DC Motors Electric Circuits / System Design
Overview:
Rube Goldberg (1883-1970) was a Pulitzer Prize-winning artist, famous for his cartoons portraying fictional inventions. Each “invention” consisted of a long, complex sequence of steps to carryout a simple task. While his inventions regularly violated some engineering principles, there was always a great deal of real engineering to be found in his humorous designs. Although he made his career as an artist, Rube Goldberg was educated as an engineer. His legacy continues today through the annual National Rube Goldberg Machine Contest held at Purdue University. The event is organized by the local chapter of Phi Chapter of Theta Tau, the National Student Engineering Organization.
In this activity, you will learn the basics of operating a small DC electric motor to achieve one step in a sequence of tasks, and you will work with other students to assemble a Rube Goldberg machine.
CAUTION:
This equipment described here is only appropriate for controlling low-voltage, low-current DC devices, such as small battery-powered motors. NEVER CONNECT THE CIRCUIT TO AC DEVICES OR TO ANY VOLTAGE SOURCE THAT EXCEEDS 12 VOLTS OR 300 mA.
PART 1 Some Basics of DC Motors and Relays
DC motors come in a large variety of sizes and shapes, but they all depend upon the interaction between electric currents and magnetic fields to produce a rotation. Most (but not all) can rotate either forward or backward depending upon the direction of the electric current.
Electromechanical relays are similar to motors in their dependence on electric currents and magnetic fields, but they are designed simply to control the back and force movement of a mechanical switch. Like motors, relays are available in a wide variety of shapes and sizes, including the 2 types shown in the photo at the top of this page. A relatively small current is needed to activate a relay, enabling the relay to control a much larger current. In many applications, relays have been replaced by transistors or other purely electronic switches which can be less expensive and which are not subject to the possible failure of a relay’s moving parts. On the other hand, relays remain important in many applications because they can be much more efficient in conducting relatively large currents with virtually no drop in voltage.
Relays are often used to control DC motors, both to turn them on and off and to control their direction of motion. The schematic circuit diagram on the next page shows an “H-Bridge,” which you will use to control a DC motor. There are also photos that show the inner workings of two types of relays. Although the relays are quite different electrically and physically, both use a current flowing through an electromagnetic to move a mechanical switch.
Circuit Schematic DiagramSPST Relay
DPDT Relay
Question 1 on the Report Form shows black-and-white versions of the motor control circuit, with the relays shown in all of their possible states. The top relay is a single-pole, single-throw (SPSP) relay, which completes an electrical pathway when the relay’s magnet is energized. The other relay is a double-pole, double-throw (DPDT) relay, which two pathways (2 poles) and which directs the electric current in either of two different directions (2 throws). In your circuit, the SPSP relay will be magnetized when your calculator sends the signal “1.” The DPDT relay will be magnetized when your calcualtor sends the signal, “2”. Both relays will be energized by the signal “3,” since 3 = 1 + 2.
· Directly on the diagrams on the Report Form, draw lines and arrows to show how current will flow from the battery positive for each of the 4 possible signals. Also indicated how the motor will respond by checking either “forward,” “reverse” or “off.”
· Use a different color of pen, pencil or marker on the diagrams above to show the path of electric current which will activate the electromagnet.for each magnetized relay.
· Answer the other question in Part 1 of the Report Form.
DC Circuits REPORT FORM (Part 1)
NAME(S) ______
1) Directly on the diagrams on the Report Form, draw lines and arrows to show how current will flow from the battery positive for each of the 4 possible signals. Also indicated how the motor will respond by checking either “forward,” “reverse” or “off.”
Signal = 0SPST relay: not magnetized
DPDT relay: not magnetized
Motor: Forward ___ Reverse ___ Off ____ / Signal = 1
SPST relay: magnetized
DPDT relay: not magnetized
Motor: Forward ___ Reverse ___ Off ____
Signal = 2
SPST relay: not magnetized
DPDT relay: magnetized
Motor: Forward ___ Reverse ___ Off ____ / Signal = 3
SPST relay: magnetized
DPDT relay: magnetized
Motor: Forward ___ Reverse ___ Off ____
2) Use a different color of pen, pencil or marker on the diagrams above to show the path of electric current which will activate the electromagnet.for each magnetized relay.
3) The SPST relay shown has a coil resistance of 250 Ω and is rated for a control voltage of 6 V. Use Ohm’s law to find the control current which will flow through the electromagnet’s coil when the relay is activated.
