D.I.Y SINGLE ARM SCARA ROBOT

In this tutorial, I'd like to share how to build a homemade Arduino based SCARA Robot. My aim is to easily assemble on my own and learn about the robotic arm by using the simple materials in hand without having to purchase 3D printed parts.

Before getting started, please check my videos below

• First testing with project description.

• Updating the pen lift.

1. Main components:

• 1pcs x Arduino Uno R3.
• 1pcs x Arduino CNC Shield V3 GRBL.
• 2pcs x Stepper Motor Driver A4988.
• 2pcs x Stepper motor NEMA 17.
• 1pcs x 50 mm L Stepper Motor Support.
• 1pcs x GT2 6mm Closed Timing Belt 400mm.
• 1pcs x GT2 6mm Closed Timing Belt 200mm.
• 2pcs x GT2 Timing Pulley 20 Teeth.
• 2pcs x GT2 Timing Pulley 60 Teeth.
• 3pcs x 608ZZ Double Metal Seal Bearings 8x22x7mm.
• 2pcs x F608ZZ Ball Flanged Shielded Bearings 8x22x7mm.
• 2pcs x Round Bar Shaft Rod Diameter 8mm, Length 100mm.
• 1pcs x Aluminum Flexible Shaft Coupling, Inner Hole Size: 5mm x 8mm.
• 1pcs x Aluminum Flexible Shaft Coupling, Inner Hole Size: 5mm x 10mm.
• 1pcs x 8mm Linear Rail Shaft Clamping Guide Support.
• 2pcs x XH2.54mm – 4P 20cm Wire Cable Double Connector.
• 1pcs x Clear Acrylic, size A4.
• 4pcs x Empty plastic coil spool. I reuse empty plastic coils that is used to coil the soldering tin wires.
• 1pcs x Power Supply 12/24 VDC
• 1pcs x Power Supply 5 VDC.
• Some small cable ties, cable spiral wrap, bolts and nuts.

2. Tools

• Drilling Machine.
• Hot Glue Gun.

Firstly, I figured out how to arrange the components to form a robotic arm. The robot consists of two arms, named LINKAGE L1 and LINKAGE L2. The two linkage arms imitates the human arm. One joint acts as a shoulder joint (JOINT 1) and the second acts as an elbow joint (JOINT 2). Arm L1 is coupled to the first shoulder stepper motor, while arm L2 is coupled to the second elbow stepper motor and connected to the elbow, to which the pen (END-EFFECTOR) is connected.

All of the above components are placed on boxes made of acrylic. The box dimension is LxWxH = 165x100x75(mm). After careful measurements, I drilled some holes for mounting shoulder stepper motor and its gear transmission, as well as, some available holes for Arduino Uno plus CNC Shield.

There are many kind of tin wire plastic coils with different center holes diameter such as: 22mm, 21mm, 20mm, 19mm and difference height. In this project, I used 4 plastic coils type as follow:

• 1pcs x plastic coil with center hole diameter 22mm, height 55mm and outer diameter 55mm for shoulder bearings.
• 1pcs x plastic coil with center hole diameter 22mm, height 23mm and outer diameter 55mm for elbow bearings.
• 1pcs x plastic coil with center hole diameter 19mm, height 23mm and outer diameter 55mm for clamping the elbow round bar 8mm.
• 1pcs x plastic coil with center hole diameter 20mm, height 23mm and outer diameter 55mm for clamping the pen.

I inserted 2 bearings into center hole diameter 22mm at top and bottom of plastic coil height 55mm. I used a ball flanged shielded bearings at top of plastic coil hole so that it is tight and strong enough to keep the shoulder gear transmission.

Then this plastic coil was mounted to the robot acrylic base through 4 small holes.

Round bar 8mm x 100mm, GT2 timing pulley 60 teeth and round bar clamping support are connected as picture below.

The stepper motor L support was mounted on the round bar clamping support.

Two acrylic plates were cut with dimension 50x210mm, thickness 5mm to build the robot arms and I drilled some holes on them.

One acrylic arm and shoulder stepper motor were mounted on the L support. In order to make this shoulder mechanism firm enough, I adjusted and placed the stepper motor bottom located on the round bar clamping support.

The round bar 8mm were put into the bearings of plastic coil and I rotated this shoulder mechanism by hand to check whether it is good.

