StepperWinch Arduino Sketch

This sketch smoothly controls four stepper motors, receiving control messages from an external source over a USB serial port. The protocol is event-driven with local filtering and motion smoothing, much like a MIDI instrument driven by note events.

The full code for the sketch spans several files; all files may be downloaded in a single archive file as StepperWinch.zip, browsed in raw form in the source folder, or browsed below.

Compiling the sketch requires installing the optional TimerOne library in the Arduino IDE for supporting timer interrupt processing.

Contents

• StepperWinch Arduino Sketch

  • Top-Level Functions

  • Main

  • Path Generator

  • Step Generator

Top-Level Functions

void setup(void)

Standard Arduino initialization function to configure the system.

void loop(void)

Standard Arduino polling function to handle all I/O and periodic processing. This loop should never be allowed to stall or block so that all tasks can be constantly serviced.

void path_poll(unsigned long interval)

Polling function called from the main event loop to update the path model and update the step generators.

void serial_input_poll(void)

Polling function to process messages arriving over the serial port. Each iteration through this polling function processes at most one character. It records the input message line into a buffer while simultaneously dividing it into ‘tokens’ delimited by whitespace. Each token is a string of non-whitespace characters, and might represent either a symbol or an integer. Once a message is complete, parse_input_message() is called.

void parse_input_message(int argc, char *argv[])

Process an input message. Unrecognized commands are silently ignored.

Parameters

• argc: number of argument tokens

• argv: array of pointers to strings, one per token

Main

The main entry points and event loop are in file StepperWinch.ino.

/// \file StepperWinch.ino
/// \brief Driver for four-channel stepper-motor capstan winch system.
///
/// \copyright Written over 2014-2018 by Garth Zeglin <garthz@cmu.edu>.  To the
/// extent possible under law, the author has dedicated all copyright and
/// related and neighboring rights to this software to the public domain
/// worldwide.  This software is distributed without any warranty.  You should
/// have received a copy of the CC0 Public Domain Dedication along with this
/// software.  If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
///
/// \details This sketch is designed to support the expressive gestural motion
/// of a kinetic fabric sculpture rather than perform precision trajectory
/// tracking as would be typical for CNC applications.  The communicates over
/// the serial port to receive motion commands and report status.  The protocol
/// is event-driven and implements parameterized gestural motions; it is
/// patterned after MIDI or OSC conventions.  This sketch assumes the following:
///
/// 1. The TimerOne library is installed in the Arduino IDE.
/// 2. Four A4988 stepper motor drivers are connected following the CNC Shield pin conventions.
/// 3. If using a CNC Shield board, A-axis jumpers are installed to connect A-DIR to D13 (also SpinDir) and A-STEP to D12 (also SpinEnable)..
/// 4. The serial communication baud rate is 115200 bps.
///
/// Typically, for 200 step/rev (1.8 deg) stepper motors the drivers are
/// configured for 1/4 step microstepping (MS2 pulled high).  However, the
/// protocol commands use integer step units so the code does not depend on
/// this.

// ================================================================
// Import the TimerOne library to support timer interrupt processing.
#include "TimerOne.h"

// Include the other modules from this sketch.
#include "cnc_shield.h"
#include "Stepper.h"
#include "Path.h"

// ================================================================
// Communication protocol.

// The message protocol is based on plain-text commands sent as keywords and
// values in a line of text.  One line is one message.  All the input message
// formats begin with a string naming a specific command or destination followed
// by one or two arguments.  The output formats are similar but include more
// general debugging output with a variable number of tokens.

// ----------------------------------------------------------------
// The following global messages are not channel-specific.

// Command	Arguments		Meaning
// ping                                 query whether the server is running
// version				query the identity of the sketch
// srate        <value>                 set the status reporting interval in milliseconds
// enable       <value>                 enable or disable all driver outputs, value is 0 or non-zero

// ----------------------------------------------------------------
// The following messages include a token representing the flag set specifying
// the affected axes.  The flag set should include one or more single-letter
// channel specifiers (regex form: "[xyza]+").  Note that the flag set cannot be
// empty.

