Using a smooth stepper driver on the esp32 in rust

Stepper drivers are used as a simpler interface to control stepper motors, here we use the esp-idf RMT peripheral to send step pulses

Context

The naive implementation of driving the 4 stepper channels through a l298n datasheet motor driver works but has several significant drawbacks.

Requires 4 RMT channels
Will overheat without current limiting
Microstepping requires PWM on the RMT, is a PITA

Solution Postulation

Content exploration led me to the Marco Reps YouTube channel who has an interesting video on motion control

The Trinamic TMC2208 (datasheet) stepper driver provides a more ergonomic step+direction interface, where a direction pin can be low/high to define which direction, and the step pin is pulsed to increment the stepper over.

Because 3d printers tend to use stepper motors there is a standard Dual Inline Pin form factor that is widely and cheaply available. This specific breakout is the Gowoops 3D Printer Stepper Motor Driver

a 16 pin breakout board with the stepper driver

Wiring

The 16 pins on this breakout and what they connect to are below:

Pin	Description	Pin	Description
GND	Ground	DIR	Direction Selector
VDD/VIO	Logic Supply Voltage	STEP	Step Trigger
M2B	Coil 2 +	NC	Not Connected
M2A	Coil 2 -	UART	Serial Configuration
M1A	Coil 1 -	NC	Not Connected
M1B	Coil 1 +	MS2	Microstep Selector 2
GND	Ground	MS1	Microstep Selector 1
VM	Motor Supply Voltage	EN	Enable stepper

Wiring sequence for an esp32:

GND -> Common Ground
ESP32 GND -> Common Ground
ESP32 3V -> VDD
Stepper Black -> M1B
Stepper Green -> M1A
Stepper Blue -> M2A
Stepper Red -> M2B
Motor Power Ground -> Common Ground
Motor Power + -> VM
EN -> Low (Ground)
Dir -> Low
Step -> Low

Reference

Tuning current

This driver limit the current set to the motor, tuned using a resistor. The specific packaging here has a trim pot on top that exposes a reference voltage (potentiometer body to ground is Vref)

table from datasheet — From the datasheet

page 50

Vref = voltage on stepper pot, 0 .. 2.5V Lrms = RMS current per phase Lrms = Lmax / 1.41

Our steppers are Nema 17, specifically 17HS4401. They accept a max current of 1.7A

Lrms = 0.71

(insert picture)

Vref = 0 .. 2.5V

Process Irms = 325mV

Lrms = (Vref _ 1.77A) / 2.5V = Vref _ 0.71
Vref = Lrms / 0.71
Vref = 0.71 / 0.71
Vref = 1

using a lead on the screwdriver turning the trimpot to sense voltage — A lead on the screwdriver does the trick

Testing Notes

We are pulling 3.3v from logic and connecting it to STEP to see if anything happens

Nothing

Attaching EN to ground enables motor
Motor starts spinning eratically
Shorting step to ground stops motor
Shorting step to high stops motor

Guess:
Hates floating

Writing code

Using esp32 my friend Benjamin wrote a codings that drives GPIO 18 high and low.

Our code drives the pin high and low 20 times per second. After powering up the stepper starts spinning very very slowly, approximately 1 rotation every 60 seconds.

Datasheet page 8:

row from datasheet demonstrating microstep configuration — MS2, MS1: 00: 1/8, 01: 1/2, 10: 1/4 11: 1/16

With MS1 and MS2 both low the stepper defaults to 1/8 microstepping, and the stepper is 200 full steps per revolution meaning

20 microsteps per second is 2.5 steps per second
200 steps per revolution / 2.5 seconds per step is 80 seconds per revolution?

Ah well, probably a software issue.