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Change servo defaults to safer defaults (#7023)
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* The default timings were chosen vastly outside specification. On COTS servos, this causes much confusion for the user, because a wide range of angle starting from 0 degress, and a just a large range of angle leading up to 180 degrees, is dead - in fact, the servo doesn't move at all, from any position.

Not investigated further for obvious reasons, as this may have also destroyed servos that ran hot trying to abide by the PWM signal but could not mechanically!

* Change timing limits to safe values. With previous default timings and safety limits, popular servos

could force against internal physical endstops, which could overload and destroy them.

* Review action: revert changes to hard boundary for min/max duty cycle timings. Internal review, fix/drop legacy comments.

* A Servo on each available ESP8266 GPIO (D0-D8), no observed interference.

* Remove possible jerk due to force-cancelling duty cycle.

* Overload attach: can specify initial angle for servo, too.

* Stricter checks for configured, and default, bounded timings.

* Missed the min/max in a comment.

* Default microsecond lower bound of 1000 causes confusing behavior - 200 is minimum enforced elsewhere, so use it here too as nearest multiple of 100 from 180.

* Comment rationale for changed defaults of min/max pulse widths.

Comment rationale for changed defaults of min/max pulse widths.
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@dok-net
dok-net authored Nov 14, 2020
1 parent c5c9f84 commit 9b437d7
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Showing 2 changed files with 44 additions and 27 deletions.
35 changes: 20 additions & 15 deletions libraries/Servo/src/Servo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -27,15 +27,15 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
uint32_t Servo::_servoMap = 0;

// similiar to map but will have increased accuracy that provides a more
// symetric api (call it and use result to reverse will provide the original value)
// symmetrical api (call it and use result to reverse will provide the original value)
int improved_map(int value, int minIn, int maxIn, int minOut, int maxOut)
{
const int rangeIn = maxIn - minIn;
const int rangeOut = maxOut - minOut;
const int deltaIn = value - minIn;
// fixed point math constants to improve accurancy of divide and rounding
const int fixedHalfDecimal = 1;
const int fixedDecimal = fixedHalfDecimal * 2;
constexpr int fixedHalfDecimal = 1;
constexpr int fixedDecimal = fixedHalfDecimal * 2;

return ((deltaIn * rangeOut * fixedDecimal) / (rangeIn) + fixedHalfDecimal) / fixedDecimal + minOut;
}
Expand All @@ -46,9 +46,9 @@ int improved_map(int value, int minIn, int maxIn, int minOut, int maxOut)
Servo::Servo()
{
_attached = false;
_valueUs = DEFAULT_PULSE_WIDTH;
_minUs = MIN_PULSE_WIDTH;
_maxUs = MAX_PULSE_WIDTH;
_valueUs = DEFAULT_NEUTRAL_PULSE_WIDTH;
_minUs = DEFAULT_MIN_PULSE_WIDTH;
_maxUs = DEFAULT_MAX_PULSE_WIDTH;
}

Servo::~Servo() {
Expand All @@ -58,10 +58,15 @@ Servo::~Servo() {

uint8_t Servo::attach(int pin)
{
return attach(pin, MIN_PULSE_WIDTH, MAX_PULSE_WIDTH);
return attach(pin, DEFAULT_MIN_PULSE_WIDTH, DEFAULT_MAX_PULSE_WIDTH);
}

uint8_t Servo::attach(int pin, uint16_t minUs, uint16_t maxUs)
{
return attach(pin, minUs, maxUs, _valueUs);
}

uint8_t Servo::attach(int pin, uint16_t minUs, uint16_t maxUs, int value)
{
if (!_attached) {
digitalWrite(pin, LOW);
Expand All @@ -76,7 +81,7 @@ uint8_t Servo::attach(int pin, uint16_t minUs, uint16_t maxUs)
_maxUs = max((uint16_t)250, min((uint16_t)3000, maxUs));
_minUs = max((uint16_t)200, min(_maxUs, minUs));

write(_valueUs);
write(value);

return pin;
}
Expand All @@ -85,27 +90,28 @@ void Servo::detach()
{
if (_attached) {
_servoMap &= ~(1 << _pin);
startWaveform(_pin, 0, REFRESH_INTERVAL, 1);
delay(REFRESH_INTERVAL / 1000); // long enough to complete active period under all circumstances.
stopWaveform(_pin);
_attached = false;
digitalWrite(_pin, LOW);
_valueUs = DEFAULT_NEUTRAL_PULSE_WIDTH;
}
}