// --------------------------------

// Absolute move. There should be an integer target value corresponding to each
// included channel; each controller target is set to the specified position.
// The motion is not coordinated; different channels may finish at different
// times.  Note that this command will enable all drivers.

//   a <flags> <offset>+
//
// Examples:
//   a xyza 100 120 -200 -50	move the axes to the specified locations
//   a x 50			move the X axis to +50

// --------------------------------

// Relative move. There should be an offset value corresponding to each included
// channel; each controller target is incremented by the specified amount.  The
// motion is not coordinated; different channels may finish at different times.
// Note that this command will enable all drivers.
//
//   d <flags> <offset>+
//
// Examples:
//   d xyza 10 10 -10 -10	move all axes ten steps (two forward, two backward)
//   d x 50			move the X axis fifty steps forward

// --------------------------------

// Reference move. There should be an offset value corresponding to each
// included channel; each controller reference value is incremented by the
// specified amount, which has the effect of applying an impulse.  Note that
// this command will enable all drivers.
//
//   r <flags> <offset>+
//
// Examples:
//   r xyza 10 10 -10 -10	move all axes reference positions ten steps (two forward, two backward)
//   r x 50			move the X axis reference position fifty steps forward

// --------------------------------
// Moot:
// Set velocity. There should be an integer velocity value corresponding to each
// included channel; each controller target velocity is set to the amount
// specified in units/sec.  Note that this command will enable all drivers.
//
//   v <flags> <value>+
//
// Examples:
//   v xyza 10 10 -10 -10	set all axes to drift 10 steps per second (two forward, two backward)
//   v x 500			set the X axis to constantly move forward at roughly half speed

// --------------------------------
// Set speed. There should be an integer speed value corresponding to each
// included channel; each controller target speed is set to the amount
// specified in units/sec.  Note that this command will enable all drivers.
//
//   s <flags> <value>+
//
// Examples:
//   s xyza 10 10 10 10		set all axes to ramp at 10 steps per second toward the target
//   s x 500			set the X axis to ramp at 500 steps/second

// --------------------------------
// Set second-order gains.  The same dynamic parameters are applied to all included channels.
//   g <flags> <frequency (Hz)> <damping-ratio>
//
// Examples:
//   g xyza 2.0 1.0		set all channels to 1 Hz natural frequency with critical damping
//   g xyza 0.1 0.5		set all channels to 0.1 Hz natural frequency and underdamping

// --------------------------------
// Set velocity and acceleration limits.  The same dynamic parameters are applied to all included channels.
//   l <flags> <maximum velocity (steps/sec)> <maximum acceleration (steps/sec/sec)>
//
// Examples:
//   l xyza 4000 40000		set all channels to 4000 steps/sec and 40000 steps/sec/sec

// ----------------------------------------------------------------
// This program generates the following messages:

// Command	Arguments		Meaning
// awake                                initialization has completed or ping was received
// txyza        <usec> <x> <y> <z> <a>  Arduino clock time in microseconds, followed by absolute step position
// dbg		<value-or-token>+	debugging message to print for user
// id		<tokens>+		tokens identifying the specific sketch

// ================================================================
// Global variables and constants.

// The baud rate is the number of bits per second transmitted over the serial port.
#define BAUD_RATE 115200

// Interval in microseconds between status messages.
static unsigned long status_poll_interval = 200000; // 5 Hz message rate to start

/// Control objects for the stepper channels.  The declaration statically
/// initializes the global state objects for the channels.  Note that this does
/// not initialize the hardware; that is performed in setup().
static Stepper x_axis(X_AXIS_STEP_PIN, X_AXIS_DIR_PIN);
static Stepper y_axis(Y_AXIS_STEP_PIN, Y_AXIS_DIR_PIN);
static Stepper z_axis(Z_AXIS_STEP_PIN, Z_AXIS_DIR_PIN);
static Stepper a_axis(A_AXIS_STEP_PIN, A_AXIS_DIR_PIN);

