Large Scale Delta drive system v2

After the initial testing of the pulley system used to reduce the speed and increase the torque of the stepper I found out that I had used the incorrect size of timing belt so anytime I tried to lift more than around 8 lbs the timing belts would slip. So I went about and created an updated version correct for this and implement a few other improvements.

Delta Mech Drawing v2

Simplified 3-D model of the updated design.

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Daisy Chaining multiple AutoDrivers

In a previous post I tested running SPI over a distance of roughly 100 feet using RS-485 over Cat5 with only a single SPI slave device, in this post I will cover the how to have multiple SPI slaves (AutoDriver with RS-485 shield) daisy chained together. The goal here is to communicate with multiple AutoDrivers over a long distance with only a single chip select signal. Continue reading

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AutoDriver SPI over RS-485 Shield v2

After verifying that the AutoDriver v1 shield was sound, I made a few changes to improve its functionality and ease of use. The biggest change was the addition of a switching power supply that can accept anywhere from 6 to 60 volts. This will allow it to be powered directly from the same power supply used to drive the stepper motors and removes the need for a separate 5v supply. The second change was switching from a transistor to a single logic inverter to invert the chip select signal. During initial testing the transistor would not turn on or off quick enough to function properly, the single logic gate would guarantee the quick switching times needed. The final change was to reorganize the signals on the connector simplify connecting up the wiring between devices.

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AutoDriver Shield v2 schematic

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SPI over RS-485 Testing

Arduino communicating with AutoDriver via SPI over RS-485 with 100' loop of CAT5.

Arduino reading the status register from the AutoDriver via SPI over RS-485 with a 100′ loop of CAT5.

In my last post, SPI over long distances, I described a method of using RS-485 to run a SPI bus over long distances. To test this setup I created two prototype shields, an Arduino SPI over RS-485 shield and an AutoDriver shield. Because the standard Arduino only has a single SPI bus and I needed two, one acting as a master and another as a slave, I would need to find a way to simulate one. Continue reading

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SPI over long distances

As part of the Large Scale Delta project I needed to control multiple stepper motors, each being over a distance of roughly 50 to 150 feet. To control each motor I chose to use the Sparkfun AutoDriver board which will drive up to 3 amps continuously and communicates over SPI. The AutoDriver, which uses STmicro’s L6470 chip, has built in current limiting, over current protection, stall protection, micro stepping and more but what is most important is that it has motion commands which handle the low level motion control aspects allowing you to command a position and it will handle the rest. This along with Sparkfun’s AutoDriver Library allow for easy and precise control of a stepper motor from an Arduino. While a SPI bus is easy to use over short distances it was not designed to run more than probably a few feet and definitely not tens or hundreds of feet, when you get to that kind of distance it introduces a few issues that need to be overcome.
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Stepper Motor Lift

Last weekend I built one rig for testing my calculations for the large scale delta project and to verify the concept. The goal was to be able to lift 22 lbs vertically using an 125 oz-in stepper motor and an AutoDriver stepper motor driver, both from SparkFun. All the mechanical bits, timing pulley, timing belt, pulley and axle, were from SPD/SI. While I was able to determine that the general concept will work it failed to lift the 22 lbs. This was due to the timing belt and pulleys not being a heavy duty enough and skipping when the load got around 8 lbs, so on the next version I will go for better a heavier duty timing pulley. See below for a video of it in action.

stepper rig 2stepper rig 1

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Visualizing the tension on wire for a suspended mass with python

I’ve been playing around with the idea of building a rig to suspend an object, like a camera or prop, using three wires and then control its position in 3-D space using a few stepper motors and a micro-controller. The general idea is similar to the skycam system they use at football games and other pro sports, or a large scale delta robot. The first thing I needed to figure out was how much I wanted this rig to be able to hold and how much tension (force) would be required to hold and move this weight around. Additional I wanted to be able to visualize the results so I could look for any peaks, valleys or other anomalies that would throw off my calculations. I ended up writing the python script below utilizing the mpmath, sympy and matplotlib libraries to calculate the tension on a wire due to a suspend mass at a specific height and plot it as the weight moves around the x/y plane. The image below is a sample of the output from this script for a 5 kg mass suspend 2.0 m above the ground with three anchor points at 3.0 m above ground level and each 120 degrees apart.

wire_tension_plot

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