Servo - November 2012

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servos. Boats could use almost any size, as could most R/C cars. Robot builders had to paw through a pile of different types in the early days to find a servo that had enough torque for their unique builds.

So, it was the analog servo that we first used. With an input pulse train of about 1.0 to 2.0 millisecond pulses, these little plastic boxes could interpret pulses from a receiver (or microcontroller) and rotate the output shaft back and forth from about 90 to 120 degrees or more, depending on the servo and its programming. With 50 20-millisecond pulse sequences per second, longer pulses would cause the output shaft to proportionally turn one way, and shorter pulses the other way. The 50 pulse sequences resulted in 50 proportional voltage pulses to the servo’s motor — either one polarity or another at a variable voltage.

Internal potentiometers gave an internal feedback to the servo’s circuitry as to direction and speed of the output shaft. Builders soon connected electronic speed controllers (ESCs) to the output of an R/C receiver and could control a motor with the received pulse trains. These ESCs were first used in boats and cars before being utilized by robot experimenters.

FIGURE 2. My tabletop with a wide variety of servos.

FIGURE 3. Robotis Dynamixel daisy-chain.

like the digital servos as they react faster and don’t seem to have that Parkinson’s buzz when not moving. The main disadvantage to digital servos is the higher current draw, plus they’re a bit costlier.

The Digital Servo

Digital servos look identical to their analog brethren and receive the same pulse trains, but they use a microcontroller to process the pulses into a higher frequency series of output pulses to drive the servo’s motor. The resulting servo output has a more constant torque level and almost no ‘dead-band’ movement.

Users can feel the dead-band in an older analog servo when they try to turn the output shaft/horn with their fingers and feel a bit of slop back and forth. They can also feel the buzzing of the output caused by the lower frequency pulses, whereas the digital servo moves very little when a signal is applied; the higher frequency buzz of up to 300 Hz is negligible.

Humanoid robot designers seem to

Dynamixel and Herkulex Servos

Back in 2005, I first saw a Bioloid robot made by the Korean company, Robotis, and its use of unique servo modules known as Dynamixel rotary actuators (more on this later). Standard servos utilize a three-wire connection, a power line, a neutral power line, and a signal line. Most complex robots such as humanoid bipedal robots or hex walkers use as many as 18 or more servos, and each servo must have the two power lines and a signal line.

Builders found that they could parallel the two power lines but were still required to have 18 signal lines running from their controller — one for each servo. The Dynamixel does not require this and interconnection is daisy-chained as shown in Figure 3.

These servos are much more than just a regular type in that each has a built-in microcontroller with an individually addressable ID. Plus, they have the ability to send information back to the controller as to speed, torque, shaft position, temperature, voltage, and load factors. There are 13 different Dynamixels in the Robotis lineup that we’ll review here.

Other Servo Factors

Other factors to take under consideration when selecting servos are the motor driving circuitry and electrical characteristics, as well as the mechanical aspects of the overall servo design. Earlier servos used what is called a three-pole motor armature with three distinct sets of wirings on three laminated metal poles.