170 lines
6.9 KiB
Markdown
170 lines
6.9 KiB
Markdown
Load Cycle Test Jig
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===================
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This is the control software for a machine that runs repeated rotational load
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cycles on (3d-printed) mechanical parts, until they fail. I was originally
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motivated to build this to test ISO 5211 valve adapters (related to [ISO 5211
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Adapter for PVC Ball Valve][iso-5211-pvc]). But it can be used to test any
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mechanical joint.
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You can use this codebase as an example for the
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[`profirust`](https://github.com/rahix/profirust) PROFIBUS DP master stack
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running on a Raspberry Pi.
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[iso-5211-pvc]: https://www.printables.com/model/605515-iso-5211-adapter-for-pvc-ball-valve
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## How it works
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A pneumatic vane cylinder twists the part back and forth at a torque set by
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the pressure regulator. The software counts each reversal and stops the run
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as soon as one of the two end-stop sensors trips, which means the part has
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given way and the cylinder is free to rotate past its normal travel.
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#### Main hardware components
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- pneumatic vane cylinder (FESTO [DSRL-40-180P-FW][festo-30658])
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* 5/3-way solenoid valve to control the cylinder
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* two inductive end-stop sensors on the cylinder (detect part failure)
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- filter-regulator maintenance unit (regulator controls the maximum torque)
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* pressure switch (detect low air pressure)
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- stack light (white, blue, orange) - see [Stack Lights][stack-light-post] on my blog
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- HD44780 LCD (20×4 characters) as the operator display
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- WAGO [750-303][wago-750-303] PROFIBUS fieldbus coupler for I/O signals
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- Raspberry Pi as the PROFIBUS master, with a MAX485 transceiver (See
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[Running RS-485 with a MAX485 on a Raspberry Pi](#running-rs-485-with-a-max485-on-a-raspberry-pi)
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below)
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[festo-30658]: https://www.festo.com/de/en/a/30658/
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[stack-light-post]: https://blog.rahix.de/stack-lights/
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[wago-750-303]: https://www.wago.com/de-en/io-systems/fieldbus-coupler-profibus-dp/p/750-303
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## Operating modes
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There are two switches which control the machine:
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- Mode selector (continuous or single)
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- Start/stop button
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The modes do the following:
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- **Continuous**: The cylinder alternates left and right every 5 seconds, the
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counter increments on each reversal, and the run ends when a limit switch
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trips. This is the standard "how many cycles until it breaks" test.
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- **Single**: One short preparation move, then a timed swing toward the opposite
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end stop. This is used to measure time to failure for a single event.
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Once failure occurs, the buzzer on the stack light pulses to let you know it's done.
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The LCD shows the current state together with either the cycle counter
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(continuous mode) or the elapsed cycle time (single mode).
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## Software architecture
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This is a pattern that I find to work well for control system software on
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hosted systems (the Linux on the Raspberry Pi in this case). It is not
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particularly efficient, but very simple and robust.
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There are three threads:
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- The main thread runs the 20 ms control loop that computes the control system
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logic (`Logic::run()`). Before and after, it copies I/O data to and from the
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fieldbus process image and display.
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- A background thread runs the PROFIBUS DP master and the cyclic data exchange
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with the WAGO coupler ([`fieldbus.rs`]). The cycle time is adjusted
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to be optimal for the selected bus speed.
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- A third thread drives the HD44780 LCD over I2C ([`display.rs`]). It is woken
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from a condvar whenever a line changes.
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For any further I/O channels, you'd add more threads. Data is shared through
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mutexes which are only locked briefly to read or update data. This is very
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important to ensure no thread is ever blocked for long periods of time.
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You can find another example of this pattern in the
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[crab-control-center](https://github.com/muccc-rs/crab-control-center) code.
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#### Source layout
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- [`main.rs`] runs the control loop and does the data plumbing.
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- [`logic.rs`] contains the actual control logic: machine mode state machine,
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cylinder sequencing, status reporting.
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- [`fieldbus.rs`] contains the PROFIBUS communication, and peripheral configuration.
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- [`display.rs`] is the HD44780 over I2C driver thread.
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- [`util.rs`] contains small helpers that mirror the ones found in industrial PLC
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programming (e.g. `TimerOn`, `TimerOff`, `PulseTimer`).
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## PROFIBUS
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The fieldbus module ([`fieldbus.rs`]) contains everything related to PROFIBUS
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communication:
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- Setup a DP peripheral with `profirust` using the Linux RS-485 PHY implementation
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- Peripheral parameters from a vendor GSD file (this was autogenerated with profirust's [`gsdtool`](https://github.com/rahix/profirust/tree/main/gsdtool))
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- Exposing the process image to the rest of the application via Mutex
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#### Running RS-485 with a MAX485 on a Raspberry Pi
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PROFIBUS uses an 11-bit character frame with even parity. The Pi's default
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"mini UART" (`/dev/ttyS0`) does not accept `PARENB` (=even parity bit) and is
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therefore not usable. The proper PL011 UART has to be enabled instead, and the
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MAX485's DE/RE driver-enable line has to be tied to the UART's RTS pin so the
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kernel can switch the transceiver in step with each frame.
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Switch the primary UART to the PL011 by adding the following to
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`/boot/firmware/config.txt` and disabling the Bluetooth HCI service:
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```
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dtoverlay=disable-bt
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```
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```bash
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sudo systemctl disable --now hciuart
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```
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After a reboot the PL011 is available as `/dev/ttyAMA0`, which is the device
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the code opens in [`fieldbus.rs`].
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Hardware flow control on the primary UART is not exposed by the stock
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overlays, so a small device tree overlay is needed that routes RTS (and
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optionally CTS) to a GPIO.
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This overlay source can be used as is: <https://github.com/HiassofT/AtariSIO/blob/master/contrib/rpi/uart-ctsrts.dts>
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Compile and install the overlay with:
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```bash
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dtc -@ -Hepapr -I dts -O dtb -o uart0-ctsrts.dtbo uart-ctsrts.dts
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sudo cp uart0-ctsrts.dtbo /boot/firmware/overlays/
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```
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Then append the following line to `/boot/firmware/config.txt` and reboot:
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```
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dtoverlay=uart0-ctsrts
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```
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Wire the MAX485 so that
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- DI to Pi's TXD (GPIO 14)
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- RO to Pi's RXD (GPIO 15)
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- combined DE and RE pins to the RTS GPIO chosen in the overlay (GPIO 17)
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Add the usual 120 ohm termination at both physical ends of the bus.
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## References
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- [`profirust`](https://github.com/rahix/profirust)
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* [Blog post about `profirust`](https://blog.rahix.de/profirust/)
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* [Blog post generally explaining PROFIBUS](https://blog.rahix.de/profibus-primer/)
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- [`hd44780-driver`](https://github.com/JohnDoneth/hd44780-driver) LCD display driver
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[`main.rs`]: ./src/main.rs
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[`logic.rs`]: ./src/logic.rs
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[`fieldbus.rs`]: ./src/fieldbus.rs
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[`display.rs`]: ./src/display.rs
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[`util.rs`]: ./src/util.rs
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## License
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Licensed under either of
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- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
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<http://www.apache.org/licenses/LICENSE-2.0>)
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- MIT license ([LICENSE-MIT](LICENSE-MIT) or
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<http://opensource.org/licenses/MIT>)
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at your option.
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