Motor FAQ

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Introduction

This page tries to answer most of the frequently asked questions related to the choice and operation of drive motors used by the Reprap.


Stepper motors vs Servo Motors vs DC Gear Motors

I'll copy and paste a quote/excepts later.

Electric motors all start with a simple concept. You apply an electric current to a coil, and it generates a magnetic field. That field causes the shaft attached to rotate. This is all an electric motor is. Through some engineering, variations on this have been created to allow some further control.

DC motors

DC (Brushed) motors= easiest to use, just connect straight to battery, only two wires. Hardest to control, you need an external feedback mechanism (optical or magnetic encoder, linear encoder, etc) and H bridge motor driver to get variable speed and direction.

DC gear motors

A DC gear motor is a DC motor which has an extension built on to it to "gear down" the rotation. Essentially, a series of gears to make the rotation of the motor move more slowly for particular uses. DC gear motors are normal DC motors which have been geared down to decrease their speed and increase their torque. The Solarbotics GM series are a popular type because of their low cost, however the plastic gears make this at a price. They include a torque limiting slip clutch to protect the plastic gears. If this clutch is disabled, the gears may break.

MagServo is a highly experimental attempt to replace the stepper motors (and their drivers) in a RepRap with low-cost DC gear motors (and their drivers and a feedback mechanism).

The original Darwin design uses DC gear motors in the extruder.

Nophead reports that his DC gear motor extruder has higher throughput than other extruders. See Nophead's Extruder Tweaks.

Servo motors

Servo motors include a closed loop feedback system usually involving an optical or magnetic rotary encoder. Higher torque than comparable steppers. The difference between a servo motor and a DC gear motor is 2 factors: servo motors usually have their drive circuitry incorporated at least, but always include a rotary encoder. They also have lightweight rotors which allow them fast wind up/wind down acceleration.

Hobby servo motors

-Hobby Servo Motors are standardized servo motors intended for robotics, RC, and other hobby applications. Hobby Servos function using information produced by a computer or a microcontroller called Pulse-width-modulation. By sending a certain repeating signal to the Servo, it will move to that certain point. In terms of a reprap, a servo would need to either be modified or purchased as a "full rotation servo". In a reprap, servos require closed loop feedback system and usually involve an optical or magnetic rotary encoder. They generally will offer higher torque than comparable steppers. Servo motors use three wires, Ground, Power, and PWM.

This is called a PID circuit, and comes in analog or digital varients. The "openservo" project. Digital has better acceleration.

The OpenEncoder OpenServo project is converting low-cost analog hobby servos into digital servos -- do these have enough precision and torque to replace stepper motors in a RepRap?

Stepper Motors

A stepper motor is another kind of special motor. This motor is powered and sent a signal to move forward x number of "steps". The current RepRap printer uses a common NEMA 17 stepper motor, which usually has 180 steps in it. These are operated in an "open loop". Stepper motors have 4 to 8 wires, and require a more complicated with two or four phases.

Stepper motors move a known interval for each pulse of power. These pulses of power are provided by a stepper driver and is referred to as a step. As each step moves the motor a known distance it makes them handy devices for repeatable positioning.

Nearly all RepRaps and RepStraps use stepper motors; see StepperMotor for details.

general motor information

The power of a motor is usually proportional to the physical size of the motor. The Darwin version of Reprap primarily used NEMA 24 stepper motors, whereas the Mendel version is designed to use either NEMA 14 or NEMA 17 motors. The more commonly used size is NEMA 17 as it is easier to find NEMA 17 motors with sufficient torque compared to NEMA 14.

Torque

The Mendel officially requires 0.137Nm torque (1400 g-cm or 1.215 lb-in) for the X, Y and Z axis. Recent designs for extruders (ExtruderController) almost exclusively require stepper motors as well, but no requirements for torque has been given in those designs.

Stepper motor's do not offer as much torque or holding force as comparable DC Servo motors or DC Gear motors. Their advantage over these motors is one of positional control. Whereas: DC motors require a closed loop feedback mechanism, as well as support circuitry to drive them, a stepper motor has positional control by it's nature of rotation via fractional increments.



friction

This page has been flagged as containing duplicate material. An editor has suggested merging this page or section into lubrication. (Discuss)

Most the electrical power fed into the motors of any CNC machine is lost due to friction. There are a variety of techniques used to reduce friction:

(There are some cases where we want to increase friction -- in particular, when two parts touch and are *not* supposed to slide against each other, such as the nuts and bolts holding the machine together, lots of friction helps keep them from unscrewing themselves).