Servos Explained

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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

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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.

* Some Sigma 5 rotaries with a pressure switch (P11, P12, P13, P14. P15, P16, and P17) can be modified to run CHC as P3 or P4. HSA - 5/14/

** Dual axis rotaries can not run on 18.29A. If a machine is using a dual axis rotary, it should not be upgraded past 18.28A. 

*** 100.21.000. and higher can run all Sigma 7 rotaries, but the ROT.zip file may need to be updated.  The procedure to do this can be found in the Rotary Service manual chapter 2.7 Update Rotary Configuration Files - NGC. Older NGC versions can run Sigma 7 rotaries if the model is present in the control.

Note: NGC mills can always be upgraded to the latest NGC version. Instruction on updating NGC software can be found in the NGC service manual Chapter 4 Software Update. 

Note: CHC mIlls with 17 series and older software can not upgrade to 18 series. 18 series requires door interlocks that are not present on older machines. 

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