The Top 5 Pain Points in Motion Control Design

Today, design engineers developing motion control functions must not only deal with technology-related questions, but also with a long list of commercial challenges and project-related questions that affect their implementation decisions.

To better understand these real-world challenges, TRINAMIC Motion Control (TMC) asked its field application engineers to identify the top five questions, problems, or issues their customers experience in motion control applications. Four of the top five design “pain points” were not surprising for anyone experienced with motion control:

time-to-market
increased miniaturization,
cost reduction,
and motion control quality.

The fifth pain point was less obvious. Many field engineers reported that there was a growing need for control interfaces that can be easily understood and used by a new generation of engineers who are primarily software-oriented. This article discusses those pain points, the challenges they present, and some solutions for how they can be addressed, using the real-world experiences of 3D printer maker BCN3D Technologies.

BCN3D began as a project of Fundació CIM, a technology center at the Universitat Politècnica de Catalunya (Barcelona Tech) specializing in digital manufacturing R&D and technology transfer. The company designs, manufactures and sells fused filament fabrication (FFF) desktop printers for professional and consumer (“prosumer”) users. Its open source approach includes providing customers with CAD files, a bill of materials including suppliers, manufacturing drawings, and source files.

The company’s engineers recently redesigned its flagship Sigma 3D professional-grade, dual-extruder, desktop printer. Two extruders make it possible to print two different materials or colors in one part, or to print a single part material with a second, water-soluble support structure material.

In most dual-extruder printers, both extruders share the same carriage on the same moving axis, so they are always next to each other. The main challenge when printing with dual extruders is ensuring that the inactive extruder does not drip molten plastic onto the part being built with the other extruder. Since both extruders are right next to each other this can’t be fully prevented.

The Sigma’s dual-extruder design takes a different approach. It has a total of six motor axes and two independent carriages so the inactive extruder can easily be moved away from the printing area (Figure 1). If any molten plastic does drip from the unused extruder it will not fall onto the part being printed. This approach fully prevents cross-material contamination, improving surface quality of the printed objects.

When BCN3D designed their next-generation 3D printer, they had to carefully balance the need for improved speed, precision and noise emissions against unit cost, manufacturability, and development time. Image credit: BCN3D

Pain Point #1 – Motion Control Quality

As the performance requirements of motor control and motion control applications increase, the overall quality of these designs also needs to increase. The concept of motion control quality encompasses multiple dimensions that affect end-product quality. In 3D printers, these are primarily noise, accuracy, efficiency, dynamic behavior, and precision.

When designing the second generation of BCN3D’s desktop printer, the Sigma R17, the company’s main goals were to increase its speed and accuracy, as well as reduce noise and vibration. In 3D printers, silence of operation and the smoothness of a printer’s movements are major selling points. Because a desktop system will be used in homes as well as offices, often sitting right next to users, it needs to operate as quietly as possible, be easy to use, and produce the best possible quality prints.

New cooling strategies for the R17, including a new fan system and new stepper motor drivers, not only reduce noise but increase the accuracy and surface quality of printed parts made with it. Given these design goals, the main challenges were 1) to develop a new stepper motor driver that was totally compatible — mechanically, electrically, and in form factor — with the previous generation of the Sigma already on the market, and 2) to design the new driver’s printed-circuit board (PCB) for optimal heat dissipation.

In the previous generation printer, the stepper driver IC was a Texas Instruments DRV8825, commonly used in 3D printers because of its wide range of current and voltage ratings. However, it is very noisy and not very intelligent, as it lacks features such as step interpolation or motor stall detection.

For its new R17 printer design, BCN3D chose the TMC2130 stepper driver IC without integrated motion controller because of its 1/16 microstepping with 1/256 interpolation, compared to the previous driver IC’s 1/32 microstepping rate. This increase in steps using step interpolation makes the motor’s movement much smoother than other driver ICs can accomplish, without affecting step generation rate. It also reduces sound dramatically. In addition, sound levels are reduced thanks to the driver IC’s advanced control of the motor windings current.