/// Path generator object for each channel.
static Path x_path, y_path, z_path, a_path;

/// The timestamp in microseconds for the last polling cycle, used to compute
/// the exact interval between stepper motor updates.
static unsigned long last_interrupt_clock = 0;

/// Identification string.
static const char version_string[] = "id StepperWinch " __DATE__;

// ================================================================
/// Enable or disable the stepper motor drivers.  The output is active-low,
/// so this inverts the sense.
static inline void set_driver_enable(int value)
{
  digitalWrite(STEPPER_ENABLE_PIN, (value != 0) ? LOW : HIGH);
}

// ================================================================
/// Interrupt handler to update all the stepper motor channels.  Note that this
/// is called from a timer interrupt context, so it should take as little time as
/// feasible and cannot use serial I/O (i.e. no debugging messages).
void stepper_output_interrupt(void)
{
  // read the clock
  unsigned long now = micros();

  // Compute the time elapsed since the last poll.  This will correctly handle wrapround of
  // the 32-bit long time value given the properties of twos-complement arithmetic.
  unsigned long interval = now - last_interrupt_clock;
  last_interrupt_clock = now;

  // Update all the stepper channels. This may emit step signals or simply
  // update the timing and state variables.
  x_axis.pollForInterval(interval);
  y_axis.pollForInterval(interval);
  z_axis.pollForInterval(interval);
  a_axis.pollForInterval(interval);
}

// ================================================================
/// Polling function called from the main event loop to update the path model
/// and update the step generators.
void path_poll(unsigned long interval)
{
  x_path.pollForInterval(interval);
  y_path.pollForInterval(interval);
  z_path.pollForInterval(interval);
  a_path.pollForInterval(interval);

  // update the step generator for new targets
  x_axis.setTarget(x_path.currentPosition());
  x_axis.setSpeed(abs(x_path.currentVelocity()));

  y_axis.setTarget(y_path.currentPosition());
  y_axis.setSpeed(abs(y_path.currentVelocity()));

  z_axis.setTarget(z_path.currentPosition());
  z_axis.setSpeed(abs(z_path.currentVelocity()));

  a_axis.setTarget(a_path.currentPosition());
  a_axis.setSpeed(abs(a_path.currentVelocity()));
}
// ================================================================
/// Return a Path object or NULL for each flag in the flag token.  As a side effect, updates
/// the source pointer, leaving it at the terminating null.
static Path *path_flag_iterator(char **tokenptr)
{
  char flag = **tokenptr;
  if (flag == 0) return NULL;
  else {
    (*tokenptr) += 1;
    switch (flag) {
    case 'x': return &x_path;
    case 'y': return &y_path;
    case 'z': return &z_path;
    case 'a': return &a_path;
    default: return NULL;
    }
  }
}

// ================================================================
/// Process an input message.  Unrecognized commands are silently ignored.
///
/// @param argc		number of argument tokens
/// @param argv		array of pointers to strings, one per token
void parse_input_message(int argc, char *argv[])
{
  if (argc == 0) return;

  // Interpret the first token as a command symbol.
  char *command = argv[0];

  if (string_equal(command, "enable")) {
    if (argc > 1) set_driver_enable(atoi(argv[1]));

