooting. The system is designed for several conditions to be met before the motor will begin to operate. A basic knowledge of electrical symbols and use of a multimeter is necessary for troubleshooting. For best performance, keep the PLC Motor Controlled Circuit in a clean environment and free from dirt and debris. Voltages Used: Motors: PLC: Velocio Branch 221 Specifications 110Vac (provided by user) 24Vdc 5Vdc 110Vac, Single Phase 5Vdc 12 digital inputs 12 digital outputs 6 analog inputs (not used) Overall Dimensions: Length: 36 Width: 30 Height: 14 Weight: 25lbs. Operating System: Windows Vista, 7, 8 or 10 (32 or 64 bit)

24 Page 23 Addendum 2 Parts List PROJECT NAME: PLC controlled Motor Starter Circuit DATE: 4/24/2016 Project Manager Name: William Heater Qty Descriptor Description Cost Extended Procured from Each Cost 1 PLC Velocio Branch 221 $ $ Velocio.net 2 24V module Convert 24V to 5V signal $19.00 $38.00 Velocio.net 2 Relay module Output Module for PLC $19.00 $38.00 Velocio.net 3 120Vac Motor Single Phase $9.95 $29.85 National Equipment 7 Pushbuttons w/ LED illumination $4.95 $34.65 AdaFruit.com AWG wire Stranded $0.25 $3.75 Mendelson's AWG wire 3 pair, twisted $0.30 $15.00 Mendelson's 2 Switches SPST $3.95 $7.90 AdaFruit.com 1 24V Power $9.95 $9.95 Mendelson's Supply 1 5 V Power $4.95 $4.95 Mendelson's Supply 5 Lamp 24V, Green $1.10 $5.50 AdaFruit.com 4 Aluminum Rail Used for base $3.00 $12.00 Lowes 1 Metal Sheet Used for base $5.00 $5.00 Mendelson's 1 Wire Ties Various sizes $10.00 $10.00 Lowes 1 Plexi Glass Used for safety covers $20.00 $20.00 Lowes 36"x36" 1 Extension Cord 16 AWG $7.00 $7.00 Lowes TOTAL COST $380.55