Project
Robotic
Hand
(Shado)
Gokul Kishore r
Karthik r
Lokesh t
Before enrolling ourselves in kaizen robotics programme, we didn’t have much idea on robotics. We
thought only Droids came under robotics and we would build one during the course. But it wasn’t
too late before our thoughts were destroyed. The very first day in lema labs, they made us realise
that droids alone are not robots and the course mainly highlighted on logics in robotics with the help
of a Motor driven Car. Yet we wanted to build a Human droid and it was one of our many dreams
which we started working on. The Level 2 programme set us a more matured platform to accomplish
the project. The first thing we made up in our minds considering several problems we had, was to
build a part of the droid first, a single part, a hand.
And that is how we started working on the project. The Walkthrough (Documentation) of our Project
is as follows:
Gate 1- The decision:
We needed to decide the way in which we were going to control the hand. We have already worked
on Gesture controlled, computer controlled and remote controlled Robots. But we thought none of
them would meet the requirements of input to the hand precisely. After surfing through the
Internet, we finally decided to work with Flex sensors – use of which, would make the robotic hand
more like a shadowing or imitating type.
Gate 2- Study on Flex Sensors:
Similar to other Sensors, the flex sensor also gives out analog output. They work as a voltage divider.
Basic Principle:
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Flex sensors are sensors that change in resistance depending on the amount of bend or
Flexure on the sensor.
They convert the change in bend to electrical resistance- the more the bend, the more the
resistance value.
Availability:
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They are available both as unidirectional and bidirectional, with varying resistance ranges
and varying Sizes (2.2” and 4.5”).
Selection:
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Based on the analysis on the requirement of the robotic hand, we decided the following
1. Unidirectional sensors were chosen, as the fingers bend only in one
direction.
2. 2.2” were chosen, as the size would be enough for an average sized human
finger and also due to the lower cost of the 2.2” flex sensor in comparison
with the 4.5” sensor.
Gate 3- calibration:
After buying the sensors, we had to check the resistance values of the sensors in the flat and the
fully bent position of the sensors and corresponding minimum and maximum values of resistances
were noted.
By applying the voltage divider rule, we got the output voltage.
Vo = Vi [R1/(R1+R2)]
Where,
Vo is the output voltage
Vi is the input voltage (5v)
R1 is the variable resistor (Flex sensor)
R2 is the Constant Resistor
The Circuit for the flex Sensor is as follows:
The Resistor R2 is chosen in such a way that the range of the output voltage is the most largest with
respect to the already measured minimum and maximum values of the variable resistor R2 (Flex
sensor).
The calculation made for choosing R2 is shown in the image below.
Gate 4- Study on Servo Motors:
Servo motors are DC motors. They have builtin gearing systems and feedback control loop circuitry.
This error sensing feedback control means that the performance of the system can be corrected by
feeding the output of the system as inputs in a closed loop system and thereby reducing the errors.
Hence they are used for accurate angular positioning. They are small in size but are very efficient in
energy.
The circuitry along with the potentiometer makes the servo to rotate according to the
commands.
Parts in a Servo Motor:
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DC motor
Potentiometer
Gear arrangement
Circuitry
External Attachments:
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Servo cable
Shaft
Servo horn
Cover
Pins of a servo:
The servo motors have three wires, two of these wires are to provide ground and positive supply to
the DC servo motor. The third wire is to provide the control signal. These wires of a servo motor
have specific Colours.
1. Red – Vcc(5v)
2. Brown GND
3. Orange (PWM signal) or White
The Orange wire is to send a logic which is square wave to the servo at a specific Duty cycle and the
servo goes to a particular angle i.e. it directly maps to servo angle. Servos are controlled by sending
an electric pulse of variable width or PWM through the control wire.
We needed Five of these, one for each finger.
Gate 5- Mapping of Servo Motors:
Mapping is used to convert the value of the obtained output analog signal from the Flex sensor (0 to
1023) with respect to the angle of the Servo Motor (0 to 180).
But the flex sensor analog values does not actually vary from 0 to 1023 after connecting it in the
voltage divider circuit. So, the Minimum and the Maximum values of the Flex sensor, at it’s normal
and the fully flexed positions are obtained using the Serial monitor and those values were mapped
to 0 to 180 of the Servo motor.
Programme syntax:
map(value, from low, from high, to low, to high);
Gate 6- Components used from the LL kit:
Breadboard, Battery, Voltage regulator, switch, Microcontroller (ATMEGA 16), Programmer, crystal
oscillator, wires, etc. were already available in the Robotics Kit provided by Lema Labs.
Gate 7- Fabrication of the Robotic hand:
Material used1. Hand
2. Rectangular wooden block
3. Anabond ( Adhesive, superglue)
4. Gloves with slots cut on it
5. Paper tape
6. Fishing string
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The Robotic Hand structure was bought online.
The hand and the Servo motors were super glued to the wooden block.
Using fishing strings, connection between the fingers of the hand and their corresponding
servo motors were made.
We also tried:
We tried to build our own hand structure using cycle chains (mild carbon steel) super glued
at alternate joints to act as links of the fingers. We succeeded in designing the fingers, and
the palm using a wooden block cut with slots for inserting the fingers. As the 9g servo motor
was not able to produce enough torque to pull the fingers, we bought a readymade plastic
hand. A bigger Servo motor would have been compatible to produce enough torque, but the
idea was dropped due to the higher cost of it.
Gate 7- Circuit Connections:
Gate 9- Debugging:
The problems Encountered and the way in which we sorted them out are discussed below:
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After the Complete setup, the hand was given supply and checked for its proper working.
When the robotic hand didn’t respond properly with respect to the motion of our hand, the
position of the flex sensors inside the glove was first checked.
After adjusting the positions, the movement was again checked.
When there was still a non-synchronisation between the motion of the robotic hand and
that of ours, the program was checked and it was found that the ranges specified in the
mapping of the servo should be changed.
Trial and error method was followed to find the suitable values and the calibration was
successfully finished, which resulted in perfect working of the Robotic hand.