  } else if (string_equal(command, "a")) {
    if (argc > 2) {
      set_driver_enable(1);
      char *flags = argv[1];
      int channel = 0;
      while (*flags) {
	Path *p = path_flag_iterator(&flags);
	if (p) {
	  if (argc > (channel+2)) {
	    p->setTarget(atol(argv[channel+2]));
	    channel++;
	  }
	}
      }
    }
  } else if (string_equal(command, "d")) {
    if (argc > 2) {
      set_driver_enable(1);
      char *flags = argv[1];
      int channel = 0;
      while (*flags) {
	Path *p = path_flag_iterator(&flags);
	if (p) {
	  if (argc > (channel+2)) {
	    p->incrementTarget(atol(argv[channel+2]));
	    channel++;
	  }
	}
      }
    }
  } else if (string_equal(command, "r")) {
    if (argc > 2) {
      set_driver_enable(1);
      char *flags = argv[1];
      int channel = 0;
      while (*flags) {
	Path *p = path_flag_iterator(&flags);
	if (p) {
	  if (argc > (channel+2)) {
	    p->incrementReference(atol(argv[channel+2]));
	    channel++;
	  }
	}
      }
    }
  } else if (string_equal(command, "v")) {
    if (argc > 2) {
      set_driver_enable(1);
      char *flags = argv[1];
      int channel = 0;
      while (*flags) {
	Path *p = path_flag_iterator(&flags);
	if (p) {
	  if (argc > (channel+2)) {
	    // p->setVelocity(atol(argv[channel+2]));
	    channel++;
	  }
	}
      }
    }
  } else if (string_equal(command, "s")) {
    if (argc > 2) {
      set_driver_enable(1);
      char *flags = argv[1];
      int channel = 0;
      while (*flags) {
	Path *p = path_flag_iterator(&flags);
	if (p) {
	  if (argc > (channel+2)) {
	    p->setSpeed(atol(argv[channel+2]));
	    channel++;
	  }
	}
      }
    }
  } else if (string_equal(command, "g")) {
    if (argc > 3) {
      char *flags = argv[1];
      float frequency = atof(argv[2]);
      float damping_ratio = atof(argv[3]);
      while (*flags) {
	Path *p = path_flag_iterator(&flags);
	if (p) p->setFreqDamping(frequency, damping_ratio);
      }
    }
  } else if (string_equal(command, "l")) {
    if (argc > 3) {
      char *flags = argv[1];
      float qdmax = atof(argv[2]);
      float qddmax = atof(argv[3]);
      while (*flags) {
	Path *p = path_flag_iterator(&flags);
	if (p) p->setLimits(qdmax, qddmax);
      }
    }
  } else if (string_equal(command, "version")) {
    send_message(version_string);

  } else if (string_equal(command, "ping")) {
    send_message("awake");

  } else if (string_equal(command, "srate")) {
    if (argc > 1) {
      long value = atol(argv[1]);
      // set the reporting interval (milliseconds -> microseconds)
      if (value > 0)  status_poll_interval = 1000*value;
      else send_debug_message("invalid srate value");
    }
  }
}

/****************************************************************/
/// Polling function to send status reports at periodic intervals.
static void status_poll(unsigned long interval)
{
  static long timer = 0;
  timer -= interval;

  if (timer < 0) {
    timer += status_poll_interval;

    // send a time and position reading
    long clock = micros();
    long x = x_axis.currentPosition();
    long y = y_axis.currentPosition();
    long z = z_axis.currentPosition();
    long a = a_axis.currentPosition();
    send_message("txyza", clock, x, y, z, a);
  }
}

/****************************************************************/
/**** Standard entry points for Arduino system ******************/
/****************************************************************/

/// Standard Arduino initialization function to configure the system.
void setup(void)
{
  // set up the CNC Shield I/O
  digitalWrite(STEPPER_ENABLE_PIN, HIGH); // initialize drivers in disabled state
  pinMode(STEPPER_ENABLE_PIN, OUTPUT);

  pinMode(X_AXIS_STEP_PIN, OUTPUT);
  pinMode(Y_AXIS_STEP_PIN, OUTPUT);
  pinMode(Z_AXIS_STEP_PIN, OUTPUT);
  pinMode(A_AXIS_STEP_PIN, OUTPUT);

  pinMode(X_AXIS_DIR_PIN, OUTPUT);
  pinMode(Y_AXIS_DIR_PIN, OUTPUT);
  pinMode(Z_AXIS_DIR_PIN, OUTPUT);
  pinMode(A_AXIS_DIR_PIN, OUTPUT);

#ifdef LED_BUILTIN
  pinMode(LED_BUILTIN, OUTPUT);
#endif

  // initialize the Serial port
  Serial.begin(BAUD_RATE);