Pain Point #2 – Increased Miniaturization

In electronics design, space constraints are always a challenge. Because the mechanical design of the machine had already been done in the prior generation, measurements were especially constrained. Also, in this case BCN3D wanted the smallest PCB possible for its driver module to reduce costs, but also needed enough space to be able to dissipate the heat that would be generated. Since the TMC2130 integrates a microstepping sequencer and power driver circuit into a single compact QFN36 package, it was possible to design a PCB that is small enough to be easily co-located with its companion motor (Figure 2).

One of the interchangeable stepper motor driver boards used in the R17 printer. Source: TRINAMIC

Until the R17 product generation, BCN3D had been designing two-layer PCBs for all of its printers’ driver modules, as well as for the previous Sigma model’s mainboard. In order to design the new driver module’s board to fit the same form factor as the previous generation Sigma yet maintain optimal heat dissipation, two additional layers were needed for main power and ground. Adding these greatly improved heat dissipation and did not produce any new design challenges.

BCN3D uses only stepper motors in its printer designs, which are integrated within the structure of the machine. Each axis of the printer is driven by a motor and a co-located stepper driver module. The motor/controller pairs are connected to the main control board via flat flexible cables (Figure 3).

Block diagram of the BCN3D R17 showing stepper driver PCBs. Source: BCN3D

This integration was chosen to physically separate the stepper driver modules from the mainboard, which has two major benefits. If a driver gets damaged, the driver module can simply be changed, instead of swapping out the mainboard. EMI performance is also improved. The stepper motor winding cables are the shortest possible, so that emissions from the high-current motor cables are reduced.

Pain Point #3 – Time-To-Market

The timeline for developing the new stepper motor driver module was around six months. Since the project was replacing a working driver module with a new one, there was no need to change the design and engineering process. Although there were some time-to-market pressures, this was not a new product design.

Pain Point #4 – Containing Costs

While the new fan system had virtually no impact on costs, the implementation of new, better stepper drivers and a more complex design (the four-layer PCB) increased the unit cost of the R17 stepper driver PCB by about 20%. However, that increment was rapidly absorbed by higher production volumes. Today the PCB’s unit cost is similar to the previous version’s stepper driver PCB.

In addition, redesigning the stepper drivers allowed BCN3D to release an upgrade kit. Because of the changes made in the stepper driver ICs to achieve faster printer speed and accuracy, as well as reduce noise and vibration, hundreds of users of the previous Sigma model bought the new stepper drivers, creating a new income stream. The small cost increment was therefore totally worth it.

BCN3D’s goal in approaching its prosumer market is to achieve a balance of quality and price: price is important, but priority is given to quality and functionality. Prices of 3D printers will continue to decline not due to technological advancements, but with ongoing massification of the technology. In general, although FFF 3D printers are not super high-technology machines, they will get cheaper as more printer manufacturers demand similar parts for their machine designs and those part volumes increase, lowering part prices.

Pain Point #5 – The Increasing Importance of Interfaces

In keeping with BCN3D’s open source philosophy, the new stepper driver modules have been designed to be mounted in any of its Sigma printers, so users of the previous generation printer can benefit from the R17’s improvements without having to modify the firmware. Consequently, although the TMC driver IC has digital interface capabilities, for this redesign BCN3D did not use them, so additional time for firmware development and its testing were not required.

Conclusion

A good motion control design platform is affordable while also powerful and capable of expansion. The ability to implement a motion control solution in an existing product and know that its capabilities can be expanded in future product generations with newer products using the same design knowledge is very attractive/desirable.

In the future, motor control design will become even easier and smarter. Offloading the main microcontroller from the motor calculations is a must. This allows the use of S-shaped ramps on multiple axes while keeping the MCU small. Also the S-shaped speed curves reduce vibrations/jerk and optimize speeds. Finally, technologies such as sensorless homing and sensorless, automatic current adaption will improve motor efficiency and reduce system costs.