This is a port of the very sophisticated DC Servomotor Controller SMC3 by Elm Chan for the ATmega328 as used with the popular Arduino Uno and Arduino Nano boards.
It includes the results of a detailed analysis of the inner workings of the source code and (hopefully) more beginner-friendly explanation of the meaning of the control parameter and how to choose sensible values for the desired application.
This analysis includes a full translation of the assembler code into C code. This was started as a tool to understand the inner workings of SMC3 but it is turning into a full project of its own right now.
The program logic is as close to the carefully optimized assembler version as possible, but it is much easier to read than the highly optimized hand-crafted assembler masterpiece by Elm Chan.
It relies on the Arduino environment for the UART part and some initializations, but the resulting code behaves identical to the original version. The main point is to show the internal logic and to be a more accessible base for modifications. The price for this convienence is a much bigger binary size: 4.8kB instead of 1.8kB.
- Originally written for the (now obsolete) ATtiny2313 microcontroller and now ported to the ATmega328.
- accepts serial command via UART
- Direct Access to Position Command Register
- G0/G1 Motion Generation
- Three Control Modes
- Absolute Positioning Mode
- Constant Speed Mode
- Constant Torque Mode
- Software Decoding of Quadrature Encoder Input
- Count rate over 100,000 counts/sec in quadrature decoding
- No External Component Required
- Two PWM Outputs for H-bridge Driver
- Supports bootstrap type FET driver (minimum duty ratio 15/16)
- Three Diagnostic Outputs
- Ready, Torque Limit and Servo Error
- On-the-Fly Servo Tuning
The version SMC3A is optimized to be a drop-in replacement for stepper motor drivers: (but not ported yet). Instead of the UART interface it supports:
- Pulse/Dir Command Input with Pulse Multiplyer
- Pulse rate over 100,000 pulses/sec
- Gear ratio from 1/256 to 255 in 1/256 step
Differences of this version to the original:
- ported to ATmega328, should be easy to adopt for other ATmega CPU
- optional decay function for the integral sum to avoid unnecessary motor current after the target position is reached.
- no support for the proprietary serial position display on the SPI pins.
Install avra (The syntax is not compatible with avr-as):
sudo apt install avra
compile:
make
upload/flash to an Arduino Uno using avrdude:
make upload
Or more in detail:
avra -I inc smc3.asm
avrdude -c arduino -p m328p -U flash:w:smc3.hex -P /dev/ttyUSB0
The avra package on Debian-based systems (Ubuntu, Mint etc) is very, very old and misses the required CPU definitions for the ATmega328. That's why m328Pdef.inc is included in this repository.
To compile the original files (they were written for the orignal AVR assembler, but avra is almost 100% compatible):
avra -I /usr/share/avra smc3.asm
| Signal | Pin ATtiny2313 | Pin m328 | Arduino Uno pin number |
|---|---|---|---|
| Enc_A | PD5/T1 | PD5/T1 | 5 |
| Enc_B | PD4/T0 | PD4/T0 | 4 |
| Enc_Z | PD3/INT1 | PD3/INT1 | 3 |
| nSTEP | PD2/INT0 | PD2/INT0 | 2 |
| DIR | PD6 | PD6/OC0A | 6 |
| TxD | PD1/TXD | PD1/TXD | 1 |
| RxD | PD0/RXD | PD0/RXD | 0 |
| SCK (Clk) | PB7/SCK/USCK | PB5/SCK | 13 |
| MISO (SegData) | PB6/MISO/DO | PB3/MOSI | 11 (intentionally swapped) |
| MOSI (DigitClk) | PB5/MOSI/DI | PB4/MISO | 12 (intentionally swapped) |
| Mot_n | PB4/OC1B | PB2/OC1B | 10 |
| Mot_p | PB3/OC1A | PB1/OC1A | 9 |
| LED_Ready | PB2/OC0A | PC2 | 16 (flexible) |
| LED_TorqueLimit | PB1 | PC3 | 17 (flexible) |
| LED_ServoError | PB0 | PB0 | 8 (flexible) |
| RESn | PA2/RES | PC6/RES | |
| Xtal2 | PA1/X2 | ||
| Xtal1 | PA0/X1 |
The three LED connections can be easily reconfigured in the source code. All other connections are very hard-coded and would require major changes.