  // set up the timer1 interrupt and attach it to the stepper motor controls
  last_interrupt_clock = micros();
  Timer1.initialize(100); // 100 microsecond intervals, e.g. 10kHz
  Timer1.attachInterrupt(stepper_output_interrupt);

  // send a wakeup message
  send_message("awake");
}

/****************************************************************/
/// Standard Arduino polling function to handle all I/O and periodic processing.
/// This loop should never be allowed to stall or block so that all tasks can be
/// constantly serviced.
void loop(void)
{
  static unsigned long last_event_loop = 0;

  // read the clock
  unsigned long now = micros();

  // Compute the time elapsed since the last polling cycle.  This will correctly handle wrapround of
  // the 32-bit long time value given the properties of twos-complement arithmetic.
  unsigned long interval = now - last_event_loop;
  last_event_loop = now;

  serial_input_poll();
  status_poll(interval);
  path_poll(interval);

  // other polled tasks can go here
}

/****************************************************************/
/****************************************************************/

Path Generator

The input filtering and path generation are implemented by Path objects. These are purely data operators, they do not directly interface to hardware.

Sources: Path.h, Path.cpp.

/// \file Path.h
/// \brief Path generator for a single motor.
///
/// \copyright Written 2018 by Garth Zeglin <garthz@cmu.edu>.  To the extent
/// possible under law, the author has dedicated all copyright and related and
/// neighboring rights to this software to the public domain worldwide.  This
/// software is distributed without any warranty.  You should have received a
/// copy of the CC0 Public Domain Dedication along with this software.  If not,
/// see <http://creativecommons.org/publicdomain/zero/1.0/>.
///
/// \details This class implements several smooth path generators intended for
/// generating gestural motions on a single motor channel.  It assumes a
/// separate controller manages the step generator of closed-loop control of the
/// physical hardware.

#ifndef __PATH_H_INCLUDED__
#define __PATH_H_INCLUDED__

#include <math.h>

// ================================================================
class Path {

private:
  float q;    	   ///< current model position, in dimensionless units (e.g. step or encoder counts)
  float qd;   	   ///< current model velocity in units/sec
  float qdd;  	   ///< current model acceleration, in units/sec/sec
  float q_d;  	   ///< current model reference position in dimensionless units
  float qd_d;  	   ///< current model reference velocity in dimensionless units/sec

  float q_d_d; 	   ///< user-specified target position in dimensionless units
  float speed;     ///< user-specified target speed in dimensionless units/sec

  float t;    	   ///< elapsed model time, in seconds
  float k;    	   ///< proportional feedback gain, in (units/sec/sec)/(units), which is (1/sec^2)
  float b;    	   ///< derivative feedback gain, in (units/sec/sec)/(units/sec), which is (1/sec)
  float qd_max;    ///< maximum allowable speed in units/sec
  float qdd_max;   ///< maximum allowable acceleration in units/sec/sec

public:

  /// Main constructor.
  Path();

  /// Path-integration polling function to be called as often as possible,
  /// typically from the main event loop.  The interval argument is the duration
  /// in microseconds since the last call.
  void pollForInterval(unsigned long interval);

  /// Add a signed offset to the target position.  The units are dimensionless
  /// 'steps'.  If using a microstepping driver, these may be less than a
  /// physical motor step.
  void incrementTarget(long offset) { q_d_d += offset; }

  /// Add a signed offset to the reference position.  This can have  the
  /// effect of applying a triangular impulse; the reference trajectory will
  /// make a step, then ramp back to the target position.
  void incrementReference(long offset) { q_d += offset; }

  /// Set the absolute target position in dimensionless units.
  void setTarget(long position) { q_d_d = position; }

  /// Set the ramp speed in dimensionless units/second.  If less than or equal to zero,
  /// it is treated as unlimited, and the
  /// reference position will move in steps instead of ramps.
  void setSpeed(long newspeed) { speed = (newspeed <= 0) ? INFINITY : newspeed; }

  /// Return the current position in dimensionless units.
  long currentPosition(void) { return (long) q; }

