Servo Stock 3D Printer Brings Closed-Loop Control To Reprap 56
A limitation of current (affordable) 3D printers is their use of open loop controllers and stepper motors which limits reliability (drove the motor too quickly and skipped a step? Your model is ruined) and precision (~300 steps per revolution). A new project, Servo Stock instead uses cheap RC Servomotors combined with Hall Effect sensors, using a closed-loop controller to precisely position the extruder. The Servo Stock is derived from the delta robot Reprap Rostock (which is pretty cool even with stepper motors). The sensors give a resolution of 4096 ticks per rotation, and the controller can currently position the motors to within +/-2 ticks. They've also simplified the printer electronics by driving as much as possible from the controlling computer using Bowler, a new communication protocol for machine control. The Servo Stock also includes sensors for the hot end, presumably to be used to control the filament feed rate and temperature. The hardware models are fully parametric, allowing reasonably straightforward scaling of the design. Source for the hardware, firmware, and software is available.
A note on the video: the extruder platform is tilted in the video, but a project update indicates it was fixed by making the support arms more rigid.
Re:Or dont overdrive the steppers (Score:4, Informative)
Or you could do ALL those things, which in non-hobbyist robotics is commonly known as "the bare minimum". I see a lot of industrial robotics in semiconductor fabs. The standard for the last 30 years has been:
Use stepper motors of adequate torque, geared appropriately so as to have very high resolution,
AND you use high-resolution encoders on the steppers,
AND you check the end effector motion against fixed "home" positions with breakbeam or similar sensors.
The software constantly compares the stepper commands with the shaft encoder reading. If they ever don't match, the tool instantly stops and thows the ubiquitous "encoder mismatch" error. If a home-position flag is ever missed, the tool freezes and throws the almost-as-common "robot position error" error. This is the bare minimum when you are moving around $10,000 wafers.
The idea that 300 steps per revolution is "not enough" resolution is so wrong as to be not-even-wrong. That's why God invented gear ratios. If there is no space for gears or timing belts, use a harmonic drive. A very common configuration on AMAT cluster tools is a 500steps/rev 5-phase stepper with a 100:1 harmonic drive giving 50,000 steps per revolution.
Re:this is cool (Score:5, Informative)
Don't think of it as "more expensive", think of it as "the price of precision."
It is not as simple as that. Stepper motors can use microstepping [wikipedia.org] to improve their precision and stability. All you need is a controller with multiple PWMs (one for each phase). A $25 Arduino will work to microstep up to three two-phase steppers. "MIssing steps" is not a problem if you don't push the motor outside its performance envelope. The head on a 3D printer is not moving against variable resistance, so that should not be a problem. You could even slap an encoder onto a stepper, so you can detect and recover from missed steps. A servo motor has its own issues, such as gear backlash, that can make it less precise than a stepper in many applications. Servo motors vs steppers may be more for marketing than for real precision.