By Dipankar Mitra, System Engineer, Texas Instruments

3D printing is a fast-emerging market with significant growth potential. 3D printers create three-dimensional objects, with complex shapes and geometries, through the successive deposition of materials directly from a computer-aided design model. The agriculture, health care, automotive, locomotive, and aviation industries are some of the first adopters of 3D printing for design prototypes and production. With the wider adoption from these industries comes an increasing demand for small, accurate, energy-efficient and silent 3D printers.

All 3D printers use multiple stepper motors to produce a good quality print. These stepper motors move the printer bed along the X-Y-Z axes, move the extruder or change the color of the printer output. Using high-performance stepper motor drivers can help printer motors move silently, precisely, and efficiently. In this article, I’ll explore how to find the right stepper motor drivers to build a 3D printer from scratch or upgrade an existing one.

3D printer stepper driver system-level requirements and performance metrics

A stepper motor moves in discrete steps during its rotation as defined by a step angle. It has two electrical current windings, each controllable with an H-bridge. As shown in Figure 1, the stepper motor driver applies current waveforms approximating a sine wave (blue) into one coil and a cosine wave (red) into the other. One (90 degree) quadrant of the current waveforms corresponds to a stepper motor rotation by one step angle – 1.8 degrees for most hybrid stepper motors used today.

Depending on the complexity and available features, a single 3D printer can contain anywhere between four and 10 stepper motors. Figure 2 shows a simplified block diagram of a 3D printer.

Let’s review the major system-level requirements of stepper drivers used in 3D printers.

 H-bridge supply voltage

Offline AC power is converted by AC-DC converters to lower-voltage DC power to operate the H-bridges of a stepper driver. The most common DC supply voltages used today are 12 V or 24 V. For the same output power, operation at 24 V halves the average current compared to 12-V operation.

Some manufacturers are designing their 3D printers to operate at even higher nominal supply voltages, such as 36 V or 48 V. A higher supply voltage, with a lower average current, leads to lower transmission losses and lighter cabling weights. A higher voltage also results in higher available torque at high speeds, which leads to faster printing speeds.

A 36-V nominal supply means that the stepper driver should be capable of tolerating at least 45 V, assuming 25% maximum tolerance on the supply voltage rail. For a 24-V nominal supply, a stepper driver with an absolute maximum voltage rating of 30 V should be sufficient.

The discrepancy between the voltage rating shown in the stepper motor datasheet and the supply voltage of the stepper motor driver can be confusing. What you must remember is that the voltage rating of the motor is simply the product of its rated current and winding resistance. The stepper driver supply voltage can safely be higher than the voltage rating of the motor.

Stepper driver current ratings

Most 3D printers use National Electrical Manufacturers Association (NEMA) 17-size stepper motors with a torque rating between 0.3 N-m and 0.5 N-m. Depending on the application, the current rating of a stepper motor can be from a few hundreds of milliamperes to 2 A, or in rare cases, even higher. Stepper motors used for the X-Y-Z movement of the printer bed or movement of the extruder tend to be rated for higher currents compared to stepper motors used to select color.

The current limit of the stepper motor driver should be higher than the maximum current draw of the stepper motor, with an acceptable factor of margin. Some systems use the same motor driver to drive both high- and low-current-rated stepper motors. An optimized solution would be to use pin-to-pin compatible stepper motor drivers rated for low and high currents to drive the various stepper motors used in a 3D printer system.

Ambient temperature and on-state resistance

Most 3D printers require that the printed circuit board surface temperature not exceed 80°C; therefore, stepper drivers must have good thermal performance. To keep the semiconductor die temperature within acceptable limits, stepper drivers with a high on-state resistance (>500 mΩ for both the high side and low side together) must use a large heat sink, which increases system costs. Some 3D printers even use gate drivers with numerous external components for cooler operating temperatures. For motors rated for 2 A, an on-state resistance close to 350 mΩ can avoid the heat sink altogether in most cases.

Microstepping, position accuracy and smooth motion

In a 3D printer, the quality of printing depends on the position accuracy of the stepper motors controlling the movement of the extruder and printer bed in the X-Y-Z directions. Operating a stepper motor in full-step mode causes the motor to jump by one step angle (1.8 degrees of mechanical rotation in most cases), resulting in overshoot, torque ripple and vibrations. As a result, most stepper motor drivers today incorporate microstepping, which splits the full-step into smaller equal segments, and therefore helps reduce vibrations by smoothing the movement of the motor to its intended location.

1/16th-level microstepping is generally considered a standard in most legacy 3D printer systems. Some of the latest drivers contain 1/32-, 1/64-, 1/128- and 1/256-level microstepping to maximize position accuracy and smoothness of motion. However, higher microstepping will result in better position accuracy only if the torque per microstep is more than the torque needed to move the load.

