145 lines
6.2 KiB
Markdown
145 lines
6.2 KiB
Markdown
project
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=======
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summary
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-------
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this clock jockey monitors the tachometer of a motor turning a clock, and regulates its speed by switching the power to it.
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technology
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the goal was to make a round clock quadrant turn.
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the quadrant is made out of wood, around 1 m heigh, and weights around 5 kg.
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it is mounted on a disco ball base.
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the bearing is strong enough to hold it and allow the clock to turn around its center.
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originally the disco ball base included a motor, but it was too fragile to withhold the forces.
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two plastic gears in the gear box broke.
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this was the state of the clock when I joined the team.
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I decided to fix the turning clock.
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the small DC or stepper motor I tried were too weak to turn the clock.
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I decided to use a universal motor salvaged from a washing machine.
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the pinout of the motor was undocumented, but not hard to figure out.
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two wires went to a small module at the end of the rotor shaft.
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this is the tachometer to measure the rotation speed.
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it is also easy to trace the two wires going to the brushes feeding the power to the rotor coils.
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next, measure the resistance of the remaining pin pairs.
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one will have 0 Ohms.
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this is the fuse to protect against over-heating.
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there will be two pairs of 0.6 Ohms, and one of 1.2 Ohms.
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these are the two coils for the stator, with a center tap.
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wire the motor with 220V AC the following way:
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AC L - fuse - rotor - stator (2 coils, not center tap) - AC N
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to limit the speed I used a 4000 W capable SCR.
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the SCR was not enough to regulate the motor speed.
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setting the potentiometer to a fixed point would results in the motor to stop turning after some time, of to speed up to fast.
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the user would have to adjust the potentiometer using then knob periodically to have a somewhat constant speed.
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to overcome this limitation I developed the clock jockey.
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this device will monitor the speed of the motor using the tachometer, and switch the power to the motor using a Solid State Relay (SSR), so to reach and stay at the predefined speed.
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the motor tachometer provides an AC signal.
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basically it's a generating motor linked to the shaft of the actual motor.
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the AC frequency (and voltage) indicates the speed of the motor.
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the AC signal is rectified using a full bridge rectifier.
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the rectified signal is input to an PC817 optocoupler (with ~ 200 Ohm inline resistor).
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the full bridge rectifier protects to optocoupler, which has a reverse breakdown voltage of 6 V, lower than the seen -10 V peaks of the AC signal.
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it also allows to get two pulses per AC period, for a more accurate speed measurement.
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the optocoupler is directly connected to the clock jockey.
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an omron G3MB-202P (5V) is also connected to the clock jockey to switch to power going to the motor.
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the SCR is still in line to limit the maximum delivered power.
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board
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=====
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the device reuses an [ST-LINK/V2 clone](https://wiki.cuvoodoo.info/doku.php?id=jtag#mini_st-link_v2).
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this board uses a STM32F103C8T6 micro-controller.
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since the device is firmware protected (against read-out), you will first need to remove this protection using an SWD adapter and running `rake remove_protection`.
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you can then flash the bootloader using SWD and application using DFU a documented in the section below.
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connections
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===========
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connect the peripherals the following way:
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- PC817X emitter, SWIM pin (PB11, pulled up to 3V3 on board)
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- PC817X collector, GND
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- G3MB-202P SSR 3+, 5V (5V required by SSR, with embedded resistor)
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- G3MB-202P SSR 4-, SWDIO (SWCLK pin in not 5V tolerant)
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all pins are configured using `define`s in the corresponding source code.
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usage
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=====
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the firmware will simply count the number of optocoupler falling edges (2 per revolution).
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it will periodically compare this count to the target value.
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if this is below, the SSR will be switched on.
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if it is above, the SSR will be switched off.
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to set this target, connect the clock jokey to a computer.
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it will appear as a serial device (using the CDC ACM USB profile).
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use a serial terminal program and connect to it (the baud rate does not matter).
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enter `tacho xx`, with xx being the target tachometer count.
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to view the set and current tachometer values, enter `tacho`.
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the target tachometer count is somewhat relative.
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it is proportional to the motor speed and tied to the periodic check (currently set to 0.1 s).
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if the tacho count it at 0 for too many seconds after boot, the clock jockey will with off the power.
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this is a safety feature since the tachometer is either not reading the speed, or the motor is stuck.
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code
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====
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dependencies
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------------
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The source code uses the [libopencm3](http://libopencm3.org/) library.
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The projects is already a git submodules.
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It will be initialized when compiling the firmware.
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Alternatively you can run once: `git submodule init` and `git submodule update`.
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firmware
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--------
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To compile the firmware run `rake`.
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documentation
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-------------
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To generate doxygen documentation run `rake doc`.
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flash
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-----
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There are two firmware images: `bootloader` and `application`.
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The `bootloader` image allows to flash the `application` over USB using the DFU protocol.
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The `bootloader` is started first and immediately jumps to the `application` if it is valid and the DFU mode is not forced (i.e. by pressing the user button on the board or requesting a DFU detach in the `application`).
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The `application` image is the main application and is implemented in `application.c`.
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It is up to the application to advertise USB DFU support (i.e. as does the provided USB CDC ACM example).
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The `bootlaoder` image will be flashed using SWD (Serial Wire Debug).
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For that you need an SWD adapter.
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The `Makefile` uses a Black Magic Probe (per default), or a ST-Link V2 along OpenOCD software.
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To flash the `booltoader` using SWD run `rake flash_booloader`.
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Once the `bootloader` is flashed it is possible to flash the `application` over USB using the DFU protocol by running `rake flash`.
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To force the bootloader to start the DFU mode, short the RST pin to the nearby ground pin.
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It is also possible to flash the `application` image using SWD by running `rake flash_application`.
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debug
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SWD also allows to debug the code running on the micro-controller using GDB.
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To start the debugging session run `rake debug`.
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USB
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---
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The firmware offers serial communication over USB (using the CDC ACM device class).
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