  /// Return the current velocity in units/second.
  long currentVelocity(void) { return (long) qd; }

  /// Configure the second-order model gains.
  void setPDgains(float k_new, float b_new) { k = k_new; b = b_new; }

  /// Convenience function to set second order model gains in terms of natural frequency and damping ratio.
  /// The frequency is in Hz, the damping ratio is 1.0 at critical damping.
  void setFreqDamping(float freq, float damping) {
    k = freq * freq * 4 * M_PI * M_PI;
    b = 2 * sqrtf(k) * damping;
  }

  /// Configure the velocity and acceleration limits.
  void setLimits(float qdmax, float qddmax) { qd_max = qdmax; qdd_max = qddmax; }
};

#endif //__PATH_H_INCLUDED__

/// \file Path.cpp
/// \brief Step and path generator for a single path motor
///
/// \copyright Written over 2014-2018 by Garth Zeglin <garthz@cmu.edu>.  To the
/// extent possible under law, the author has dedicated all copyright and
/// related and neighboring rights to this software to the public domain
/// worldwide.  This software is distributed without any warranty.  You should
/// have received a copy of the CC0 Public Domain Dedication along with this
/// software.  If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.

#include <Arduino.h>
#include <math.h>
#include <stdint.h>

#include "Path.h"

//================================================================
Path::Path()
{
  q    = 0.0;
  qd   = 0.0;
  qdd  = 0.0;
  q_d  = 0.0;
  qd_d = 0.0;

  q_d_d = 0.0;
  speed = INFINITY;

  t    = 0;
  k    = 4*M_PI*M_PI;   // 1 Hz; freq = (1/2*pi) * sqrt(k/m); k = (freq*2*pi)^2
  b    = 1.0;           // damping (would be 2*sqrt(k) for critical damping)

  qd_max  = 3500.0;     // typical physical limit for 4x microstepping
  qdd_max = 35000.0;
}

//================================================================
// Path integration running from main event loop.
void Path::pollForInterval(unsigned long interval)
{
  float dt = 1e-6 * interval;

  // calcuate the derivatives
  float qdd = k * (q_d - q) + b * (qd_d - qd);

  // clamp the acceleration within range for safety
  qdd = constrain(qdd, -qdd_max, qdd_max);

  // integrate one time step
  q  += qd  * dt;
  qd += qdd * dt;
  t += dt;

  // clamp the model velocity within range for safety
  qd = constrain(qd, -qd_max, qd_max);

  // Update the reference trajectory using linear interpolation.  This can
  // create steps or ramps.  This calculates the maximum desired step, bounds it
  // to the speed, then applies the sign to move in the correct direction.
  float q_d_err = q_d_d - q_d;  // maximum error step

  if (q_d_err == 0.0) {
    qd_d = 0.0;  // make sure reference velocity is zero, leave reference position unchanged

  } else {
    if (isinf(speed)) {
      q_d = q_d_d;      // infinite speed, always adjust reference in one step
      qd_d = 0.0;       // then assume zero velocity
    } else {            // else calculate a ramp step
      float d_q_d_max = speed * dt; // maximum linear step, possibly infinite
      if (q_d_err > 0.0) {
	float d_q_d = min(d_q_d_max, q_d_err); // reference position step
	q_d += d_q_d;
	qd_d = speed; // reference velocity
      } else {
	float d_q_d = min(d_q_d_max, -q_d_err); // absolute value of reference position step
	q_d -= d_q_d;
	qd_d = -speed; // reference velocity
      }
    }
  }
}
//================================================================

Step Generator

The hardware drivers are operated by the step generator running on a fast timer interrupt. The step targets are updated from the path generator outputs in the main event loop.

Sources: Stepper.h, Stepper.cpp.