The channel-to-channel current matching of the stepper motor driver impacts the overall position accuracy. Having one coil current be a pure sine wave and the other coil current be a pure cosine wave will ensure that the output current is constant and the incremental angle is the same in each microstep. Any mismatch from ideal values will cause nonuniformity in the angular position increments and lead to uneven output torque, position inaccuracies, and an increase in motor vibrations.

The choice of driver decay mode also plays a big role in determining system accuracy. Any ripple in the current waveform is a deviation from the desired shape and manifests as vibration and poor accuracy. Operating a stepper motor on a slow decay mode whenever possible instead of fast or mixed decay will reduce ripple. Because of back-electromotive force, however, current waveforms with only slow decay become distorted at high operating speeds. Therefore, a slow decay mode that can adapt itself to high speeds is the best way to improve accuracy. Smooth operation of the stepper motor should result in the monotonic position accuracy plot shown in Figure 3a, not the sudden change in step angle shown in Figure 3b.

Audible noise

3D printers, especially older models, can be so loud that it can be hard to be in the same room while the printer is in use. The noise largely comes from fans, stepper motors and other moving mechanical parts and might warrant expensive noise suppression methods such as rubber isolators, which do not completely eliminate the noise. Higher-quality stepper motor drivers can help reduce motor noise significantly, which minimizes the amount of noise produced by the printer.

A slow decay mode results in the least amount of motor noise by minimizing the current ripple. Low-resolution step modes, like full or half stepping, cause the rotor to overshoot and oscillate around the next position, leading to mechanical vibrations and noise. Microstepping drastically reduces overshoots and undershoots, leading to much quieter operation. Also, using an effective step frequency beyond the audible frequency range (approximately 20 kHz) vastly reduces the noise coming from stepper motors.

Stall detection

Most 3D printers include a component called an endstop. By sensing the motor position, endstops ensure that the printer head stops when it reaches the end point of movement in a given direction. Most endstops are either mechanical or optical. Unfortunately, optical endstops suffer from poor accuracy and are difficult to configure, and mechanical endstops can wear out prematurely and require periodic maintenance. These external components designed to monitor motor position increase overall system costs. In addition, some systems severely overdrive the motor beyond the desired end point to ensure the end point is reached, reducing system efficiency and causing audible noise and mechanical failures.

A sensorless stall detection method integrated within the stepper motor driver can replace endstops and address these issues. A stall detection scheme should be able to detect the motor end point reliably across supply voltage, temperature, motor speed, and motor parameter variations. Integrated sensorless stall detection also provides an immediate response when a stall occurs compared to position sensor solutions that require a timeout mechanism.

Texas Instruments offers a stepper driver family specifically designed for next-generation 3D printer designs. The smart tune ripple control decay mode in these drivers operates with slow decay but also adapts itself to high-speed operation, enabling silent and precise 3D printers. The drivers feature up to 1/256-level microstepping, resulting in excellent current regulation. The ±2.5% channel-to-channel current matching also helps achieve high positional accuracy.

Specifically, the DRV8424 and DRV8434 provide a 330-mΩ on-state resistance (rated for 2.5 A), while the DRV8426 offers a 900-mΩ on-state resistance (rated for 1.5 A). All three devices are pin-to-pin compatible, which adds flexibility to the choice of stepper driver.

The DRV8434S (with Serial Peripheral Interface) and DRV8434A (with an analog general-purpose input/output interface) feature sensorless stall detection that eliminates the need for endstops. The 330-mΩ on-state resistance also lends itself to designs with no heat sinks.

The device family is available in both thermally enhanced thin shrink small-outline (HTSSOP) and compact 4-mm-by-4-mm quad flat no-lead (QFN) packages, which help designers trade-off solution size with thermal performance.

Using stepper motor drivers with the right feature set can significantly reduce audible noise, improve motion accuracy and increase the energy efficiency of 3D printers. Stepper motor drivers such as the DRV8424, DRV8426, DRV8434S, and DRV8434A provide 3D printer designers with near-silent operation.

 

Additional resources

1. Nagaraj, Sudhir. 2014. “Stepper motors made easy with smart tune.” Texas Instruments technical white paper, literature No. SLYY066C, December 2014.
2. Lockridge, James. 2018. “Smart Tuning for Efficient Stepper Driving.” Texas Instruments application note, literature No. SLVAE58, November 2018.
3. Mitra, Dipankar. “Sensorless Stall Detection With the DRV8889-Q1.” Texas Instruments application report, literature No. SLVAEI3, January 2020.
4. Eaker, Madison, and Dipankar Mitra. “How to Reduce Audible Noise in Stepper Motors.” Texas Instruments application report, literature No. SLVAES8, May 2020.
5. Mitra, Dipankar. “How to Improve Motion Smoothness and Accuracy of Stepper Motors.” Texas Instruments application report, literature No. SLOA293, September 2020.