DIY Dual H-Bridge to control a Pen Plotter

In my previous post I pulled apart an old pen plotter revealing it to be controlled by two brushed DC motors.  In order to power each DC motor a H-bridge was required but I had none on hand. However it turns out a H bridge can be constructed from a few transistors as can be seen in this useful guide.

What I came up with uses a Darlington array (ULN2003) to serve as the NPN transistors for both H-Bridges while 2N3906 transistors are used for the PNP side. These PNP transistors are switched using PN5856 NPN transistors so that the positive voltage output of the Arduino can be used. Its powered by 12V DC running at around 150mA (300mA stall current).

Throw in a few LED’s so that its clear when each direction is activated and the pen plotter now has bidirectional control for the x and y axis. Video of it in action below:

Please note this design was only thrown together to make use of what I had on hand and there are far more effective ways of building an H-bridge.

In the video you can see the two 5kOhm potentiometers. When connected to the ADC on the Arduino they give a full scale range of 726 (256 to 982). With the largest travel distance being 260mm for the x axis this gives an upper resolution limit of approximately 0.36mm. It seems likely that mechanical limitations would limit the resolution to a value far greater than this technical upper limit.

By writing a sketch to record each pot’s value and then pulse the motors until this value is reached it is quite simple to instruct the pen plotter to move to a position and then hold there.

To take this one step further I added two external potentiometers and used them to turn the pen plotter into an overly complicated Etch A Sketch.

You can find a copy of the sketch used to control the plotter here.

I might have a go some time at using the Arduino maths library to get the pen plotter to draw some interesting shapes.

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Old-school Scientific Pen Plotter Teardown

I recently acquired an old Rikadenki pen plotter from a lab I work in and have decided to pull it apart to investigate how it works. It turns out to be quite unlike any of the Cartesian bots currently used by the DIY community.

The pen plotter was originally used to plot magnetic hysteresis loops of magnetic materials onto paper using a pen fixed in a holder. A voltage from a scientific instrument was fed to each axis with the size and polarity of the voltage dictating the position of the pen.

Note that all images can be enlarged by clicking on them. 

In the image above you can see the pen plotter with its x and y axis. The pen is fixed in the holder attached to the y axis and can move vertically only. The entire Y axis assembly can then move horizontally to form the x axis.

With the pen plotter turned over and its base removed the internal electronics can be seen.

It appears to be quite old in its design. As I dont have an electrical engineering background I didnt attempt to repair its existing electronics and so they were removed.

With the electronics removed two brushed DC motors and two potentiometers can be seen, one for each axis.

A thin gauge wire is used to move each axis. The potentiometers are also connected to the motors and geared such that a full movement of an axis from one end to another results in 1 full rotation of the potentiometer. It appears the potentiometers were used as position feedback. Each axis slides on a stainless steel shaft with out the use of bearings.

Looking side on these gear reductions can be seen more clearly.

Note the wire is guided by rotating spindles (seen in the bottom left) and spools around the drive wheel to ensure it doesnt slip. Springs are used to maintain tension on the wire where they are fixed to the centre of some of the hubs.

The path of each of the thin gauge wires, one for each axis, is hard to see in these images so I have tried to draw it schematically in sketchup. Note that the actual path in the pen plotter differs, but the idea is the same. This is the x axis.

When the motor turns it pulls the wire through a series of rotating guides to form a continues loop. The whole y axis assembly (the vertical grey rectangle in the image) is attached to the wires at the red points and is then moved left or right depending on the direction of the wire.

Below is the Y Axis.

When the y axis motor turns it pulls on the y axis carriage from the top of the image while also letting out wire for the carriage at the bottom of the image. This causes the y axis to move up, or down if the motor direction is reversed. If the X axis moves the whole y axis moves to the left or to the right. However, the y axis carriage does not move in the vertical direction as the y axis motor is stationary and so wire is pulled in and let out from the right side of the image where its fixed.

Here is one last higher quality photo with more parts removed.

Needless to say the movement of the x and yaxis is a lot more complex than a typical DIY belt driven 3D printer. However this complexity does have its advantages. For example, I imagine wire bought in bulk could be far cheaper than timing belts. Also DC motors a lot easier to find/buy than stepper motors. Perhaps of most interest though is the ultra low weight of the x and y assembly which could allow for very fast direction changes.

The next step is to put a H bridge together with an arduino and see if this pen plotter can be brought back to life.

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Playing around with a heated chamber design.

