hi guys,

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i'm a beginner in robotic. only knw a bit about coding (programming).

anyway, i'm starting my project today, 6 arm robot with a gripper, 7 servo actually as i need 1 small servo to control the gripper to hold the load. the length of the arm would be 70cm. i'm expecting to be longer like 150cm total with payload 1kg.

here is my design in attachment. sorry it look like back to 19th century but yes, i'm a programmer, not a cad and even i don't have any idea about autocad and 3ds max. perhaps i should giveit a try next time. fortunately i have a nice backup... a lovely workshop which will do my nasty design into reality

and here is the servo i got from the market (in the attachment also), with D shaped shaft, r: 4mm.
those servo are 100kgfcm (10Nm) and 180kgfcm (18NM), daisy chain, digital, the seller claimed that the servo can give the feedback about bla bla bla which i just need the current angel position. but the servo don't come with a specific built controller instead of normal H bridge or kind of servo shield for arduino uno or mega. not sure.
*in short: i don't know about how to choose the correct position sensor/solution
**sure... the seller is cute by saying "ouw... our servo is accurate, digital, digit, can connect to other servo like daisy chain and u can call each servo by each ID". honesty, i don't have experience on this but still it seems this kind of servo is my best bet to go for this project. what do you think? any opinion would be apreciated. i'm a beginner.

anyway, i have basic several questions.

1. arm joint construction. it seems i have problem. the servo looks wow with that torque but after i did my simple calculation, gosh... i need even bigger torque around 50NM or even more if i use longer arm. should i use pulley and belt solution with let's say 10:1 ratio? then how can i detect the recent angle of my arm as this is my main concern for robot arm. that's really painful. any idea about this? 180kgfcm using belt and pulley 10:1 so i can have kgfcm in the shoulder? perhaps there is some generous guys here want to share a bit about belt and pulley implementation as i'm absoluetely zero in this field. in my dummy head, it's only "attach the servo with horn, then screw it in one side of the arm". that works if the arm is small but just wondering how to synch the rotation between right side and left side of the arm chassis? using rod? pipe? shaft? i don't even know the name of the components to move the joints.

2. can i move the robot arm manually by hand, for teaching? so i can record the position and save it for certain movement set. i see servo.read() can only return the previous position but not the actual angle if i move the arm manually by hand, even when the power is on (power must be on coz i need to keep the arduino alive and my best bet is i move the hand and arduino record it if i press kind of save button )

that's all guys. please share your idea. i'll use arduino due at 3.3V for the sake of more computing power and not sure, i'll bet for AC servo with 1 controller for each servo. but i don't have any idea how to wire/connect all 7 controllers to 1 cute tiny arduino. i wish i can stick with my first plan and go with that.

thanks guys in advance and pls help for opinion, strategy like position sensor, or anything.

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Servos Explained

A servomechanism (servo) can refer to quite a few different machines that have been around longer than most may realize. Essentially, a servo is any motor-driven system with a feedback element built in. Servos are found everywhere from heavy machinery, to power steering in vehicles, to robotics and a wide variety of electronics.

Here we take a look at the three main servo sizes, and a couple of quick and simple builds to demonstrate what servos can do.

How does a servo work?

If you open up a standard hobby servo motor, you will almost always find three core components: a DC motor, a controller circuit, and a potentiometer or similar feedback mechanism. The DC motor is attached to a gearbox and output/drive shaft to increase the speed and torque of the motor. The DC motor drives the output shaft. The controller circuit interprets signals sent by the controller, and the potentiometer acts as the feedback for the controller circuit to monitor the position of the output shaft. Nearly all hobby servos have a standard three-pin, 0.1&#;-spaced connector to power and control the servo. The color coding can vary between brands, but the pins are almost universally in the same order. When combined together, you can power and control the direction, speed and position of the output shaft with just three wires.

Inside a standard hobby servo

Controlling a servo

In order to move a servo to a position along its movement arc, or, in the case of continuous rotation servos the speed and direction of the motor, the controller needs to send a precisely timed signal for the servo to interpret. Typical hobby servos expect to see a pulse every 20ms, and the width of this signal determines the position. This width is usually between one and two milliseconds. This type of signal control is frequently referred to as Pulse Width Modulation, abbreviated as PWM. A servo controller will normally be a dedicated piece of hardware that can take inputs from other components like a joystick, potentiometer or sensor feedback to set the control signal for the servo. Other control options include using the PWM-capable pins on a microcontroller to send that signal directly to the servo.

A servo being controlled by SparkFun 9DoF IMU Breakout

Powering a servo

Depending on the size and torque output of your servo the input voltage will vary, but most hobby servos will work fine with 5V from your preferred microcontroller or battery circuit. More important than voltage is the current draw a servo can pull while moving and with a load attached. When unloaded, a common hobby servo can pull as little as 10mA, but larger servos under load can pull in excess of an Ampere or more. It is important to check the specifications of the servo you intend to use to make sure your power supply has the proper voltage range and can deliver enough current to move the servo with your load attached.

Let's take a look at an example :

An exerpt from a datasheet for the Hitec HS-422

We've pulled out a few key power specs from this datasheet for the Hitec HS-422 (Standard Size) servo. The first thing to notice is that this servo lists a Test Voltage, often labeled Operating Voltage, from 4.8V to 6V. The datasheet shows that at 4.8V the servo can move 60° at a speed of .21 seconds without any limiting force factors (load). Also operating at 4.8V, this servo motor can drive a load up to 3.3kg/cm (Stall Torque). At 6V, the high end of the test voltage, the servo can move 60° at a speed of .16 seconds with no load, and has a higher torque limit of 4.1kg/cm. It is worth noting that any measurement between no load and the stall torque spec will more than likely slow down the operating speed listed.

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