The shoulder stepper motor was mounted to acrylic box. The 60 teeth pulley of shoulder mechanism was coupled to the 20 teeth pulley of stepper motor by GT2 timing belt 200mm. Then I checked its rotation again.

To ensure the mechanism can't move up and down, I locked the round bar at plastic coil bottom by one pulley 8mm hole diameter.

To build the elbow of robot, I clamped the flexible coupling 5x8mm to the remaining round bar 8mm. I used the second plastic coil with height 23mm and center hole diameter 19mm then inserted the flexible coupling 5x8mm with round bar into the plastic coil hole. Because the outer diameter of flexible coupling is also 19mm, so it is very tight when I inserted it into plastic hole.

Two bearings were inserted into center hole diameter 22mm at top and bottom of the third plastic coil and prepared the 60 teeth pulley for the robot elbow.

All acrylic plates, 2nd and 3rd plastic coil, 60 teeth pulley were connected together. The elbow stepper motor was coupled to 60 teeth pulley of elbow mechanism by GT2 timing belt 400mm.

I checked the shoulder & elbow rotation and tightened all bolts.

Finally Arduino Uno and CNC shield were mounted to the side of acrylic box and connected the wires from A4988 to the stepper motors.

For pen clamping, I used one plastic cable glands. By this way, I can do hand-tightening a pencil easily.

To keep the pen tighter, I used one more plastic coil and inserted 2 cable glands at top and bottom of center hole.

This is another arrangement of the elbow stepper motor. It is mounted on top.

To build drawing surface, I reused my kid's plastic chess box. After measuring the robot height, I glued 6 PVC pipe straight connectors diameter 27mm at the chess box bottom.

Inside the chess box, I glued 4 neodymium magnets taken from old HDD drives and they are located following to the A4 paper size.

I prepared A4 paper for testing.

Done. I didn't assemble the pen lift part because I wanted to test how the SCARA robot arm works.

After checking robot arm working, I decided to build the pen lift part as follows:

• Preparing the fourth plastic coil with center hole diameter 19mm, height 23mm and outer diameter 55mm and drilling 4 holes on top.

• Inserting a flexible coupling 5x10mm into center hole at bottom of plastic coil.

• Putting one ballpoint pen core into 5mm hole of flexible coupling and it was locked at the 10mm hole side.

• I glued a servo at bottom of plastic coil.

• I connected 4pcs x long bolt M3x40mm from plastic coil to elbow arm. We can easily adjust the height of pen tip by this way.

I also glued the robot acrylic box to plotting surface - chess box to prevent the robot arm moving when it works.

I have referenced to many articles/ codes as well as comments on some robotic forums to learn about how to program a robotic arm.

We need to do the following steps to install SCARA-GRBL firmware for Arduino Uno

• Download SCARA-GRBL firmware files.
• Copy GRBL to C:\Users\Administrator\Documents\Arduino\libraries\
• Open Arduino IDE, from File menu click Examples ‣ SCARA-GRBL ‣ grblUpload.
• Select the correct Port and Board (Arduino Uno), Compile and Upload the code to Arduino Uno.

1. GRBL parameters for my SCARA single arm robot are as follows:

Notes: Setting $1 = 255 keeps stepper motors always enabled and prevent the motors from moving when the robot arm is stationary. We can use this command to hold the axis, otherwise the vibrations may cause a drift.

2. STEP per DEGREE setting:

Normally, the command to stepper motors via GRBL firmware is given as Cartesian coordinates (X, Y) in mm, and base on parameter STEP/MM it is calculated to number of steps instead of angles, so it is necessary to convert angle into number of steps.

A Steps per Degree (SPD) parameter is defined for each stepper motors in order to determine the number of steps is required to move per degree. The SPD for each motor is dependent on the step angle, gear ratio and micro-stepping ratio of the drivers for the steeper motors according to the formula.

Steps per Degree = 1/((Steps Angle)*(1/Micro-steps)*(Ns/Nd))

GRBL $100 & $101 are calculated by following table: Two stepper motors have a 1.8° step, which means 200 steps for 1 complete revolution. Each motor is geared down with timing belts by factor 3 (20/60). With the A4988 drivers set to 1/8th micro-stepping that makes 13.3 steps per degree.

