Rajiv Panday
Stephen Carter

ME 5642 - Mechatronics
New York University - Polytechnic School of Engineering


Abstract

A conceptual proof of concept experiment was designed to improve the portability and decrease the cost of the industrial CNC Plasma Cutter. Methods included triangulating the mechanism upon which the plasma cutter traverses along the work area. A microcontroller with a pair of stepper motors, lead screws, analog potentiometers, and limit switches were used to control the device. G-code was employed to send the path commands to activate the system. As a result, a sketch of “NYU” was demonstrated in a continuous smooth motion.


Introduction

In the contemporary workforce, the manipulation and manufacturing of heavy metals and alloys are critical to the many industries that uses these metals. Precisely formed metal components are used in transportation industries to fabricate automobiles, aircraft, buildings, bridges, robots, and tools. Metals are used simply because of their malleability and hardness. Therefore to manipulate and form metals into specialized components, it necessary to use a powerful and precise machine such as a plasma cutter. The plasma cutter works by sending a pressurized gas, such as nitrogen, argon, or air through a small channel that contains a negatively charged electrode. When power is applied to this negative electrode and its tip touches the metal piece, a circuit connection is created. As a result, a spark is created that heats the channeled gas until it reaches the plasma state. A shielding gas or a high pressurized burst of air is also used around the plasma beam to regulate this unstable state. At this state, the plasma’s temperature can reach 30,000o F moving at 20,000 feet per second, powerful enough to melt and cut away the metal.

As shown in Figure 1, the plasma cutter itself is computer numerically controlled (CNC) by a robotic arm. A pre-constructed shape is loaded into the CNC software and the plasma cutter automatically cuts it without the need for human interference. A Cartesian coordinate system is used to traverse from one point to the next. To traverse in this way, the plasma cutter glides horizontally along a rail which then also moves vertically by the two guide rails positioned at the sides. This system is usually placed on a CNC cutting table to provide stability and control the torch head resulting in greater precision.

Figure 1 – Robotic Arm of CNC Plasma Cutter

Figure 2 – Baileigh 2’x2’ CNC Plasma Cutter

Figure 3 – Preliminary Sketch

Figure 4 – Triangular Coordinate System

Figure 5 – Path Movement

¼”-16 ACME Threaded Rod – used as the lead screw to translate turning motion to linear motion. The ACME thread form a 29o thread angle which can carry a heavier load and offer greater precision.

 

¼”-16 ACME Hex Nut – employed as the fastening mechanism to the ¼”-16 ACME Threaded Rod.

NEMA 17 Stepper Motor – this stepper motor is bipolar and has 1.8o per step, or 200 steps per revolution. The shaft features a five millimeter diameter and is twenty four millimeters long. It is rated at 12V and requires 350mA of current.

NEMA 17 EasyDriver Shield– this driver is used to regulate the speed and torque of the stepper motor.

1K Linear Taper Rotary Potentiometer – these potentiometers work as a voltage divider which creates a signal that can be linearly mapped to the specific angle created by the base and the given lead screw.

FunduinoUno R3– this microcontroller board features fourteen digital input/output pins, six analog inputs, operates a 5V and can source 40 mA of current.

Proto Shield Prototype Kit Shield - platform to easily connect the Funduino Uno microcontroller to all the necessary external circuit components.

Momentary Push Button Switch – S.P.S.T. Normally open pushbutton switch will act as the limit switch to prevent the stepper motors from rotating within a certain angle range and ensure that the lead screws do not fall out of the mounting hex nut.



AC Adapter Power Supply – provides regulated voltage to power the prototype system.

Electrical Outlet Power Strip – feature an on/off switch to be simulated as the emergency switch to disable the stepper motors.

CNC Motor Shaft Coupler – this aluminum coupler is used to link one end of the threaded rod to the shaft of the stepper motor.

Angle Iron – L shaped bracket used to mount the stepper motor.

3" Lazy Susan Swivel Bearing Turntable – this bearing is connected to each angle iron stepper motor mount and will hold the drawing utensil.

Square Tubing – houses the potentiometer and act as the base upon which the hex nuts will be fixed.

Suction Cup – Restraining mechanism upon which the potentiometer is allowed to rotate. Each suction cup is connected to the lead screw/nut setup thereby fixing the distance between them.

Hose Clamp – tightens the potentiometer to the suction cup.

Steel Washer – Acts a limiting device upon which the limit switch will be activated when pressed.

Multicolor Wire – connects electrical parts to the microcontroller.

Expo Dry-Erase Marker, Ultra-Fine Point, 4-Pack – will be used to simulate the plasma cutter.

Plumbing and Barbed Hose Fitting – used to hold and support the dry-erase marker upright.


Bill of Materials

From Table 1 below, a comprehensive list of the necessary parts and materials, as well as the quantity and cost for each item is compiled. Our total project cost was about $140, which is significantly cheaper than many current industrial CNC plasma cutters of similar cutting areas.



Fabrication and Assembly

After all the necessary parts and materials were gathered, they were assembled using the appropriate tools and machining equipment. First, the holder from each suction cup was removed so that the potentiometer could fit into the grooved socket. Next, using a drill press, holes were drilled in the interior ground plane of the square tubing with a milling machine to fit the potentiometer through. A washer was welded on the backside of the square tubing to hold the limit switch which was secured with a lock washer and corresponding nut (Figure 10). The pair of hex nuts were evenly spaced and welded to the exterior top of the square tubing. As evident in Figure 10, the lead screw can now be feed through the pair of hex nuts and fixed into the suction cup through the potentiometer.