4) The SPST relay is rated for a “maximum switched current” of 500 mA. Why is this value not the same as the current calculated in question 3?
5) The 2 relays used here are SPST and DPDT. Describe with words or a diagram how a SPDT relay would operate.
PART 2 Building and Testing the Control Circuit
CAUTION: The motor here must use a DC voltage of 12 V or less and it must use a current of 500 mA or less. Be extremely alert for short circuits, which can cause fires and can cause the batteries to over heat and spill harmful chemicals. DO NOT INSERT BATTERIES INTO THE BATTERY PACK UNTIL AFTER THE CIRCUIT HAS BEEN CHECKED AND APPROVED BY YOUR INSTRUCTOR.
The pictures on the next page show a recommended board layout for using a SPST relayt and a DPDT relay to control a DC motor. The green lines represent metal clips inside a solderless breadboard, such as Radio Shack’s #276-175. Solid 22AWG wire should fit easily into any of the holes on the breadboard to complete other connections. If your motor or battery pack uses stranded wire, it may be necessary either to make the attachment to the board using a short jumper wire with alligator clips.
· Assemble the components and connection on the breadboard as shown. Note that the coil connections on the DPDT relay are furthest from the other pins and that these go to the left in the suggested layout. This means that the lettering on the DPDT relay will face towards the back.
· After the breadboard has been approved by your instructor, use the calculator, CBL2 and Binary Basic Trainer to test the operation of your motor, verifying that it turns both directions and goes off with the appropriate calculator instructions. If the system does not function as it should, use the troubleshooting guide which follows the pictures to find and correct the problem. Describe the results of your tests on Part 2 of the Report Form.
Board Layout
DC Circuits REPORT FORM (Part 2)
NAME(S) ______
1) Describe below the steps you took to determine whether your control circuit is functioning as as it should.
2) Describe any problems you encountered with the system and explain how you attempted to identify and correct the problems. Also desribe the final result of your troubleshooting.
3) In your system, what is the source of energy which powers the motor?
4) In your system, what is the source of energy which powers the relays?
PART 3 A Rube Goldberg Megasystem
CAUTION: The motors here must use a DC voltage of 12 V or less and must use a current of 500 mA or less. Be extremely alert for short circuits, which can cause fires or cause the batteries to over heat and spill harmful chemicals.
In the spirit of Rube Goldberg, the goal of this section is to move a ping pong ball from one end of the room to the other in the most complex and entertaining way possible. Work with others in the class to design a megasystem which satisfies these requirements:
· There must be at least 5 distinct and different steps to the ball’s motion. (The more steps, the better.)
· The entire motion must be planned in advance, with control of each step assigned specifically to one team. Control may only be exercised by entering appropriate signal codes into a graphing calculator.
· Once the trial begins, no one except the instructor may touch the ping pong ball directly.
Complete Part 3 of the Report Form to describe the Megasystem.
DC Circuits REPORT FORM (Part 3)
NAME(S) ______
1. In the table below, describe each step in moving the ping pong ball. Add more pages as needed.
Step # / People Responsible / Description1.
2.
3.
4.
5.
6.
7.
8.
9.
Trouble Shooting
Sooner or later, you will encounter problems with the control circuit. Although it can certainly be frustrating to find that an experiment or demonstration is not working as expected, most problems (like those in most other electrical systems) will be bad connections that can be quickly corrected. Real-world systems don't always work perfectly, and it is extremely valuable to know how to deal with the inevitable problems.
The key strategy for diagnosing and repairing problems with the control circuit depends upon understanding that the circuit merges a flow of information from the calculator and the CBL with one or more flows of energy from the batteries. If you don't see the desired action, it is possible that a faulty or damaged component is causing the problem, but it is far more likely that the flow of either information or energy is blocked at some point. You need to find the blockage and eliminate it.
Check the information path:
Measurement / Expected result / Problem solutionTo check the calculator-CBL2 connection, exit the SIGNAL program if it is running and restart the program. The program checks to verify that the CBL2 (or LabPro) is responding to the calculator and gives an error message if the two devices are not communicating. / If there is no error, the SIGNAL program goes quickly to the welcome screen. When you actually send a signal, the yellow and green LEDs on the CBL2 or LabPro should flash. / · The most common problems involve the link cable between the calculator and CBL2 or LabPro. Be sure it is inserted fully into both devices. You may also need to try a difference cable, particularly if the tip is bent.
· Be sure the CBL2 or LabPro has good batteries or that it is connected to the proper AC adapter.
