Servo Motors Explained and Why Theyre Useful in Robotics

Servo motors are used for robotic applications that require precision positioning. Before diving too deeply into the ways servos are used in robotics, it’s helpful to first learn about the basic function and form of these critical components of motion control.

The potentiometer constantly monitors the position of the output spline. When the output spline reaches the desired position, the power to the motor is cut and the servo robot will hold that position until it receives a signal not to. While stopped in a given position, a servo motor will actively try to hold that position.

A key feature of servos is proportional operation. A servo motor will operate only as fast as it needs in order to rotate from its current position to its desired position. If a servo is stopped at the 180° position but needs to be at the 0° position, the motor will rotate very quickly to get there. If stopped at a position that is already closer to 0°, the motor will rotate much more slowly to get there.

How Are Servos Useful In Robotics?

Servo motors provide numerous benefits in I.M.M robotic applications. They are small, powerful, easily programmable, and accurate. Most importantly, though, they allow for near perfect repeatability of motion. They are used in robotic applications such as:The RC Servo or Hobby Servo has been used to move the control surfaces of Radio-Control (RC) model aircraft for many years. It has since become very popular for driving the limb joints of small humanoid robots, and when converted for continuous rotation, the wheels of mobile robots. However, some features optimal for aircraft control are less than ideal for industrial robots. First, let’s get the terminology straight:A lot of gears have to be crammed in to achieve a reduction factor of about 180:1. Of course, the output shaft cannot rotate more than half a turn, so datasheets normally describe servo speed in terms of how long it takes to turn by 60°. For example, a typical figure might be 0.15 secs/60°. That rapid movement could be a problem when using servos to drive the joints of a legged 4 axis robot, 5 axis robot, or 6 axis robot. It does tend to make it lurch about like a mechanical toy but is easily solved by ensuring that movement from one angle to another doesn’t take place in one go. Instead, incremental changes in the PWM signal are made, each separated by a short delay until the target angle is reached.

Power supply

RC servo datasheets often quote values for torque and speed at two different power supply voltages. These are usually +4.8 and +6.0 volts corresponding to the power packs found in model aircraft containing four or five 1.2 volt NiCd rechargeable batteries. The +5 volt logic supply of a robot application will normally be adequate for one or two servos, but a separate supply will be needed for any more. When a servo is holding position under load, its DC motor is drawing a stall current of perhaps 500mA or more, and digital chips sharing the supply are vulnerable to glitches caused by sudden current surges.