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My Digital Setting Circles, Bluetooth Ek's Box and Magnetic Rotary Encoders
My DSC system is based on the Digital Setting Circles project by Dave Ek. It's an aged but yet really effective system for DIY push-to projects.
I wanted to implement and use the very small AMS AS5040 Magnetic Rotary Encoders (MRE) I had already at home (here my page that introduce these encoders). They are available at AMS's site as free sample (max three items) or on the web for a few bucks. Their resolution is 1024 tics (10bit) but a 12bit version is also available (AS5045) with 4096 ticks per revolution.
I started some time ago with a couple of prototypes (here and here, two old videos showing the first encoder's testing) and then I continued trying to optimize the controller's and encoder's housing hardware and components layout.
DSC Controller and Encoders electronics
I designed a dedicated PCB (using Fritzing, then printed via EasyEDA cheap online PCB production service) that fits exactly into a easily commercially available plastic enclosure, leaving room for a 9V battery also. Here below you can find a complete list of all the components needed to build my controller. The controller is an Ek's DSC based Bluetooth version mainly with two RJ9 jacks for the encoders, a main power switch, optional LEDs for testing data feedback, a Bluetooth HC-06 module for communication and is powered by a 9V battery.
- RJ9-4p jack
- Switch
- HC-06 module
- Plastic enclosure
- SSOP-16 adapter board for soldering the AS5040 chip
- MXL gears, shaft adapter and belt
The encoders are SSOP-16 chips that have to be soldered on a breakout board adapter so they can be easily mounted up-side-down on top of the polarized magnet that in turn is fixed on top of the pulley rotating shaft. The electronics is very simple, a 1uF electrolitic capacitor and a diagnostics LED.
According with the MRE design, this LED will go on only if the magnet will be in a out of range distance situation (too close or too far from the AS5040 chip surface, take a look at the encoder specifications for details).
With these two components mounted directly on the encoder support board, the remaining pins that need to be wired to the controller box are the two quadrature channels A & B togheter with 5V and GND.
The two small MRE's PCB support boards I currently use have been home made (home etching) but I just designed a Gerber version that can be ordered on PCB online services, togheter with the main controller board. More on this later.
Encoders' Gears mechanics
For the MRE's support board I built a small 'carrier' that holds the 20T pulley internally (aluminium square profile tube 20x20mm) and act as a slider with respect to the remaining structure. A short stainless steel shaft (5mm diameter) is used to keep the pulley and the MRE's polarized magnet (small 6mm diameter cylinder) on top of the shaft by means of a small rubber (Meccano spare part) tube.
For the encoders accomodation, the original idea was to build a simple pulleys/timing belt system to drive the encoder's magnet rotation but soon I realised that the mechanical inaccuracy of my wooden home-made dobsonian mount was causing the belt to not mantain the needed constant tension. So I decided to go for a much more reliable solution: direct gears transmission, with spring.
In principle, the encoders' housing has one fixed part (mounted directly on the dob's structure) and another one (holding the actual encoder components like sensor chip, magnet and mechanical support structure) that is free to slide precisely (and constrained to one linear movement) with respect to the fixed one. A steel loaded-spring helds in constant contact the two gears.
The first attempt involved the direct contact between the two gears (MXL standard) but this wasn't a completely successfull solution because of an unavoidable gap (backlash of about 0.1mm or even less but enough to introduce significant errors in the encoders precision reading) between the gear's teeth. The final and entirely satisfactory solution was to cover the 'big' gear (the one linked to the telescope rotation axes) with a timing belt having the same outer diameter (luckily, I found two of them at home, 96T) so that, with a slight finger's pressure, they could be firmly set around the plastic gear. Now the mechanical trasmission is absolutely no backlash.
Testing the Controller
Links to Ek's pages for software downloads and information:
Bluetooth DSC firmware (hex file for the PIC microcontroller)
EkBoxTester (go to "Testing" chapter)
DSCWin Ek's software
Sky charts and Astronomy compatible software
I was already confident about the reliability of these MRE encoders, starting from the very first tests I made in the past, that have shown clearly their goodness. As you can see in the video below, the DSC's controller is able to read and manage quite precisely the quadrature's data coming from our AS5040 encoders. The AS5040 chip is 1024 ticks per revolution and the gears ratio I use is 96/20 teeth so the final encoder resolution is 4915.2 (approximated to 4915). One complete revolution of the telescope axis is 4915 steps.
Here below at left, a video showing the testing with Ek's testing program, EkBoxTester. At right, the controller connected to SkySafari Plus for Android on a Samsung Tab2 7".
The gDSC controller works fine with many Sky Charts softwares, like Cartes Du Ciel and Stellarium.
These programs needs the Ek's ASCOM driver (see above) to be installed on your computer. For CdC, open the ASCOM Telescope Chooser, under Telescope Control Panel and select the Tangent-Compatible Digital Setting Circles driver then click on Properties...
The driver settings window will open. Choose the Dave Ek's Digital Setting Circles interface type from the drop-down list then set the encoders resolution (for me is 4915). Choose also the COM port that correspond to your HC-06 Bluetooth module.
Cartes Du Ciel
Stellarium
...
Work in progress ...
Gerber files for boards production will be added soon in the PCB page ...
November 2017
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