I know that servos and stepper motors are used for more precise usages, but I don’t understand how they work.
In: Engineering
A servo motor is a motor with some sort of feedback device attached to it. There are couple of common feedback devices such as tachometers, encoders, and resolvesrs which work on different principles but all serve the same purpose. They give the machine some way of determining the motor’s position. You can tell a servo to move 3 rotations, and it will do it because it has some means of actually measuring those rotations.
A stepper motor is usually open loop. This means there isn’t any feedback device and it can potentially lose track of where it is. Steppers are built so that they can essentially “index” some number of degrees. You send a pulse of power, and it activates the winding, pulling the shaft to it’s next position. Send a bunch of pulses immediately after each other and it rotates like a normal motor. Again, there’s no feedback so it becomes possible to stall a stepper motor, and your system won’t realize that the motor isn’t where it’s supposed to be. This is called losing steps.
Stepper motors and servos have very different ways of moving. If you provide constant power to a servo motor it will rotate continuously. If you provide constant power to the wires of a stepper motor then it will hold a position and prevent you from turning it. This is because of how the motor is designed and how it is meant to be controlled.
A servo has a brushed DC motor which means that as long as it has power, it will automatically create a changing magnetic field by redirecting the electricity inside its wires. This is done mechanically by moving electrical contacts inside called “brushes”.
A stepper has multiple sets of windings but they are not mechanically switched by the rotation of the shaft. You have to use an electrical controller to switch power on and off of each set of windings to change which way the magnets get pulled. If you send power to only one winding it will move to that step and stay there.
How you control these motors is thusly different. A servo usually has electronic circuits inside which convert a signal into commands for the motor. It also has a sensor to detect its current position. If there is a difference between its current position and the signal then it will power the motor in one direction or the other by simply closing a switch until it reaches the right position.
A stepper will move very predictably when you change the power to its wires in a particular way. These are called steps. A common stepper motor will turn 1.8 degrees per step. If you tell it to move a certain way then you can be very certain that it has reached the exact position by counting the steps you send. The issue is that if something happens and the motor misses a step then you have no way of knowing where the motor *actually* is and you will be in trouble. Even worse, you will not know that you missed a step. Some steppers can have encoders added onto them for verification but this is not standard.
Additionally, the torque of a stepper motor is usually pretty constant at any position. The torque of a servo motor is usually depending on how far from its desired position it is. This means that when it is at the desired position a servo will use no electricity and will provide no resistance. A stepper will be constantly using electricity as it sits still.
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