New Project - 3D Printer

I decided to make a 3D printer. 3D printing seems to be the wave of the future for producing many parts. There are a few different types of 3D printers, one of which, the extrusion type, seems to be the most popular on the market. This kind of 3D printer takes in filament, which is essentially plastic wire, and extrudes it into a small bead of plastic. The printer moves the extrusion nozzle on a table to draw a part. This printer is basically a fancy hot-glue gun that makes 3D shapes. 

The consumer grade 3D printers on the market are pretty slow and they don't produce the best quality parts. In addition, most of them don't use support material. Support material is a second material used by the 3D printer with the standard plastic. The support material can be dissolved so you can make parts with overhangs or other features that require additional support.

The concept for this 3D printer is to use high power hobby brushless motors, similar in size and power to those used in the heavy weight combat robot I built last semester. These motors aren't meant to be used as servos, so the printer needs custom motor controllers as well as additional hardware to properly drive the motors.

The brushless motors have 3 wires: phases A, B, and C. Brushless motors have magnetic coils in multiples of 3, one set for each of the 3 phases. These coils are connected together in a few different configurations, such as wye or delta, which provide different characteristics to the motor. Driving the motor is the same regardless of the coil configuration. Phases A, B, and C must be pulled to supply voltage, ground, or floated to make the motor rotate. A nice resource for this would be AN857 from Microchip. Microchip makes microprocessors, transistors, and other components used in motor controllers. AN857 is one of their application notes on brushless motor control.

Most hobby brushless motors are sensorless, so there is nothing added to the motor to help a controller determine when to switch which phases are being powered. Since a servo needs an encoder, I decided to use this to perform the commutation, or timing for when the motor controller switches which coils are powered.

Here's a picture of the custom motor controller. This board uses power MOSFETs. MOSFETs are a type of transistor that is great for high frequency switching applications. There are 3 pairs of power MOSFETs; each pair corresponds to one of the motor's phases. One MOSFET in each pair is connected to the power line, and the other in each pair is connected to the ground line. If a high-side MOSFET is on, that phase will be connected to the power line. If a low-side MOSFET is on, that phase will be connected to ground. If neither is on, the phase will be disconnected from power and ground. If bot MOSFETs are on, power fill flow from the power line to ground. In this case the MOSFETs will form a short circuit. With a large power source, batteries or computer power supply, the MOSFETs will fry from too much current.

Portions of this project became a group effort for final projects in two of my electronics classes. The motor controller and its design are part of an advanced electronics course. The logic portion of the control, which is built on an FPGA and microprocessor board is part of a digital electronics and embedded systems course.

The logic portion of the motor controller is implemented on an FPGA. This includes PID control, which is being temporarily used until the mechanical portion is built; motor commutation; and coordinate input. An FPGA is a chip that has programmable logic gates. An FPGA allows the user to implement hardware design on the chip. Certain features like encoder counting, and motor commutation are very convenient to implement on an FPGA. Since these features are in hardware, certain situations can be made impossible. One example of this would be short circuiting two of the MOSFETs together.

Here is a quick video of the motor. The motor controller is connected to a variable voltage supply. The motor controller is just switching which phases are powered depending on where the motor is positioned. The voltage determines the peak speed of the motor. I can't remember if this test went up to 12,000 rpm, but the motor and encoder functioned up to this rpm.

This shows the output of the encoder at 12,000 RPM. The signal is very clean. There are two square waves because this encoder is a quadrature encoder. There are two sensors, 90 degrees out of phase from each other, which read lines on the encoder. This lets the controller decode a resolution four times higher than the line count on the encoder. This encoder can be read at up to 8192 counts per rotation.

The mechanics for this project are meant to be relatively cheap to make. Linear slides in the gantry are built using hardened round rails and ball bearing bushings. These components are commonly used in other 3D printers and can be found very cheaply on sites like Ebay. The design for the printer isn't completely finished. There table motion system and mount, material mount, extruder, and heater block have yet to be designed. Thankfully the project proposed for class only required the motor controller, not the full 3D printer. I will likely finish these parts over the winter break when I have more free time.

Here's a screen shot of the CAD in the 3D printer's current state. The gantry design is done and looks pretty sweet. The table still needs some work, and the extruder doesn't exist.

These are the bearing blocks used in the gantry. Each one holds a round rod ball bearing bushing. The bushings are tight slip fits, so you can put them in by hand, but it does require some force to push though. Each of the bushings are held in with snap rings. Most of the dimensions on this printer are a mix of english and metric units. Linear slides are all metric, but screws and part dimensions are english. I wanted to use existing stock and screws for most of this project to keep my costs lower, otherwise I would have made every dimension metric.

Here are most of the components in the gantry. Locating dowel pins are not pressed into the bearing blocks or the blocks that hold the X axis rails.

The frame is made from MDF. MDF was chosen because it is pretty cheap, it cuts nicely on a CNC router, it is relatively dense, and it doesn't produce splinters like other woods. If I get around to paining the frame it will look even cleaner than it does now.

Eventually I will glue the frame together, but for now it's going to be held together with clamps. There are still a few features that need to be cut into the frame, so I want to be able to replace or modify the 6 panels.

