Mechanical challenge accepted
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So I was asked to make the mechanical ECG Heart monitor for the London and also the New York stage show of Stranger Things.
The first job was to work out exactly how this unit would fit into the existing hospital trolley that the stage show had purchased. It had to basically spool paper out of the end with a heart trace on it and the paper had to pile up on the floor, rather than stay within the machine (like a normal ECG would). I say normal ECG, they don't make them like this in modern times. If we had been given more time, then the easiest way would have been to purchase a second hand ECG machine and alter this for our needs. But, time was against me and they also did not want to constantly re-load it with paper. I CAD designed a basic idea. A large spool that stored the paper roll (which isn't paper, but more on that later). A second geared motor ran a much smaller spindle that pulled the paper off the roller. |
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This turned out to actually be quite complex. A series of moving roller clamps were required to make sure the paper stayed under tension.
I 3d printed most of the mounting hardware and this all got mounted on a timber plinth/module. This fitted behind an opening in the timber troller cabinet.
The front of the timer was clad in white Acrylic to provide a smooth running surface and an aluminium frame added to guide the paper.
I 3d printed most of the mounting hardware and this all got mounted on a timber plinth/module. This fitted behind an opening in the timber troller cabinet.
The front of the timer was clad in white Acrylic to provide a smooth running surface and an aluminium frame added to guide the paper.
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The roller had laser cut end pieces on an aluminium tube core.
Driven by a GT2 belt from a geared 60RPM motor. The paper was actually Duckram. This is re-enforcing material used at the bottom of large curtains. Flexible, tough, but a good replacement for paper. Getting this thing to roll up straight proved to be a nightmare. The slightest offset, and the paper would not roll up neatly, so a lot of adjustment had to be built in. |
The ECG trace is already drawn on the paper. The fake needle that bounces around on the front of the paper just gives the illusion that it is drawing the line.
When you ask it to pay out the paper, the spool motor runs slightly slower than the geared motor at the left hand end that is pulling the paper out of the machine. This keeps the paper tight across the front of the machine.
The pair of geared motors were controlled through a H-Bridge motor controllers.
A servo also opens the rollers at the spool end to 'release' the paper tension. A second servo closes on the left had 'pulling' end of the mechanism to add a clamping roller to the motorised pull spool (so it grips the paper).
When you ask it to pay out the paper, the spool motor runs slightly slower than the geared motor at the left hand end that is pulling the paper out of the machine. This keeps the paper tight across the front of the machine.
The pair of geared motors were controlled through a H-Bridge motor controllers.
A servo also opens the rollers at the spool end to 'release' the paper tension. A second servo closes on the left had 'pulling' end of the mechanism to add a clamping roller to the motorised pull spool (so it grips the paper).
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A speed control knob adjusts the paper payout speed.
When the paper gets towards the end of the spool, a black marker strip on the paper covers an IR sensor and this stops the machine. Flick the direction switch and the servo clamp roller on the left opens to release the tension on the paper. The second servo on the right now closes a second set of clamp rollers to provide tension for the spool motor to pull the paper back in. A second IR sensor stops the paper when it reaches the home position. |
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Add some LEDs on the front aluminium front runners and the fake servo needle and it's complete.
The whole unit ran off a 12v battery. The system was controlled by an Arduino Mega, fitted into a breakout board. A Gyro was added to stop the paper spooling out if the machine was tilted or fell over. |
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