• Srabanti Chakrabarty

Chapter II Of Yethinder's Journey

One month in, I am right on track to complete the first phase of my project. I faced a few conundra and most of which involved the logistics. However, I couldn't avert from the enigmas and wanted to expedite my work.

The first of those problems was selecting the right servo motor, commonly known as servo(s). For a novice, they are a type of motor which can be precisely controlled (angles/position), unlike the conventional motors. Servo motors come in many sizes and three basic types. The three types include positional rotation, continuous rotation, and linear. Standard, hobby-grade servos used in projects are mostly positional rotation servos; the output shaft rotates in about half of a circle or 180 degrees and it has physical stops placed in the gear mechanism to prevent turning beyond these limits to protect the rotational sensor.

Such servos usually can handle only small loads but I wanted a servo that can resist larger loads (10-12 kg/cm). Conspicuously such servos are available on the internet, however, they weren't ready to ship to my location. I had to find a panacea for an unfortunate problem, so I bought a continuous rotation metal gear servo hoping that it would ameliorate my situation. I wanted a precise rotation of just 80 degrees upon giving a signal, but, I couldn't stop the continuous rotation servo at will like I would control a positional rotation servo.

Fig. 1: Internal construction of potentiometer used a position encoder

Generally, the positional rotation servos (180) have a potentiometer connected to the shaft which measures its absolute position, whereas, in the continuous rotation servos, such potentiometers are absent as they can't be rotated by more than one revolution. Thus, the servo's position cannot be measured by that means, instead, some manufacturers integrate a Hall Effect sensor and an encoder pin to track the position and the number of rotations. Unfortunately, the servo I bought did not have this feature. I surfed through the Internet to find a solution for the problem I faced and I soon stumbled upon a video where he controls the rotational direction of the servo using ASCII characters through Arduino. This still did not solve the issue of angle control. Moreover, it was very slow as it uses many reduction gears to achieve high torque. I realized it would not suit my application, hence, I returned it and made a special request to a seller to send me a 180 degree (positional rotation) servo with similar torque values (10-12 kg/cm).

In summary, positional rotation servos should work for most of the applications which require rotary position control. If a higher torque output is necessary, choose a metal gear positional rotation servo or a continuous rotation servo with dedicated sensors, however, bear in mind that they are slow, comparatively.

Yethinder Ragav

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