Robot kinematics are mainly including two types: forward kinematics and inverse kinematics. In forward kinematics, the length of each linkage arm and the angle of each joint are given and we have to calculate the position of robot end effector. In inverse kinematics, the length of each linkage arm and position of end effector are given and we have to calculate the angle of each joint.
The following code performs conversion from Cartesian coordinates to Scara angles which is called Inverse Kinematics

void inverse_kinematics(float const *cartesian, float *f_scara)
{
    float SCARA_pos[2];

    static float SCARA_C2, SCARA_S2, SCARA_K1, SCARA_K2, SCARA_theta, SCARA_psi;

    SCARA_pos[X_AXIS] = -cartesian[X_AXIS] - SCARA_OFFSET_X;
    SCARA_pos[Y_AXIS] = cartesian[Y_AXIS] + SCARA_OFFSET_Y;

    SCARA_C2 =   ( sq(SCARA_pos[X_AXIS]) + sq(SCARA_pos[Y_AXIS]) - L1_2 - L2_2 ) /(2*L1*L2);
    SCARA_S2 = sqrtf( 1 - sq(SCARA_C2) );

    SCARA_K1 = L1 + L2 * SCARA_C2;
    SCARA_K2 = L2 * SCARA_S2;

    SCARA_theta = ( atan2f(SCARA_K1, SCARA_K2)-atan2f(SCARA_pos[X_AXIS],SCARA_pos[Y_AXIS]) ) ;
    SCARA_psi   =   atan2f(SCARA_S2,SCARA_C2) + SCARA_theta; 
		
	if(!angle_mode)
	{
		f_scara[X_AXIS] = DEGREES(SCARA_theta);
		f_scara[Y_AXIS] = DEGREES(SCARA_psi);
		f_scara[Z_AXIS] = cartesian[Z_AXIS];
	}
	else
	{
		f_scara[X_AXIS] = cartesian[X_AXIS]; 
		f_scara[Y_AXIS] = cartesian[Y_AXIS]; 
		f_scara[Z_AXIS] = cartesian[Z_AXIS];		
		
	}
}

The inverse kinematics determine the angles THETA and PSI of Joint 1 and Joint 2 respectively to bring the end-effector to the position setpoint (Px, Py). Cartesian coordinates of the desired end effector position are entered in my Excel sheet which calculate the angles and simulate by graph.

If the Cartesian coordinates of desired end-effector location are given by P(199.73, 217.97) and both L1 and L2 linkage lengths are 160 mm, the shoulder angle THETA and elbow angle PSI is calculated and shown by picture below.

Inverse Kinematics Simulation

I wrote a small SCARA simulation in Excel template to check the forward & inverse kinematics rules based on the Arduino code above. For the inverse kinematics, there is a little difference between Excel template and Arduino code, that is coordinates X is reversed in the Arduino inverse kinematics code. When coordinates X is reversed, it is shown like below.

1. Calibration

In the file "scara.h" we have to edit the parameters according to our configuration.

#define SCARA_LINKAGE_1 	160.0f	// mm
#define SCARA_LINKAGE_2 	160.0f 	// mm

#define SCARA_OFFSET_X 		-245.0 	// mm
#define SCARA_OFFSET_Y 		85.0	// mm	

#define MANUAL_X_HOME_POS 	0.0f	// Theta
#define MANUAL_Y_HOME_POS 	0.0f 	// PSI
#define MANUAL_Z_HOME_POS 	0.0f

The pen lift is controlled by pin 13 (PORTB - BIT5) on Arduino Uno, it is declared in "cpu_map_atmega328p.h".

2. Testing

To generate the G-code from text or image, I used Inkscape software. And after we have an executable G-code file from Inkscape, to stream and send G-Code file to Arduino Uno, we can use Universal Gcode Sender - UGS.

My project used 2 stepper motors and A4988 drivers. You can refer to my instructable: BACK TO BASIC-MINI CNC PLOTTER at STEP 5 for setting up micro-stepping and current limit of stepper driver A4988.

I tested how the robotic arm can do the text writing.

And here is the result.

And this is my test with pen lift updating.

Actually, my robot arm hasn't been properly calibrated yet but I love this writing style. The characters are tilted and curved depending on the "home" position, looking like artistic text.

You can see some more pictures of this project.

Thank you very much for reading my work and hope you enjoyed my article this time!