lab 10 mechanical motion

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Lab 10 – Mechanical Motion Figure 10.0. Tachometer Apparatus to Measure Motor Speed  The ability to translate electrical signals into motion in the real world combined with the ability to measure position can help you exploit the power of the computer to generate computer automation the source of much of the modern world’s conveniences. Goal: In this experiment, use the power capacity of the NI ELVIS II variable power supply to run and control the speed of a small DC motor. Using a modified free space IR link, build a tachometer to measure the speed of the motor. By combining the motor and tachometer with a LabVIEW program, you can incorporate computer automation in the system. Required Soft Front Panels (SFPs) Variable power supply (VPS) Oscilloscope (Scope) LabVIEW Required Components 1 k  resistor (brown, black, red) 10 k  resistor (brown, black, orange)  IR LED/phototransistor module OR an IR LED and a separate Phototransistor  

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7/27/2019 Lab 10 Mechanical Motion

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Lab 10 – Mechanical Motion

Figure 10.0. Tachometer Apparatus to Measure Motor Speed

The ability to translate electrical signals into motion in the real world combined with theability to measure position can help you exploit the power of the computer to generate

computer automation − the source of much of the modern world’s conveniences.

Goal: In this experiment, use the power capacity of the NI ELVIS II variable powersupply to run and control the speed of a small DC motor. Using a modified free space IR

link, build a tachometer to measure the speed of the motor. By combining the motor and

tachometer with a LabVIEW program, you can incorporate computer automation in the

system.

Required Soft Front Panels (SFPs) Variable power supply (VPS)

Oscilloscope (Scope)

LabVIEW

Required Components

1 k Ω resistor (brown, black, red)

10 k Ω resistor (brown, black, orange) IR LED/phototransistor module OR an IR LED and a separate Phototransistor

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DC motor

Small punch or drill Glue

Optional: Several combs with varying numbers of teeth per inch

Exercise 10.1: Start Your Engine

You can purchase a small, inexpensive DC motor at Radio Shack or many hobby stores.

These motors usually require a voltage source from 0 to 12 V, and produce a maximumRPM of about 15,000 at 12 V. With no load, the current requirement is usually about 300

mA. The NI ELVIS II VPS can supply up to 500 mA at 12 V. Also, by changing the

polarity of the applied voltage, you can change the direction of rotation.

Complete the following steps to install and run a motor on an NI ELVIS II

protoboard.

1. Connect a DC motor to the VPS output terminals, (SUPPLY+ and GROUND).

2. Launch the NI ELVIS II Instrument Launcher strip and

select Variable Power Supply (VPS).

3. From either the workstation’s manual VPS controls or the SFP controls, test

the motor.

NOTE: It is a good idea to look up the specifications of the motor being used, if they are

not already known, to make sure that too great of a load is not applied to it.

In the following example, manual control has been selected by clicking on the Manual

box []. Read the VPS voltage by clicking on the Measure Supply Outputs box [],

applying power to the protoboard, and clicking Run.

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Figure 10.1. VPS Supply + Configured to Manually Drive a DC Motor

End of Exercise 10.1

Exercise 10.2: The Tachometer

Using an IR LED and phototransistor or an integrated LED/phototransistor module, youcan build a simple motion sensor.

Complete the following steps to build a simple motion sensor.

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1. On the protoboard, insert the components shown in the Figure 10.2 circuit

diagram.

+ 5 V

Gnd

To ACH4

+

-

10 k Ω

DetectorEmitter

1 kΩ

Figure 10.2. Circuit for Operation of an Integrated LED/Phototransistor Module

In the case of an LED/phototransistor module, an internal LED is used for the optical

source. Power it from the +5 V power supply through a 1 k Ω current limiting resistor

Then connect a 10 k Ω resistor from the phototransistor emitter to ground and connect thesame +5 V power supply to the phototransistor collector. The voltage developed across

the 10 k Ω resistor is the phototransistor or tachometer signal.

2. Connect leads from the 10 k Ω resistor to the AI 0+ and AI 0- pin sockets.

3. Select Scope from the NI ELVIS Instrument Launcher strip and select thesettings, as shown in Figure 10.3.

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Figure 10.3. Tachometer Signal Viewed on the Oscilloscope

4. Power on the protoboard and run the oscilloscope SFP.

5. Pass a piece of paper through the IR motion sensor. You should see the

oscilloscope trace change (HI-LO-HI). With a narrow piece of paper, you

might catch the pulse generated as you drag it through the sensor.

6. Optional: Place a comb with many teeth in the sensor IR beam. Dragging it

through the sensor generates a train of pulses. You can even run it back and

forth like a saw to generate a continuous stream of pulses as shown in Figure

10.3.

It is interesting to try combs with different sizes and numbers of teeth. Each combgenerates its own signature waveform.

End of Exercise 10.2

Exercise 10.3: Building a Rotary Motion System

The rotary motion demonstration system consists of the DC motor controlled by the

variable power supply and the IR motion sensor configured as a tachometer. To

complete the tachometer, you must attach a disk with a 2 in. diameter, to the shaft of the

motor by completing the following steps.

