Motor FAQ

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.

-Stepper=positional control for cheap but open loop

-Servo motors=require closed loop feedback system usually involving an optical or magnetic rotary encoder. Higher torque than comparable steppers.

-DC gear motors=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. 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.

Stepper Motors

There is a good article on wikipedia explaining the technology behind stepper motors. The physical size of stepper motors are usually described via a US based standard called Nema, the reprap site has an article explaining the standard.

The pages related to building a Mendel has a list of suppliers of stepping motors.

The power of a motor is usually proportional to the physical size of the motor, The Darwin version of Reprap primarily used NEMA 24 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 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.

Power and current

All stepper motors will have a certain specifications for voltage and current, typically 2.8V and 1.68A, as long as the stepper driver/controller does current control you can use any supply voltage greater than the motor's rated voltage. In fact, a large difference is advantageous to the top speed of the motor. If the motor dirver/controller does not do current control, you must use a supply voltage fairly close to the motor voltage (no more than 2x the voltage specified by the manufacturer) or the motor will overheat and burn out its winding insulation or demagnetize its rotor.

The 2.3 version of the Reprap axis controllers do have current control.

Stepper drivers vs Stepper Controllers

To run a stepper motor, two things are normally required: a controller to create step and direction signals(at +-5V normally) and driver circuit which can generate the necessary current/amperage to drive the motor. In some cases: a very small stepper may be driven directly from the controller, or the controller and driver circuits may be combined on to one board.

PWM and Stepper Drivers

From Wikipedia:[[1]]: Pulse-width modulation (PWM) is a very efficient way of providing intermediate amounts of electrical power between fully on and fully off. A simple power switch with a typical power source provides full power only, when switched on. PWM is a comparatively recent technique, made practical by modern electronic power switches.

Stepper Drivers normally work by chopping up a supply voltage using an embedded PWM chip. These chips do require minor support circuitry which is the primary thing you pay for when you buy a stepper driver. The PWM chips themselves usually have a unit price below $10USD depending mostly on their rated current.

Some example chips include:

Chip	Verified	Max Amperage	Comments
[L293D	Yes	.6amp(s)	Multiples can be stacked on top of each other to divide up amperage.
[A3967]	No	.75amp(s)	Slightly underpowered, at only 750mA/Phase
[A4983]	Yes	2amp(s)	Can get very warm, active cooling is needed
[Allegro 3977]	No	2.5amp(s)
[TB6560]	No	2.5-3amp(s)

Stepper drivers

Sourcing stepper motor drivers can be a bit difficult, the 2.3 stepper drivers for the Reprap is very hard to purchase pre-assembled, sourcing the individual parts and assembling the controllers can be done with just a little bit of skill, for those without skills or materials to assemble the boards, generic stepper drivers purchased from the web. In Europe it will usually be more cost-effective to purchase pre-assembled boards compared to purchasing the individual parts and perform a DIY assembly.

Alternative sources for stepper drivers

Manufacturer	Verified	Location	Max Amperage	Microstepping	Comments
[RepRap Stepper Motor Driver 2.3]	Yes	US	2amp(s)	Half	Listed for comparison.
[Sparkfun]	Yes	US	.75amp(s)	1/8	Slightly underpowered compared to other drivers, at only 750mA/Phase. Has plenty sufficient power for Mendel. Recommended.
[Polulo]	Yes	US	2amp(s)	1/16	Can get very warm, active fan cooling or passive small heatsink is needed above ~.5A. Recommended.
[DIY CNC]	No	GB	2.5amp(s)	1/8	Can drive 1 stepper, discount when buying several.
[Arduino Motor Shield]	No	US	.6amp(s)	?	Requires Arduino as controller. Can drive 2 servos, 4 DC, or 2 (bipolar or unipolar)steppers,
[TB6560AHQ based]	No	GB	2.5-3amp(s)	?	can drive 4 or 5 steppers depending on model
[Gecko Drive	Yes	US	3.5amp(s)]	?	Can drive 4 steppers

Micro stepping

Microstepping between the pole-positions is made with lower torque than with full-stepping, but has much lower tendency for mechanical oszillation around the step-positions and you can drive with much higer frequencies.

If your motors are near to mechanical limitations and you have high friction or dynamics, you won't receive much more accuracy. When your motors are 'overpowered' and/or you don't have much friction, then you can transfer the higher positioning accuracy to moving accuracy too.