Project: Arduino XZ dip coater

This project outlines, without going into too much detail, how to build a small XZ robot based on Arduino Uno and Firgelli miniature linear actuators. The electronics and programming skills needed are low. Mechanical skills required (to build something sturdy) are average to okay, meaning, you have to be able to make something solid ;)

(Note: this is an old project, so stuff might be outdated.)

What you'll need:

• Arduino Uno
• Motor shield:
  • Velleman KA03 (kit) or VMA03 (prebuilt)
  • Any other L298P-based shield (source code changes needed)
• LCD:
  • 20x4 LCD03 from Robot Electronics
  • Any other I²C LCD (source code changes needed)
• 12V/2A power supply and a power connector for the Arduino
• Either a lot of jumper wires or some pcb, pinheaders and wires
• Two Firgelli linear actuators (12V, linear position feedback, choose stroke and ratio to your needs): L12-P and L16-P
• Ball bearings, aluminium strips and screws+bolts
• (Update 2018: I see that Firgelli sells drawer slides nowadays! What a coincidence :) )
• The Arduino sketches and libraries. I've used Arduino version 1.0.6 to compile the firmware (newer versions might not work, for some reason).

Put it all together

The complete system is drawn schematically in the figure below. The Arduino will hold the firmware to control the motor shield and read the current position of the linear actuators, while displaying any useful information on the LCD.

Motor shield

The Velleman motor shield is a nice product, apart from the fact that Arduino pin A5 is used to monitor the external power supply, since we need pin A5 for I²C control of the LCD. So, if you plan to install an LCD (or other I²C hardware), remove (or don't mount) R9, R10 and C3 (10k, 100k and 22uF) from the Velleman board. Jumper settings for this project are: PWMA:6, DIRA:7, PWMB:9, DIRB:8, power:EXT. After the motor shield is correctly assembled and configured, place it on the Arduino board.
Alternative, the official Arduino motor shield can be used. In this case, change PWM and DIR pins in /libraries/XY/XY.cpp to the pins used by this shield. Also, disable (cut) the current sensing jumpers on the back of the board.

Linear actuators

The Firgelli linear actuators are composed of two parts. A servo motor controls the speed at which the actuator moves. A multiturn potentiometer is connected to the servo motor and is used for position feedback. Simply: the resistance of the potentiometer gives the position of the actuator. Since we want to accurately control the position of the actuators, the firmware has a so-called PID controller that reads the current position (from the potentiometer) and sets the speed of the actuator until the actual position equals the desired position.
For each actuator, connect the red wire to output 1/3 of the motor shield. Black wire goes to 2/4. Connect the yellow wires to Arduino's AREF pin, and the orange wires to GND. The purple wires each go to an Arduino analog input: A0 and A1.

	yellow	black	red	purple	orange
X-axis	Arduino::AREF	MotorShield::2	MotorShield::1	Arduino::A1	Arduino::GND
Z-axis	Arduino::AREF	MotorShield::4	MotorShield::3	Arduino::A0	Arduino::GND

LCD

If you want to connect an I²C LCD, connect SDA and SCL to Arduino pins A4 and A5, respectively, and connect the power supply wires to GND and 5V. For LCDs with the LCD03 protocol, the firmware works out of the box. For different LCDs, change /sketches/dipcoater.ino to suit your needs: change the include at the top to the desired library, then change the instantiation and initialization of the LCD in the code.

Power supply

Connect the 12V power supply both to the Arduino power input and to the motor shield power input.

Mechanical hardware 

From a local hardware shop, I got some ball bearings that are used for drawers (the thingies that make drawers slide in/out). I sandwiched these between aluminum strips which are held apart with some screws and bolts. This works amazingly well!

Running the dip coater

The idea behind this dip coater is that it can submerge something in a solution for a certain time, a second solution for a certain time, a third ... etc., while repeating this cycle as often as you want. The firmware (/sketches/dipcoater.ino) has few parameters to control the sequence, which are stated at the top of the code just below the includes. A short explanation follows.

Parameter	Type	Unit	Description
NUM_CYCLES	int	#	The total number of cycles the dip coater has to run.
NUM_STEP	int	#	The total number of steps in each cycle.
x_dist[NUM_STEP]	double	cm	Distance between each dipping container. Starting at X=X_START, the dipping sequence advances by x_dist(current_step) between steps.
x_wait[NUM_STEP]	int	sec	The number of seconds the dip coater is down (dipping) at each step.
X_START	double	cm	Starting position in the X direction. Should only be changed for a good reason (e.g. obstruction of the dip coater at X=0). If the first dipping jar is not at X=0, set the position using the first number in the x_dist[] array.
Y_UP	double	cm	Up position of the dip coater.
Y_DOWN	double	cm	Down position of the dip coater.

Look at this little video to get an impression:

Running your own program

The sketch /sketches/random/random.ino contains the bare minimum code, and does random walks over the entire range the actuators can span. Use this sketch if you want to build your own functionality into anything containing one or two linear actuators. IMPORTANT:If you change the linear actuator stroke (mine are 10 and 14 cm for the L12-P and L16-P, respectively), you have to change the TRAVERSE_X and TRAVERSE_Y constants in /libraries/XY/XY.h to match the maximum actual stroke difference you measure between fully retracted and fully contracted actuator shafts.

Fine-tuning the PID

Adjust the PID values according to the forces the actuators have to exert on the system you built them into. To do this, change the values in /libraries/XY/XY.cpp. Parameters given to the functions PIDX.SetTunings(0.1,0.1,0) and PIDY.SetTunings(0.1,0.1,0) are P, I and D, respectively. A quick guide to heuristic PID tuning can be found here.