Serial interface for controlling robotic arm.

Dependencies:   Axis mbed


This program uses a LPC1768 processor for controlling a robotic arm. The basis for the program is the Axis () class which uses a PID controller to actuate a DC motor with quadrature encoder feedback.

Youtube video of robotic arm using the 6-axis controller performing an internally programmed gate, then performing the home function.

The Axis Class has 3 dependencies (MotCon, LS7366LIB, and PID). The class encapsulates the required functionality of controlling a DC motor with encoder feedback through pin assignments, an SPI bus, and a pointer for the limit switch source.

The LS7366 encoder interface IC off-loads the critical time and counting requirements from the processor using an SPI bus interface for the class. The Axis class then uses a state machine to perform trapezoidal movement profiles with a Ticker class. Parameters can be adjusted through the serial interface using a FT232RL USB to serial interface IC for computer communication.

The MotCon class is a basic class that defines a PWM output pin and a single direction signal intended to control an H-Bridge motor driver IC. I used an MC33926 motor driver for each motor which are rated at 5.0-28V and 5.0 amp peak, with an RDSon max resistance of 225 milli-ohms. This part also has 3.0V to 5V TTL/CMOS inputs logic levels and various protection circuitry on board. I also liked this particular motor driver chip because you can use a PWM frequency of up to 20KHz, getting the frequency out of the audio range.

/media/uploads/jebradshaw/20151217_141308.jpg .

Above is the prototype for the controller. Originally, a PCF8574 I/O expander was used to read the limit switches by the I2C bus. This has now been re-written to use 6 external interrupts directly for the limit/homing switches. Six motor driver breakout boards using the MC33926 motor driver chip were used to drive the motors.

I use the mbed online compiler to generate the .bin file, use bin2hex to convert it and upload the hex file using Flash Magic to the processor with the serial bootloader. I prefer to use the FT232RL usb to serial converter IC for PC comms due to the high level of reliability and USB driver support (typically already built in Windows 7+). I've started putting this on a PCB and hope to finish by the end of the month (Dec 2015).

Well 3 months later, I've completed the first PCB prototype. A few minor errors but it's working!!

/media/uploads/jebradshaw/pcb_artwork.jpg Express PCB Artwork

/media/uploads/jebradshaw/6axiscontroller_innerpowerlayer.jpg Inner Power Layer Breakup for motor current

/media/uploads/jebradshaw/6axiscontroller_pcb_prototype_trimmed.jpg First Prototype





Latest documentation (schematic and parts layout) can be found here Download and open with Adobe /media/uploads/jebradshaw/axiscontroller_schematics_v2.0.pdf

Latest PCB File (Express PCB) /media/uploads/jebradshaw/lpc1768_axiscontroller_20161216.pcb

Parts Layout /media/uploads/jebradshaw/silkscreen_top_stencil.jpg /media/uploads/jebradshaw/axiscontroller_bottommirrorimage_20161216.jpg

Python script for converting mbed .bin output to intel hex format (no bin2hex 64K limit)

Example batch script for speeding up conversion process for FlashMagic programming of board /media/uploads/jebradshaw/axisconvert.bat

Latest firmware: 20180706 Hex File /media/uploads/jebradshaw/axis_pqr5sNW.hex , binary file /media/uploads/jebradshaw/axis_9uOSEf9.bin