This instructable was created in fulfillment of the project requirement of the Makecourse at the University of South Florida (www.makecourse.com). In this instructable you will learn how to make a damper dynamometer to create a force vs. velocity curve from which a damping ratio can be extracted. Other than having access to a 3D printer, this project can be done with basic tools, such as a soldering iron and a dremel.

Step 1: Tools & Items Required

Tools Required

3D printer
Soldering iron
Solder
Dremel
Vernier Calipers
Power Drill
Drill bit set
Hot glue gun

Parts Needed

Black Box (for electronics)
Arduino UNO
Small breadboard
Assortment of jumper wires
Push button
10k ohm rotary potentiometer
LCD Display
Two 220 ohm resistors
One 1N4007 diode
One mosfet IRF520 transistor
One 6-9V DC motor
5kg load cell
Sparfun HX711 load cell amplifier
Two M5 fully threaded bolts
One 8mm x 15mm x 24mm linear bearing
Two 3/8" OD by 0.049" wall thickness steel tubes
Traxxas damper
USB cable
2" x 2" wood
Paper clip

Step 2: Load Cell Preparation

In order to prepare the load cell for wiring, the following needs to be completed:

Solder 9 male pins to the HX711 load cell amplifier.
Solder 4 female connections to the four wires coming out of the load cell.
Using the HX711 library and calibration code found on SparkFun, calibrate the load cell using a known weight.

Step 3: Black Box Preparation

In order to prepare the black box for assembly, the following needs to be completed:

Using a dremel, cut a square hole in the box's lid for the LCD screen.
Using a set of drill bits, cut 5 holes to the lid.
- One hole for the push button
- One hole for the rotary potentiometer
- One hole for the crank-slider mechanism
- Two holes for the steel tubes that support the damper mounting
Drill a hole on the side of the box to fit a USB cable which will be used for data transfer from the Arduino, as well as a power source.
Drill another hole on the side of the box to route the load cell wires.

Step 4: Component Manufacturing

The following components need to be 3D printed:

Linear bearing holder
Damper collar
Damper collar spacers
Load cell spacer
Piston
Connecting rod
Spinning disk

***NOTE: All STL files have been included. The linear bearing is press fitted, so pay close attention to tolerances.

Step 5: Component & Black Box Assembly

To assemble the 3D printed components, follow these steps:

Glue the spinning disk to the output shaft of the DC motor.
Cut two pieces from a paper clip that will be used as connecting pins for the connecting rod.
Line up the holes of the spinning disk and the connecting rod and secure the two with the piece of paper clip.
- It is recommended that you put a drop of solder on both ends of the paper clip to prevent the connecting pins from falling out of place.
Repeat step 3 for the connecting rod and the piston.
Press the linear bearing into the holder.
Cut 3 holes on the 2" x 2" piece of wood, spaced by about 1.3"
- Two of the holes will be used for the steel tubing and the third hole will be used for the load cell mounting.
Using an M5 bolt and the load cell spacer, screw the load cell into the piece of wood.
On the other end of the load cell, and using an M5 bolt, secure the 3D printed collar to the load cell.
Once the collar has been attached to the load cell, use the collar spacers to attach the damper to the collar.

To assemble the black box, follow these steps:

Using a hot glue gun, secure the push button and the rotary potentiometer in place
Press the LCD screen into the square hole that was previously cut. Note: the screen should have a tight fit. In case that the screen does not fit, sand the hole accordingly.
Line up the linear bering with its corresponding hole that had been drilled in the lid previously.
Glue the bearing holder in place.
Using another piece of wood, glue the DC motor with the crank slider to the base of the box. Be careful to align the piston with the linear bearing!
Guide the crank-slider mechanism through the hole in the lid and into the linear bearing.

Step 6: Wiring

Follow the Fritzing diagram to wire all the electrical components to the breadboard and the Arduino. The accompanying picture shows the packaging and electrical component arrangement in the black box.

Wiring Guidelines:

Wire 5V and ground to the Arduino. Make sure that the 5V and ground is passed to the secondary power rail in the breadboard.
Wire the rotary potentiometer to analog pin number 3 (A3).
Wire the push button to digital pin number 10.
Wire the load cell white and green wires to digital pins 2 and 3 respectively.
Wire the LCD's green and yellow wires to pins SCL and SDA respectively.
Wire the motor output to digital pin number 11 (PWM)

**NOTE: make sure that the code has this same pinout setting. Otherwise, modify the code depending on where you place your wires in the Arduino.

Step 7: Code

The Arduino sketch is provided here. Make sure that all the pins that you have selected in the "Wiring" step are defined correctly within your sketch. Additionally, use the calibration factor for the load cell that you previously found.
The video included describes the code and its functionalities. The video serves as a guideline to the sketch that has already been provided.

Main_Code_REV01.ino
Download

Step 8: Final Assembly and Use!

Once the wiring is done, the program uploaded and the components all assembled, follow these steps to finalise the dynamometer:

Fasten the lid of the black box using 4 screws.
Insert the steel tubes through the holes that were previously drilled.
Press the wooden piece into the steel tubes.
Attach the shaft of the damper to the piston of the crank slider mechanism.
Select the speed setting through the rotary potentiometer and press the push button to start the system.
Open the serial monitor to save the data recorder by the load cell.
Post-process the data in order to determine the damping ratio through a force vs, velocity curve.

Introduction: RC Car Damper Dynamometer