· Reload the SIGNAL program to verify that it has not been changed by accident.
Check to see if the Binary Basic Trainer is responding to the CBL2 or LabPro by using the SIGNAL program’s “Count 0 to 15” option. / If there is no error, all 4 LEDs on the Binary Basic Trainer should light and in turn and go out again according to the binary sequence. / · Verify that the cable from the CBL2 to the Binary Basic Trainer is snapped completely into place at both ends.
· Verify that the Binary Basic Trainer’s batteries are good and in place.
· If using the LabPro, verify that the cable is attached to DIG/SONIC1, not DIG/SONIC2.
Check to see if information is reaching the SPST power relay, by using the SIGNAL program’s “Key Press” option to press alternately “0” and “1.” / By listening closely or putting your finger on the tube-shaped SPST power relay, you should be able to detect the faint sound of the relay clicking off when you switch from “1” to “0.” (It is very difficult to hear the relay switch “on” when “1” is sent.) / · Trace the wires which lead from one of the Binary Basic Trainer’s “+” terminals to the relay’s input and reseat as needed.
· Verify that the SPST relay’s 4 pins are all in the correct holes of the breadboard.
· If necessary, use a voltmeter to find the break in the connection. Send a continuous “1” from the calculator and place the voltmeter’s negative probe where the wire from the Binary Basic Trainer’s “1” attaches to the breadboard. Move the voltmeter’s positive probe along the current path, starting where the wire from the Binary Basic Trainer’s “+” terminal attaches to the breadboard. The voltmeter should read about 5 volts until you reach the relay and it should read 0 volts after the relay. When you find an exception, reattach the wire.
Check to see if information is reaching the DPDT direction relay, by using the SIGNAL program’s “Key Press” option to press alternately “0” and “2.” / By listening closely or putting your finger on the rectangular DPDT direction relay, you should detect the sound of the relay coming on when “2” is sent and going off when “0” is sent. / · Trace the wires which lead from one of the Binary Basic Trainer’s “+” terminals to the DPDT relay’s input and reseat as needed.
· Verify that the DPDT relay’s 8 pins are all in the correct holes of the breadboard.
If necessary, use a voltmeter to find the break in the connection. Send a continuous “2” from the calculator and place the voltmeter’s negative probe where the wire from the Binary Basic Trainer’s “2” attaches to the breadboard. Move the voltmeter’s positive probe along the current path, starting where the wire from the Binary Basic Trainer’s “+” terminal attaches to the breadboard. The voltmeter should read about 5 volts until you reach the relay and it should read 0 volts after the relay. When you find an exception, reattach the wire.
Check the energy path:
Measurement / Expected result / Problem solutionVerify that energy is reaching the breadboard by using a voltmeter to measure the voltage where the 2 wires from the battery pack attach to the breadboard. / The voltage at the breadboard should be close to the nominal value expected from the batteries (about 1.5 V time the number of batteries). / · Remove the batteries and reinstall them, making sure none are backwards
· Verify the batteries are not dead. Replace them if needed.
· Check for a short circuit on the breadboard. If the battery pack leads show 6 volts when they are not connected to the breadboard but the voltage drops when the are connected to the board, there may be an undesired connection from positive to ground on the board.
Verify that the main batteries can operate the motor by connecting the motor leads directly at the point where the battery leads attach to the breadboard. / The motor should one continuously in one direction regardless of any signal from the calculator. / · Verify that the voltage and current requirements of the motor match the capability of the batteries.
· Use an ohmmeter to check for a short circuit, and open circuit in the motor and replace the motor if needed.
Verify that the SPST power relay can switch the motor on and off, by moving the motor’s positive lead connect with the opposite side of the SPST relay. Use the SIGNAL program’s “Key Press” option to send alternate signals of “0” and “1.” / The motor should run in one direction when you send “1” and go off when you send “0.” / · Check that all 4 pins of the SPST relay are seated correctly and that the connecting wire are in the correct location
· Replace the SPST relay, if needed.
Verify that the DPDT direction relay can switch the motor’s direction, by reconnecting the motor’s positive lead to the correct output of the DPDT relay. Use the SIGNAL program’s “Key Press” option to send alternate signals of “1” and “3.” / The motor should run in one direction when you send “1” and in the opposite direction when you send “3.” / · Check that all 8 pins of the DPDT relay are seated correctly and that the connecting wires are in the correct location
· Replace the DPDT relay, if needed.
DC Motors