Fix negative currentStep for small changes in position

[klaasdc]

I am using this library to drive an automotive gauge. Sometimes currentStep seems to ty to become negative (underflow), and the motor blocks until currentStep again enters the domain of valid step positions.

Simple test below.
During the test, the velocity variable vel is increased very quickly, so the destination step is reached but velocity did not ramp down quickly enough. This leads to the if (currentStep==targetStep && vel==0)-condition not triggering, which in turn leads to dir being -1 when currentStep reaches 0, inside the stepTo function.

Therefore it is better to limit vel to the delta to the target position.

#include <SwitecX12.h>

const int STEPS = 315 * 12;
const int A_STEP = 8;
const int A_DIR = 9;
const int RESET = 10;

SwitecX12 motor1(STEPS, A_STEP, A_DIR);

int state = 0;

void setup() {
  Serial.begin(9600);
  digitalWrite(RESET, HIGH);
  motor1.zero();
  motor1.setPosition(STEPS/2);
}

void loop() {
  if (state == 0){
    motor1.setPosition(0);
    Serial.println("go to 0");
    state = 1;
  } else if (state == 1){
    if (motor1.stopped){
      Serial.println("stopped1");
      state = 2;
    }
  } else if (state == 2){
    motor1.setPosition(500);
    Serial.println("go to 500");
    state = 3;
  } else if (state == 3){
    if (motor1.stopped){
      Serial.println("stopped2");
      state = 0;
    }
  }
  motor1.update();
}

[guyc]

Thanks for the PR.