/// \file Stepper.h
/// \brief Step generator for a single stepper motor.
///
/// \copyright Written over 2014-2018 by Garth Zeglin <garthz@cmu.edu>.  To the
/// extent possible under law, the author has dedicated all copyright and
/// related and neighboring rights to this software to the public domain
/// worldwide.  This software is distributed without any warranty.  You should
/// have received a copy of the CC0 Public Domain Dedication along with this
/// software.  If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
///
/// \details This class implements fast constant-velocity stepping.  It is
/// possible to use this directly, but the overall sketch pairs this with a
/// interpolating path generator which frequently updates the position and
/// velocity setpoints.

#ifndef __STEPPER_H_INCLUDED__
#define __STEPPER_H_INCLUDED__

#include <stdint.h>

/// An instance of this class manages generation of step and direction signals
/// for one stepper motor.
class Stepper {

private:
  /****************************************************************/
  // The following instance variables may only be modified from a non-interrupt
  // context, i.e., not within poll().

  /// the I/O pins for this channel designated using the Arduino convention
  uint8_t step_pin, dir_pin;

  /// the target position in dimensionless step counts
  long target;

  /// the interval in microseconds between steps
  unsigned long step_interval;

  /****************************************************************/
  // The following instance variables may be modified within poll() from an interrupt context.

  /// the current position in dimensionless step counts
  long position;

  /// the time elapsed in microseconds since the last step occurred
  unsigned long elapsed;

  /****************************************************************/
public:

  /// Main constructor.  The arguments are the pin numbers for the step and
  /// direction outputs. Note: this does not initialize the underlying hardware.
  Stepper(uint8_t step_pin, uint8_t dir_pin);

  /// Step-generator polling function to be called as often as possible.  This
  /// is typically called from a timer interrupt.  The interval argument is the
  /// duration in microseconds since the last call.
  void pollForInterval(unsigned long interval);

  /// Add a signed offset to the target position.  The units are dimensionless
  /// 'steps'.  If using a microstepping driver, these may be less than a
  /// physical motor step.
  void incrementTarget(long offset) { target += offset; }

  /// Set the absolute target position.
  void setTarget(long position) { target = position; }

  /// Return the current position in dimensionless 'steps'.
  long currentPosition(void) { return position; }

  /// Set a constant speed in steps/second.  Note that the value must be
  /// non-zero and positive.  The maximum rate available is a function of the
  /// polling rate.
  void setSpeed(int speed) {
    // (1000000 microseconds/second) / (steps/second) = (microseconds/step)
    if (speed > 0) {
      step_interval = 1000000 / speed;
      if (step_interval == 0) step_interval = 1;
    }
  }

};

#endif //__STEPPER_H_INCLUDED__

/// \file Stepper.cpp
/// \brief Step and path generator for a single stepper motor
///
/// \copyright Written over 2014-2018 by Garth Zeglin <garthz@cmu.edu>.  To the
/// extent possible under law, the author has dedicated all copyright and
/// related and neighboring rights to this software to the public domain
/// worldwide.  This software is distributed without any warranty.  You should
/// have received a copy of the CC0 Public Domain Dedication along with this
/// software.  If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.

#include "Stepper.h"
#include <Arduino.h>

Stepper::Stepper(uint8_t _step_pin, uint8_t _dir_pin)
{
  step_pin = _step_pin;
  dir_pin  = _dir_pin;
  position = 0;
  target   = 0;
  elapsed  = 0;

  step_interval = 200;  // 200 microseconds = 5000 steps/sec
}
//================================================================
// Step generator running on fast timer interrupt.
void Stepper::pollForInterval(unsigned long interval)
{
  // Accumulated the time elapsed since the step.
  elapsed += interval;

  if (elapsed >= step_interval) {
    // reset the timer according to the target interval to produce a correct an
    // average rate even if extra time has passed
    elapsed -= step_interval;

    // check whether to emit a step
    if (position != target) {

      // always set the direction to match the sign of motion
      digitalWrite(dir_pin, (position < target) ? HIGH : LOW);

      // emit a step
      digitalWrite(step_pin, HIGH);

      // update the position count
      if (position < target) position++;
      else position--;
      digitalWrite(step_pin, LOW);
    }
  }
}
//================================================================