 Here is the servo mount. The motor is a 350KV outrunner motor that should be able to output 1200 watts. This is total overkill for the printer. This size motor is used because of the low KV which makes it easier to control at a particular voltage. It also eliminates the need for a gear box, which can introduce backlash to a system. The encoder is mounted on top. The gantry uses an H-Bot design. This design uses one loop of belt within the gantry. The servos can be stationary and mounted to the frame with this design. This makes the gantry much lighter than a traditional gantry design where each motor separately controls each axis. In the H-Bot design the position of the gantry is a linear combination of the two motor positions. The gantry moves in the X axis when both motors spin the same direction. The gantry moves in the Y axis when both motors spin in opposite directions. If only one motor is spun, the gantry moves equally in both the X and Y directions.

 Here is the FPGA board and motor control board wired up to the system for testing. The final system will be wired in a much cleaner way...

I'll try to keep up to date on the progress over break. I have this and a number of other projects to start and complete, so I should post more often than during this semester.

Guitar Assembly

It's been in this state for over a month. I'm slowly coming to the sad realization that its going to be at least another month before it gets past this point. The good news is that I have it assembled and playing even though I'm missing most of the electronics and most of the pieces that make up the body. The bad news is it's somewhat difficult to hold while playing in it's current state.

The last big piece I needed to make it function was the main body piece. I was very fortunate to get access to a machining center. Always take up opportunities to make parts on a real CNC. The body was 90% machined in 2 setups. One setup to get the top and one to get the bottom. While getting the tool paths generated took some time, the machine time was a few minutes for each side. That's pretty fast considering the amount of material removed. Auto-tool changers and flood coolant really make aluminum parts fast to machine. 

This is one of my favorite set-ups of all time. I had trouble coming up with a good way to run the wiring from the pickups to the electronics area without the need for a large plate to hide the slots for the wires. My best idea was to drill a hole straight through half of the body. I got to use a 12" long .1875" diameter drill bit. It wasn't really any different from using a normal drill bit... I just thought it was cool. Once I get around to finishing the body I'll counter bore the end of the guitar body so the connector will seat cleanly in the end. The end result will be a very clean looking electronics setup, but it isn't what I would call friendly to machine.

Everything bolted together nicely. I chose to run the lightest gauge strings I could find at my local music store.     I strung the guitar over a month ago so I honestly don't remember which size I used. They took a lot less tension than I had expected. The neck can easily handle thicker strings but I figured I would try out some thinner strings and see how they work with the incredibly rigid aluminum construction.

 Alright... It doesn't quite look like a guitar yet, but its getting there. If you only look at the neck you couldn't tell it's incomplete. The main reason for assembling it without all of the pieces, aside from my lack of patience, was to make sure it sounded decent. The all aluminum design is kind of risky. Most guitars are made from wood, which is a dead material. Dead materials make a thud when hit. Aluminum will ring for quite a while when it isn't damped by something like a hand. Tap-testing a number of the guitar parts including the body made deafening rings that lasted for minutes. After assembling everything the guitar became dead. My guess is that it's due to the bolted joints and the dissimilar materials. In any case it doesn't resonate badly.

 One of my favorite parts of this guitar is the string alignment system. I had to align the strings before passing over the nut. My system uses quite a few stainless parts, but it looks pretty cool and seems to provide near-frictionless alignment.

Overall the sound of the guitar so far is pretty amazing. It has a brighter sound to it than my other guitars. My wooden guitars sound muffled. Since this guitar has different pickups, electronics, and strings along with the aluminum construction I don't know what makes the difference. I do know that I like this guitar's sound better than my other guitars. I've been playing it for over a month now and I just can't will myself to play my wooden guitars. The metal neck has some serious mass to it. When playing solos the wooden guitars can't compare. The thinner strings probably contribute to how easy it is to play, but the solid feeling when I press down on the fret board really makes a big difference as well. The wooden guitars just feel light weight and cheap in my hand. The best way to describe the feeling is holding a solid metal product in one hand and a plastic one in the other hand. The metal almost always feels higher quality. I'm sure most people that play a wooden instrument fear that a neck might snap if it's dropped or mistreated. Not with this guitar. If someone were to try to smash a guitar like this at the end of the concert they would be left with a broken stage rather than a broken guitar.

Pros (so far):

-Brighter sound

-Quality feeling

-Easier to play fast (may be due to the strings, but the solid feel definitely helps.)

-Unlikely to break

Cons (so far):

-Hard to hold (it's missing most of its body)

-Lack of adjustability (It currently has one pickup directly wired to the amp. No volume or pickup selection yet.)

-Thermal stability. The guitar has a warm-up period where it goes out of tune until you play it for a bit. The tuning goes sharp as I play, so it can't be the strings slipping. Since the strings are steel and the neck is aluminum, the neck should expand faster than the strings with an increase in temperature. This should put more tension on the strings and raise the pitch. I haven't confirmed this as the reason for the tuning shift, but I can't come up with a better reason given the conditions.

I'm pretty excited to finish this project up and see how others feel it compares to normal guitars.