Robotic Arm having two degrees of freedom Its development, applications and advantages over a conventional X-Y table

The X-Y Table A two guided system controlled by two stepper motors forms this assembly. A two guided system controlled by two stepper motors forms this assembly. A pointer attached to the top of the table can access any given vertex in a 2-D plane. A pointer attached to the top of the table can access any given vertex in a 2-D plane.

Major Drawbacks in the X-Y approach If another degree of freedom is to be incorporated, both X-Y tables will have to be moved/rotated together in another plane. If another degree of freedom is to be incorporated, both X-Y tables will have to be moved/rotated together in another plane. The cost of manufacturing such a robot for a simple application of accessing 2-D coordinates is quite large. The cost of manufacturing such a robot for a simple application of accessing 2-D coordinates is quite large. Both stepper motors need to be of same power as they are required to translate similar tables, thus power consumption is also large. Both stepper motors need to be of same power as they are required to translate similar tables, thus power consumption is also large.

The Polar Coordinate System To eliminate the shortcomings faced by the conventional X-Y table operated robotic arm, polar coordinate system is adopted, in which the X and Y coordinates are calculated in terms of radius r and angle θ by using the formulae given below: To eliminate the shortcomings faced by the conventional X-Y table operated robotic arm, polar coordinate system is adopted, in which the X and Y coordinates are calculated in terms of radius r and angle θ by using the formulae given below: X coordinate = R sin θ Y coordinate = R cos θ Where, R 2 = (x 2 + y 2 ) R 2 = (x 2 + y 2 ) θ = tan -1 (x/y) θ = tan -1 (x/y) To access any cell, the arm will rotate by an angle θ and then shall translate the pointer by “R-d” distance. To access any cell, the arm will rotate by an angle θ and then shall translate the pointer by “R-d” distance.

Designing the Arm A stepper motor of large power rating for getting angle θ. A stepper motor of large power rating for getting angle θ. A stepper motor of much less power rating for getting linear displacement R. A stepper motor of much less power rating for getting linear displacement R. A guide (grooved shaft) to convert rotary motion into linear. Also a table attached to this shaft. A guide (grooved shaft) to convert rotary motion into linear. Also a table attached to this shaft. Bearings to transfer the weight and to prevent horizontal displacement of the shafts. Bearings to transfer the weight and to prevent horizontal displacement of the shafts. A stand on which entire assembly can be mounted. A stand on which entire assembly can be mounted.

Rotary motion into linear displacement A grooved shaft attached to the motor from one end and to the table from the other. A grooved shaft attached to the motor from one end and to the table from the other. Roller guides to smoothen the movement. Roller guides to smoothen the movement. Bearing holder in which bearings are embedded and also to hold the motor. Bearing holder in which bearings are embedded and also to hold the motor.

How bearings help… Two bearings are embedded into the bearing holder from one end. The other end is coupled to the shaft/s. Therefore, only rotary motion is permitted and vertical/horizonta l movement due to improper handling can be prevented. Also all the weight is transferred to the ground and load on the motor is far less. Two bearings are embedded into the bearing holder from one end. The other end is coupled to the shaft/s. Therefore, only rotary motion is permitted and vertical/horizonta l movement due to improper handling can be prevented. Also all the weight is transferred to the ground and load on the motor is far less.

The Tables and Rollers To smoothen the linear movement, rollers are inserted in between the two plates/tables. To smoothen the linear movement, rollers are inserted in between the two plates/tables. One of the plate is fixed while the other travels over it. A screw couples the shaft and the moving plate.

The Rods/Shafts

Electronics Used (Initial attempt) Problems Faced Four BJTs and four power amplifiers were used to run a single motor. Four BJTs and four power amplifiers were used to run a single motor. Many resistors were required for biasing these transistors which made the circuit cumbersome. Many resistors were required for biasing these transistors which made the circuit cumbersome. Soldering was complicated owing to a large number of components and not feasible for controlling 2-3 motors. Soldering was complicated owing to a large number of components and not feasible for controlling 2-3 motors.

The Circuitry Employed

PWM for Current Control (How L297 and L298N help…) The L297 has two PWM choppers. In each chopper the flip flop is set by each pulse from the oscillator, enabling the output and allowing the load current to increase. As it increases the voltage across the sensing resistor increases, and when this voltage reaches Vref the flip flop is reset, disabling the output until the next oscillator pulse arrives. The output of this circuit is therefore a constant rate PWM signal. Note that Vref determines the peak load current. The L297 has two PWM choppers. In each chopper the flip flop is set by each pulse from the oscillator, enabling the output and allowing the load current to increase. As it increases the voltage across the sensing resistor increases, and when this voltage reaches Vref the flip flop is reset, disabling the output until the next oscillator pulse arrives. The output of this circuit is therefore a constant rate PWM signal. Note that Vref determines the peak load current.

What the circuit looks like

Circuitry set-up

The Robotic Arm

Advantages of this Robotic Arm The design is simple enough to incorporate subsequent degrees of freedom, since there is just one grooved shaft to be moved in other planes. The design is simple enough to incorporate subsequent degrees of freedom, since there is just one grooved shaft to be moved in other planes. The cost of manufacturing is greatly reduced owing to the use of just one shaft-controlled table. The cost of manufacturing is greatly reduced owing to the use of just one shaft-controlled table. The motors required are of different power ratings. The motors required are of different power ratings. The arm is lighter in weight and occupies lesser floor area to accomplish a given task. The arm is lighter in weight and occupies lesser floor area to accomplish a given task. A larger area can be swept, i.e. more cells can be visited by the arm even if the length of the grooved shaft is same as that of the X-Y table. A larger area can be swept, i.e. more cells can be visited by the arm even if the length of the grooved shaft is same as that of the X-Y table.

Areas of Applications It can be used to fill liquids of varying amounts in test-tubes that are arranged in a matrix. It can be used to fill liquids of varying amounts in test-tubes that are arranged in a matrix. It can also be used to make incisions at specified locations, the concept vastly applicable in PCB design, making engraved patterns on metal, soldering components on the PCB. It can also be used to make incisions at specified locations, the concept vastly applicable in PCB design, making engraved patterns on metal, soldering components on the PCB. Jobs like welding, picking/putting objects to a particular place, screwing nuts at inaccessible locations, etc can be easily accomplished using this robot. Jobs like welding, picking/putting objects to a particular place, screwing nuts at inaccessible locations, etc can be easily accomplished using this robot.

References Books on mechanical design of robots. (Machining Fundamentals: From basic to advanced techniques by John R. Walker) Books on mechanical design of robots. (Machining Fundamentals: From basic to advanced techniques by John R. Walker) Basic machines and how they work by Henry T. Brown. Basic machines and how they work by Henry T. Brown. Datasheets of L297 and L298N from S.T.Microelectronics. Datasheets of L297 and L298N from S.T.Microelectronics. Stepper motor concepts from study material taught in the college ( V.Deltoro) and internet. Stepper motor concepts from study material taught in the college ( V.Deltoro) and internet.