Next, another hex nut was welded to the steel washer which was attached to the back end of each lead screw (Figure 11). Soon after, the horizontal base of the L-shaped angle irons were welded to two consecutive corners of the turntable in such a way that the continuation of line from each lead screw passes directly over the center of the inner circle. This was to ensure that the system can be easily calibrated to a 45o-45o-90o triangle upon initialization. Subsequently, the stepper motors were screwed on to the vertical base of angle iron through which the shaft was linked to the respective lead screw by a shaft coupler.


Wiring

After the design was fabricated and assembled, the appropriate wiring had to be completed so that it could be integrated with the microcontroller. First, the stepper motors had a six wire configuration but only four are used for a bipolar motor configuration. These four wires are connected to the Easy Driver as should in the top right. Next, all ground and power connections are illustrated in black and red wiring respectively. Each Easy Driver has two controls for the stepper motor: step and direction. Furthermore, each potentiometer is connected to the analog inputs. In addition, limit switches, A and B, are connected to Pin 2 and Pin 3 respectively. See Figure 12 for the entire circuit and wiring diagram.

Line Algorithm

Because our design was triangulated from the Cartesian coordinate system, the optimization of the movement from the current position to a target coordinate required some trigonometric analysis. First, the current position was defined with the rectangular coordinates (cx,cy). Similarly the target position’s coordinates were (dx,dy). The value of the (cx,cy) is (0,0) as the algorithm is in a relative mode and each movement is broken down to each step the motor takes. Thus to take the next step, there are four options along which the marker can travel. They are labeled as Ain, Aout, Bin, and Bout. Ain refers to motor A that retracts lead screw A “inwards” toward the base. Aout denotes to motor A extending lead screw A “outwards” towards the top. Similarly, Bin refers to motor B retracting lead screw B “inwards” towards the base and Bout denotes motor B extending lead screw B “outwards” towards the top. These positions are labeled as 1, 2, 3, and 4 in Figure 15. Next, each of these step sizes are resolved into their x and y components and thus form triangles that all have the same point in common, which is the current position. Because of the high resolution of the stepper motors, these triangles can be approximated as right triangles that share the angle the lead screw makes with the base. Thus the respective angles A and B are labeled in each of these four triangles in Figure 15. Using the trigonometric functions, the distances of each of these components can be calculated and thus the coordinates of these components is known. Next, the distance formula is used between Point (dx,dy) and each of the four options to eliminate the options that diverge the marker away from the target coordinate. Of the two remaining options, the option that stays closest to the line between the initial and target coordinates, as indicated by the green line, is selected. To do this, the orthogonal projection of each of these options to the line is calculated as shown in red in Figure 15. The option corresponding to the shorter orthogonal projection is chosen. As a result, the marker would step to that option and this algorithm repeats until the target coordinate is reached. In this example, to translate from the current position (cx,cy) to the next step, motor A would step outwards to Point 2, while motor B remains stationary as both motors cannot step simultaneously.




Calibration Algorithm

Another crucial part of our design, was to properly calibrate the mechanism to an initial starting position. To do this first requires calibration of the analog potentiometers, which can register an analog output range from 0 to 1023. Each potentiometer was mapped to its given range of motion. Because of the symmetric properties of the 45o-45o-90o triangle, our lead screws would be initialized to this configuration. As defined in Figure 16, first the stepper motors would fully extend both lead screws outwards. Then the stepper motors would retract both lead screws at the same rate to preserve the isosceles triangle configuration. During this process, the potentiometers are monitoring the angles of the triangle. In addition, each step that each motor takes to retract is counted to determine the lengths of the corresponding triangular configuration. When both potentiometers reads 45o, the stepper motors stop stepping and the lengths of the 45o-45o-90o are calculated and the coordinates of the marker are initialized to (0,0). It is important to note that this calibration to the initial point is occurring as long as either potentiometer does not read 30o at which point the corresponding code triggers a fault and the operation ceases.




Serial Interfacing

For the scope of the design, a simple sketch of the letters “NYU” was demonstrated. This movement was controlled with G-code, a computer numerically controlled programming language that tells the machining tool where to move, how fast to move, and through what path to move. To use G-code for this application required vectorizing the letters “NYU” as shown in Figure 17. These coordinates are listed in order that traces the lettering from left to right as shown in Figure 18.


















Pros vs. Cons



Conclusion

The CNC Plasma Cutter is an invaluable industrial tool used to fabricate many metals and alloys. Traditional CNC Plasma Cutters are bulky and quite expensive with the smallest footprint starting upwards of a few thousand dollars. Additionally, the mechanism traverse on a Cartesian coordinate system relying on robust horizontal and vertical guide rails. Our CNC Prototype design is triangulated with a pair of stepper motors coupled with lead screws which eliminates the need for guide rails and increasing portability as well as functionality. An analog potentiometer corresponding to each stepper motor is used to monitor the changing angles of the triangular configuration. Code is used to deactivate the system when either lead screw is 30 o or less with the base of the drawing areas and thus the operation limits become unsafe. A self-calibration mechanism was employed to initiate the configuration to a 45o-45o-90o triangle. Subsequently, the mechanism undergoes a line algorithm to optimize the transition from one step to the next. G-code was used to send the path commands from one coordinate to the next. For our demonstration, a sketch of “NYU” was drawn in a continuous motion. For further improvement, a linear servo motor could be implemented to actuate the marker along the z-axis so overlapping lines could be avoided during the sketching operation. The prototype proved the concept and lines were drawn as they would be with a conventional coordinate system. The portability is key and this system can easily be transported into the field where others are immobile.


Applications