void Servo::write(int value)
{
// treat values less than 544 as angles in degrees (valid values in microseconds are handled as microseconds)
if (value < _minUs) {
// treat any value less than 200 as angle in degrees (values equal or larger are handled as microseconds)
if (value < 200) {
// assumed to be 0-180 degrees servo
value = constrain(value, 0, 180);
// writeMicroseconds will contrain the calculated value for us
// for any user defined min and max, but we must use default min max
value = improved_map(value, 0, 180, _minUs, _maxUs);
}
writeMicroseconds(value);
}

void Servo::writeMicroseconds(int value)
{
value = constrain(value, _minUs, _maxUs);
_valueUs = value;
if (_attached) {
_servoMap &= ~(1 << _pin);
Expand All @@ -117,8 +123,7 @@ void Servo::writeMicroseconds(int value)

int Servo::read() // return the value as degrees
{
// read returns the angle for an assumed 0-180, so we calculate using
// the normal min/max constants and not user defined ones
// read returns the angle for an assumed 0-180
return improved_map(readMicroseconds(), _minUs, _maxUs, 0, 180);
}

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36 changes: 24 additions & 12 deletions libraries/Servo/src/Servo.h
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Original file line number Diff line number Diff line change
Expand Up @@ -27,9 +27,9 @@
//
// Servo - Class for manipulating servo motors connected to Arduino pins.
//
// attach(pin ) - Attaches a servo motor to an i/o pin.
// attach(pin, min, max ) - Attaches to a pin setting min and max values in microseconds
// default min is 544, max is 2400
// attach(pin) - Attaches a servo motor to an i/o pin.
// attach(pin, min, max) - Attaches to a pin setting min and max values in microseconds
// default min is 1000, max is 2000
//
// write() - Sets the servo angle in degrees. (invalid angle that is valid as pulse in microseconds is treated as microseconds)
// writeMicroseconds() - Sets the servo pulse width in microseconds
Expand All @@ -44,13 +44,17 @@

#include <Arduino.h>

// the following are in us (microseconds)
//
#define MIN_PULSE_WIDTH 544 // the shortest pulse sent to a servo
#define MAX_PULSE_WIDTH 2400 // the longest pulse sent to a servo
#define DEFAULT_PULSE_WIDTH 1500 // default pulse width when servo is attached
#define REFRESH_INTERVAL 20000 // minumim time to refresh servos in microseconds
#define MAX_SERVOS 12
// The following values are in us (microseconds).
// Since the defaults can be overwritten in the new attach() member function,
// they were modified from the Arduino AVR defaults to be in the safe range
// of publically available specifications. While this implies that many 180°
// servos do not operate the full 0° to 180° sweep using these, it also prevents
// unsuspecting damage. For Arduino AVR, the same change is being discussed.
#define DEFAULT_MIN_PULSE_WIDTH 1000 // uncalibrated default, the shortest duty cycle sent to a servo
#define DEFAULT_MAX_PULSE_WIDTH 2000 // uncalibrated default, the longest duty cycle sent to a servo
#define DEFAULT_NEUTRAL_PULSE_WIDTH 1500 // default duty cycle when servo is attached
#define REFRESH_INTERVAL 20000 // classic default period to refresh servos in microseconds
#define MAX_SERVOS 9 // D0-D8

#if !defined(ESP8266)

Expand All @@ -63,8 +67,16 @@ class Servo
public:
Servo();
~Servo();
uint8_t attach(int pin); // attach the given pin to the next free channel, sets pinMode, returns channel number or 0 if failure
uint8_t attach(int pin, uint16_t min, uint16_t max); // as above but also sets min and max values for writes.
// attach the given pin to the next free channel, sets pinMode, returns channel number or 0 if failure.
// returns channel number or 0 if failure.
uint8_t attach(int pin);
// attach the given pin to the next free channel, sets pinMode, min, and max values for write().
// returns channel number or 0 if failure.
uint8_t attach(int pin, uint16_t min, uint16_t max);
// attach the given pin to the next free channel, sets pinMode, min, and max values for write(),
// and sets the initial value, the same as write().
// returns channel number or 0 if failure.
uint8_t attach(int pin, uint16_t min, uint16_t max, int value);
void detach();
void write(int value); // if value is < 200 its treated as an angle, otherwise as pulse width in microseconds
void writeMicroseconds(int value); // Write pulse width in microseconds
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