1. Speed Sensor Calibration
Lab 4
Speed Sensor Calibration
Circuits Lab 2

2. Lab Safety Do not stand on chairs, or sit or stand on the tables
Know the location of the first-aid kit
Report all injuries occurring in the lab to the instructional staff
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3. Agenda Background on Calibration Overview of Lab
Physics Calculation Calibration Technique
Camera Calibration Technique
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device.
The device with the known or assigned correctness is called the standard. The second device is the test instrument (TI), or any of several other names for the device being calibrated. --Wikipedia
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4. Sensors Many products use sensors
Some products use sensors to detect the presence of some condition, such as:
to automatically open doors when someone approaches
to change traffic signals when cars are waiting
These types of sensors typically have two states, “on” and “off”.
Some products use sensors to measure and display physical characteristics such as temperature, weight, light intensity, etc.
This type of sensor must:
reliably and repeatably create a signal directly related to the characteristic being measured
be calibrated against a known standard so that the readout is accurate
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5. Coaster Application Software
“Coaster” app measures up to eight sensors’ outputs and records the pulse duration for each.
The speed is found by dividing the diameter of the ball by the pulse duration.
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6. Coaster Application Software
This software is designed to accept a correction factor for determining the accurate speed of the ball passing through the sensors.
This lab uses three different techniques for determining this correction factor.
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7. Calculating Speeds
The clear plastic around the LED is a lens that focuses the light into a beam. The beam has a spread of about 6 degrees. Because of this and because the edges of the ball are not flat, the measured time is shorter than the actual time.

8. SPEED CALCULATIONS Measure (t2-t1)
Leading Edge
Trailing Edge
Diameter of Ball (D)
Time axis
Measure (t2-t1)
The correction factor for the LED/Phototransistor pair and the 1” Dia. coaster ball was measured to be 8.3%. So the speed can be calculated using:
s=D/[1.083*(t2-t1)]
When ball passes through Point A - we trigger – transition from 0 to 1 (leading edge of square wave)
When ball passes through Point B – we reset – transition from 1 to 0 (trailing edge of square wave)
We observe this square waveform using a Virtual Oscilloscope (same as in ENG-181)
The problem in building such a circuit boils to:
Sensor circuits: Sensing the ball moving over Point A and/or Point B – we will use optic and mechanical switches for this purposes. These circuits will generate signals when ball passes through these points. In our lab set up, we will use a mechanical switch at Point A and a optical switch at Point B. They could be interchanged. Variations of the switch circuits could also be used (laser-based, different micro-switch, some form of pressure transducer, etc.) .
Square wave generation: Generating a square waveform based on the signals we obtain from the sensor circuits. We will use what is commonly called a D-Flip flop for this purpose. We will not examine the internal design/working of a D-flip flop. All we will know is :
It is an IC (chip) - We will use 5 of its pins for building one speed trap.
Lets call its output Q – where the square wave is generated (Q can be 0 Volts or 5 volts - equivalently 0 or 1)
It has SET and RESET input pins – where we will connect the output of our 2 sensor circuits
While the flip flop is powered on, it is in a unknown state – we set it to a known state (Q=0) by another small mechanical switch circuit (similar to the ones used in our sensor circuit)
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9. Technique 1 – Physics Calculations
You will calculate values of speed based on physics principles compared to the speed measured by the software.
You will analyze the results and determine a correction factor to calibrate the speed sensor.
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10. Physics Calculations Ball rolling down a ramp
Total energy at top of ramp = Potential
Total energy at bottom of ramp = Translational Kinetic plus rotational kinetic
Conservation of Energy (including rolling friction losses and assuming there is no sliding)
Solving for ball velocity at the bottom of ramp
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11. Physics Calculation Technique
Lowest rung
Highest rung
Middle rung
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12. Technique 2 – Video capture
Use a camera and computer to capture video images at a known rate of frames per second.
Measure how far the ball travels between frames near the speed sensor to determine its accurate speed.
Determine a correction factor based on this technique.
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13. Camera Calibration Technique
Needed Adjustment
Camera Setup
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14. Measurement of ball position
Click here to move the menu.
144 mm
Align the left edge of the “Play Speed Settings” Menu with the leading of the ball to obtain an accurate position of the ball.
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15. Analysis of Camera Data
Finding the position of the ball for all frames with the ball in view allows you to generate a graph of speed vs. position.
The linear trend from this data can be extrapolated to find the speed of the ball at the LED Speed Sensor.
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16. Lab logistics
SuperGroup 1
SuperGroup 2
SuperGroup 3 A B G D E F
Each group will record their data on the spreadsheet at each piece of equipment in their supergroup.
The instructor will provide the spreadsheets at each table and collect and send the composite results to all students.
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17. Bring It All Together
The correction factor for each method is determined by calculating the ratio of the measured speed for each calibration technique to the measured speed of the LED speed sensor.

18. Assignment Due Today: Initial Paper Design Initial Project Schedule
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19. Assigned Work: Lab Memo Next week due: Circuit Prototyping Lab Memo
At the end of your lab procedure, review the requirements for the lab memo
the post lab calculations by Tuesday next week
Next week due:
Circuit Prototyping Lab Memo
The week after:
Sensor Calibration Lab Memo
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