Stepper Motors vs Servo Motors: What Makes a Better FDM 3D Printer?

When you look under the hood at any FDM 3D printer, you’ll find that they all rely on some form of a motor to control the printing process. Whether it’s moving the print head, raising and lowering the z-stage, or moving filament through an extruder, motors are an integral part of the FDM process.

Within this, the two most standard forms of motors found in an FDM printer are stepper motors and/or servo motors. While both types achieve the same end result, there are significant differences between the two that make them each better suited for different product types.

In this piece, we’ll take a look at both stepper and servo motors to understand their differences and how their roles in FDM printers.

Stepper Motor: A Background

Stepper motors are a form of motor that rotates in incremental steps of a fixed degree. These motors consist of a single rotating shaft (i.e., a rotor) that is positioned inside a stator. The stator itself consists of a number of evenly-spaced teeth, each of which is surrounded by coiled wire known as motor windings. The motor windings act as electromagnets: when they are energized (i.e., a voltage is applied to them), they become magnetized and attracted the rotor to the energized coil.

Cross section of a stepper motor. Image from Monolithic Power.

Operating a stepper motor requires the use of a dedicated stepper motor driver integrated circuit (IC) which is controlled by a microcontroller (MCU) or processor. The stepper motor driver selectively applies voltages to the motor windings to control the rotor’s movement to the desired position.

Controlling a stepper motor in wave mode. Image from Monolithic Power.

In the context of 3D printing, you’ll typically find at least one stepper motor for each axis. For X and Y, it is common for stepper motors to be attached to a belt system which then drives a gantry, while the Z axis may be directly attached to a screw to raise/lower the Z-stage.

Stepper Motor: Pros and Cons

For FDM printers, stepper motors offer a number of advantages and disadvantages.

One of the major advantages of stepper motors is that they are very affordable, making them a good choice for consumer-facing desktop FDM printers. Additionally, stepper motors have the advantage of extremely high torque at low speeds, which is ideal for 3D printing. By the same token, steppers have the advantage of high holding torque – meaning that they can easily hold position during print pauses. 

One downside of a stepper motor is that the positional accuracy of the motor is limited by the number of windings in the motor. To further confound things, stepper motors do not inherently offer feedback – meaning that it’s impossible to know the exact position of the motor (without the use of an encoder). This can limit the dimensional accuracy in prints.

Additionally, stepper motors are plagued by the fact that their torque decreases as speed increases. The result of this is that stepper-motor-based 3D printers lose reliability and accuracy as their print speed increases.

Servo Motor

Servo motors, on the other hand, are a form of electric motor that consists of a DC motor, gears, controlling circuitry, and positioning feedback all enclosed into one unit. Because of the integrated control circuitry and positioning feedback, servos offer extremely high positional accuracy as compared to stepper motors.

Cross section of a servo motor. Image from Sparkfun.

Controlling a servo motor requires the use of an MCU which sends pulse-width modulation (PWM) commands to the motor. The PWM control signal typically consists of a series of pulses, where the duration (width) of each pulse determines the desired position of the servo motor.

The typical range for the pulse width is 1 ms to 2 ms, with a period of around 20 ms. In most cases, servo motors only have a 180-degree range of motion.

In FDM 3D printers, servos are used to control the X and Y movement of the print head as well as the raising and lowering of the Z-stage.

Servo Motor: Pros and Cons

In general, the major advantages of servo motors for FDM printers are their exceptional positional accuracy and speed. 

Since they offer uniform torque regardless of speed, servo motors allow for faster printing with greater control. At the same time, the integrated feedback loop of servo motors allows for precise accuracy and control over positioning, meaning that dimensional accuracy and print quality are improved.

The major disadvantage of servos in FDM printers is that they’re much more expensive than stepper motors. For this reason, they are only really used in high-end industrial-quality FDM printers.

Which is better?

Both stepper and servo motors offer their own unique set of advantages and disadvantages when it comes to FDM printing. 

While there is no clear “winner” between the two, the more common choice for most printers is stepper motors. While servos certainly offer the ability for greater control and accuracy, more affordable stepper motors generally suffice for most consumer-level applications, and as such they are the more popular choice. However, for top-quality, industrial printers where price is less of a factor, servos may be the way t go.

The post Stepper Motors vs Servo Motors: What Makes a Better FDM 3D Printer? appeared first on 3DPrint.com | The Voice of 3D Printing / Additive Manufacturing.

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