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1. Cut a 2 in. diameter disk from a piece of thin but sturdy cardboard or plastic.

2. Cut a slot about 0.25 in. wide and 0.25 in. deep near the circumference of thedisk.

3. Punch or drill a small hole at the center point.

4. Glue the disk to the end of the motor shaft.

5. Mount the motor so that the slot lines up with the IR transmitter/receiver beam.

In operation, each revolution generates one pulse.

+ 5 V

Gnd

To ACH4

+

-

10 k Ω

DetectorEmitter

1 k Ω12 V DC Motor

VPS+

Gnd

Figure 10.4. Motion Sensor Circuit and Motor Parts

NOTE: You can also use the CD and motor of Lab 6. Instead of a small magnet

triggering the sensor, you can drill a hole about the size of the transmitter/receiver beam

(3 mm) near the edge of the CD. Align the IR sensor so that the beam passes through the

hole.

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Figure 10.5. Apparatus to Measure the Speed of a Spinning CD

End of Exercise 10.3

Exercise 10.4: Testing the Rotary Motion System

Complete the following steps to test the rotary motion system.

1. Power on the protoboard and run the motor using the NI ELVIS II VPS SFPto control the motor speed.

2. Adjust the motor position so that the disk does not touch the sensor slot.

3. Observe on the oscilloscope the pulses generated by the rotating motor, as

shown in Figure 10.6.

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Figure 10.6. Typical Tachometer Waveform

4. Read the pulse frequency (Freq:) from the measurement row CHO Meas: at

the bottom of the oscilloscope screen. Take frequency measurements for avariety of power supply levels. A plot of frequency versus VPS voltage level

demonstrates the linearity of your rotary motion system.

5. Close NI ELVIS and all SFPs.

End of Exercise 10.4

Exercise 10.5: A LabVIEW Measurement of RPM

LabVIEW has several VIs located at Functions»Programming»Analog

Waveform»Waveform Measurements that are convenient for measuring thetiming periods of a continuous waveform. You can use the Pulse Measurements.vi

to measure the period, pulse duration, or duty cycle from a waveform array.

Figure 10.7. Period Measurement Converted to kRPM

You can convert the period measurement to revolutions per minute by inverting the

period to get frequency and multiplying by 60 to get rpm. For scaling, divide by 1000 to

get krpm.

NOTE: You can also use the Express template for Timing and Transitions Measurements

and get the frequency directly. Then convert the frequency to rpm as discussed above.

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Figure 10.8. kRPM Measurements using an Express VI

Launch the LabVIEW program RPM.vi. Open the Block Diagram window and study theprogram.

NOTE: This LabVIEW program is configured to connect to“Dev1” for your NI ELVIS

workstation. If your device is configured to another device name, you need to rename your NI

ELVIS workstation to “Dev1,” in Measurement and Automation Explorer (MAX) or modify the

LabVIEW programs to your current device name.

Figure 10.9. Block Diagram of program RPM.vi

Use the DAQ Assistant to collect 1000 V samples for the tachometer graph and providean input signal array for the Pulse Measurements.vi. The rpm signal is sent to a front

panel meter and displayed in krpm. The rpm signal also goes to a shift register with five

elements. This provides an averaged rpm signal for the front panel. You manually control

the motor speed with the front panel knob labeled Setpoint. Also available on the front

panel is a graph of the tachometer signal as a function of time.

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Run this VI and take your motor for a spin. See and hear how responsive the motor is to a

rapid change in the rpm setpoint.

Figure 10.10. LabVIEW Tachometer and Motor Control Circuit Front Panel

End of Exercise 10.5

LabVIEW Challenge: Computer Automation of the Rotary Motion

System

National Instruments offers the LabVIEW PID and Fuzzy Logic Toolkit, which features

additional LabVIEW VIs you can use to add computer automation to your rotary system.PID stands for “proportional integral derivative.” These control algorithms move a

system from one setpoint (initial rpm) to another setpoint (final rpm) in an optimizedmanner. The addition of a single VI (PID.vi) provides optimal control to your program.

The algorithm compares the target rpm (final rpm) with the current rpm (averaged rpm

signal) to generate a DC error signal, which drives the VPS. Integration and

differentiation parameters adjust the VPS voltage smoothly from one measurement to thenext.

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Figure 10.11. PID subVI for Control Applications

If you are more familiar with control, you can use another VI (PID Autotuning.vi) to set

the initial PID parameters automatically. Then you can fine-tune the parameters to yourspecific system. Search for additional LabVIEW PID resources at http://www.ni.com.

Figure 10.12. Setpoint (yellow) and RPM (red) Traces show Optimal Control PID in

Action

In Figure 10.12, the setpoint (yellow trace) is changed suddenly from 3300 to 4500 krpm.The system rpm (red trace) responds by moving the motor speed smoothly and optimally

from the current setpoint to the target setpoint.