Speed Sensor Calibration Lab 4 Speed Sensor Calibration Circuits Lab 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 Circuits Lab 2

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 Circuits Lab 2

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 Circuits Lab 2

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. Circuits Lab 2

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. Circuits Lab 2

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.

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) Circuits Lab 1

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. Circuits Lab 2

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 Circuits Lab 2

Physics Calculation Technique Lowest rung Highest rung Middle rung Circuits Lab 2

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. Circuits Lab 2

Camera Calibration Technique Needed Adjustment Camera Setup Circuits Lab 2

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. Circuits Lab 2

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. Circuits Lab 2

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. Circuits Lab 2

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.  

Assignment Due Today: Initial Paper Design Initial Project Schedule Circuits Lab 2

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 Email the post lab calculations by Tuesday next week Next week due: Circuit Prototyping Lab Memo The week after: Sensor Calibration Lab Memo Circuits Lab 2