For quite some time I have lusted after a 3D printer with the following specifications:

  • A 20x20x15cm build area
  • A heating print bed
  • A heated build chamber (ambient to 100C) to possibly eliminate warping.
  • A respectable print resolution and speed

Over 12 months ago I made my first serious attempt at satisfying these by designing and half building a rep-strap based on the mantis CNC design. However serious limitations in the design and a lack of free time have resulted in the ‘gunstrap’ collecting dust for over 12 months.  Among others, the problems with the gunstrap were:

  • High rolling resistance due to metal on metal sleeve baring
  • Slow speed due to needing to physically move the heavy print bed and print head. assembly.
  • Poor resolution due to the design of the Z and Y axis.
  • Extruder stepper located within the build chamber.

To overcome each of these limitations I have spent some time designing a replacement in SketcUp, as seen below.

What you see above is a fully enclosed build chamber that will be constructed from 12mm wood fibre board or similar. The blue transparent section is a double layer glass viewing window that is opened by the handle below it.

When opened, the two axis print head and print bed are accessible. To the right of the window will be a 16 character 2 line LCD display for temperature readouts and the like.

This design features the following:

  • All electrical components and motors (ex end stops) are located outside of the heated build chamber.
  • Print head weight has been reduced to as little as possible to increase print speed and resolution.
  • Rolling resistance is lowered through the use of ball bearings.
  • Scissor lift Z axis for increased stability

The scissor lift Z axis will be constructed by modifying a  lab jack similar to the one shown here. If the wooden shell, which also acts as the main structure of the printer, is removed then the workings become more clear. The modified lab scissor jack coupled to a stepper motor can be seen below (click image to enlarge).

Looking from the front top down on the two axis print head stage its seen that its composed of stainless steel shafts for guides like a Mendel, a PTFE sleeve bearing for the print head holder similar to a Ultimaker. Rather than an expensive belt I plan on sourcing some fine braided wire to use as a pulley which I have seen work quite well on older mechanical pen plotters for lab work. I plan on cutting box section aluminium from corner to comer to make L pieces to hold the guide bearings.

Feeding filament into a wades extruder on the side of the printer will be a mounted filament spool. The wades extruder will force the filament up a PTFE tube which enters the printer at a hole located at the top of the printer.

Finally, in a side compartment insulated by double thickness paralleling will be the electronics. This includes a RAMPS based stepper driver system, ATX powersupply and cooling fans.

As no low melting point plastic components or electrical equipment is contained within the build chamber I believe the high build chamber environment of 100C should be achievable. The heat will be provided by the heated print bed and print head only and will be actively circulated by a fan at the top of the chamber.

I am plananing on sourcing the bearings from smallparts.com.au and modifing them to include a flange. The stepper motors will come from robotgear.com.au for around $85 for 4,  including shipping within Australia. I have a month off before starting a PhD in 2012 so hopefully that allows enough time to get this all built and calibrated.

You can find a copy of the 3D model from here. Some parts of the model were sourced from Googles 3D warehouse including the Steep Reel, bolt, Arduino Mega, character display and ATX powersupply.

I would love to hear what people think of this design and so welcome all comments and criticisms. If you have any suggestions for improvements or alternative ideas please leave a comment!

Happy new year to all!

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MIT: 3-D printing with variable densities

By variable densities this video is referring to the density of the infill, not the density of the material extruded. At least that was my take on it.

Hasn’t the DIY 3D printing scene been doing this for quite some time now with the use of spars internal structures for solid objects?

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Mike Biddle: We can recycle plastic

An interesting TED talk.

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Printable PCB motor?

The holy grail of a self replicating 3D printer is the ability to print its own drive train. In the long term this may be possible with some form of multi-metal stintering system that can produce a stator layer by layer. However in the near term a more viable DIY option may be to use a piezoelectric “PCB Motor”.

The printing, routing or embedding of wires to form a circuit board track is now common place and with pick and place of components well on its well this could become feasible sooner than thought.

There was already an an entry on the RepRap wiki regarding PCB motors but I cant find any evidence that they have been successfully used.

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Cryogenic Granular Grinding

I recently came across this industrial grade meet grinder. Its located in a laboratory used for, among other things, injection moulding.

I’m told that its used to grind plastic granules down into a fine powder. What is particularly interesting though is that this will only work if liquid nitrogen is poured in while it is grinding or else it will quickly become jammed. It seems the liquid nitrogen cools to the plastic (−196 °C, 77 K , −321 °F) and so reduces its fracture toughness and allowing it to be broken into smaller pieces.

So is this a practical approach for those at home wanting to attempting to recycle their own plastic for use with a 3D printing? Obviously not all of us have access to liquid nitrogen or even dry ice, so would a home freezer provide much of an advantage?

I was unable to find any useful information on the deformation properties of HDPE, ABS or PLA at low temperatures in the limited time I have available and so its difficult to tell if a domestic freezer (-20°C?) would be cold enough to make a usable difference.

If anyone wants to put on their science hat and undertake a few home tensile tests on a short lengths of 3mm filament to produce a stress-strain curve at low temperatures and at room temperature it could provide interesting results.

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