Connect the position encoder and the motor driver, start a serial terminal with 38400N1. Check if the motor runs on a 50% PWM signal:
SMC type 3
%m0
%s128
Turn off the motor:
%s0
The higher modes require the definition of some motor control parameters. These values depend on the motor, the supply voltage, the encoder and the masses involved.
Follow this guide in the project wiki on how to measure and calculate them. Once you have a set of numbers define and save it:
%p0 20
%p1 256
%p2 2560
%p3 13
%p4 240
%p5 9
%p6 4480
%p7 16
%w0
These values are stored in EEPROM and can be recalled any time by
%r0
Now let's move! But be careful: This can physically crash your machine if the speed and torque limits or the feedback values are incorrectly set!
Move to position 1000 following the speed ramp defined by P6 and P7:
g0 1000
Move to position 100 at a constant speed of 10:
g1 100 10
Jump to 500 at maximum speed (P0) and acceleration (only limited by the current/torque limit P4):
j 500
Positions are in encoder counts and speeds are in encoder counts per millisecond. A linear encoder strip from an old DeskJet printer with 150 lpi (lines per inch) gives 600 encoder counts per inch = 42.3um/count.
- Position 1000 is at 42.3mm
- Speed 10 cnt/ms is 10000 cnt/s = 423mm/s.
For rotational encoders it all scales to rounds and rpm or rps instead of distance and distance/time.
The original source supports an optional 7-segment display with 8 digits to show the current position. The supported serial protocol (see Chan Elm's page for details) is unfortunatly not compatible with the cheap serial LED modules you can find on Aliexpress.
Since this is not an off-the-shelf standard display anyway I didn't bother to port that code (yet?) and swopped these two serial pins for the Arduino pin definition in a way that the builtin SPI could be used.
The AT90S2313 got replaced by the ATtiny2313 a long time ago. Both are very similar, only some register names changed slightly and the interrupt table got a little bigger. See Atmel/Microchip AVR091 for details on migration.
Today I would choose one of the more modern CPUs in the 40 or 50 cent class like ATtiny404, ATtiny44 or ATtiny45. 4kB flash should even allow for C code and the hardware multiplier would help as well.
Using an ATmega328 is ridiculous overkill, but that's what I have laying around in my drawer...
atmega/: the ATmega port of SMC3
cport/: A translation of the the assembler code into valid C code. The program logic is as close to the carefully optimized assembler version as possible. It compiles and the resulting code behaves identical to the original version. The main point is to show the internal logic and to be a more accessible base for modifications. The price for this convienence is a much bigger binary size: 4.8kB instead of 1.8kB. Carefully crafted assembler code can do real magic.
The original archives are unpacked for reference into chan/smc and chan/smc3.
chan/smc/smc.asm: v0.2 of the motor controller (AT90S2313)
chan/smc/smc2.asm: v0.2 of the motor controller with position display on a 7-segment display connected via SPI to the ISP connector. (AT90S2313)
chan/smc3/SMC3.ASM: v0.3, ATtiny2313. Supports serial interface, but not the stepper-like dir/step input and no LED display. 'E' command not implemented, echo always on.
chan/smc3/SMC3A.ASM: v0.3a, ATtiny2313. Supports the stepper-like dir/step interface, but no serial interface and no LED display.
- Internal operations of SCM3 in the project wiki
- ATtiny2313 data sheet
- AVR091 Replacing AT90S2313 by ATtiny2313.pdf
- AVR Assember user guide (was known as Atmel doc1022 before the merger with Microchip)
- Theory of sensorless speed control of brushed DC motors: AB-026 : Sensorless Speed Stabiliser for a DC Motor by Precision Microdrives.
The original work was released under the Creative Commons Attribution 3.0 Unported (CC-BY 3.0) by Elm Chan.