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Analog Feedback Servos Created by Bill Earl https://learn.adafruit.com/analog-feedback-servos Last updated on 2021-11-15 06:03:37 PM EST ©Adafruit Industries Page 1 of 15 Table of Contents About Servos and Feedback 3 • What is a Servo? • Open and Closed Loops 3 3 Using Feedback 5 • • • • • Reading the feedback Calibrating the feedback Using feedback in your code Seeking to a position Finding out where you are 7 7 8 8 8 Servos as Input Devices 9 • To run the Servo Record/Play Demo Sketch: 12 Using With CircuitPython 12 • Wiring • Reading the feedback 12 13 Calibrating The Feedback 13 Finding and Seeking 14 ©Adafruit Industries Page 2 of 15 About Servos and Feedback What is a Servo? The word 'servo' means more than just those little RC Servo Motors we usually think of. Servo is a general term for a closed loop control system using negative feedback. The cruise control in a car is one example of a servo system. It measures your speed and feeds that back into a control circuit which adjusts the accelerator to maintain speed. For the familiar RC Servo motor, the position of the output shaft is measured and fed back to the internal control circuit which adjusts current to the motor to maintain position. Open and Closed Loops An "Open Loop" system has no feedback, so there is no way to verify that it is performing as expected. A common expression among control engineers is "You can't control what you can't measure.". ©Adafruit Industries Page 3 of 15 A "Closed Loop" system can use the feedback signal to adjust the speed and direction of the motor to achieve the desired result. In the case of an RC servo motor, the feedback is in the form of a potentiometer (pot) connected to the output shaft of the motor. The output of the pot is proportional to the position of the servo shaft. The problem with controlling a standard RC servo motor from a microcontroller is that it is 'closed loop' inside the servo motor case, but 'open loop' with respect to your microcontroller. You can tell the servo control circuit how you want the shaft positioned, but you have no way to confirm if or when this actually happens. ©Adafruit Industries Page 4 of 15 The Feedback Servos allow you to close this outer loop by providing the feedback signal to the microcontroller too! Using Feedback If a servo motor does what it is told to do, why do we need feedback? RC servos usually do what they are told to do, but there are many cases where a servo motor might not. These can include: • Insufficient motor size • Insufficient power supply • Physical interference • Electrical interference • loose connection In these cases, feedback could alert you to the problem. But even if the servo is adequately sized and functioning normally, it still takes some time to respond to a position command, and in many applications it is just as ©Adafruit Industries Page 5 of 15 important to know when the position is reached. This following code snippet is from the "Sweep" example in the Servo library. Note the arbitrary 15 millisecond delay after "myservo.write(val)". void loop() { val = analogRead(potpin); (value between 0 and 1023) val = map(val, 0, 1023, 0, 179); between 0 and 180) myservo.write(val); scaled value delay(15); } // reads the value of the potentiometer // scale it to use it with the servo (value // sets the servo position according to the // waits for the servo to get there Without feedback, most servo programming has to make some assumptions about how long a particular move will take. Adding fixed-time delays to servo code works OK for simple applications, but can result in slow and/or jerky performance when trying to coordinate multiple servo motions or interactions between servos and other sensors or actuators. Or worse: If the delays are not long enough, your servos may not reach the desired position in time. This can cause malfunctions and/or damage to your project. Timing problems are a big problem in battery-powered projects because the motors will run slower as the battery power fades. ©Adafruit Industries Page 6 of 15 Reading the feedback The feedback signal is tapped off the position pot attached to the servo shaft. You can connect the white feedback wire to any of the analog input pins and read the feedback value using analogRead(). int feedback = analogRead(feedbackPin); Calibrating the feedback The raw feedback signal is a voltage. In order to convert that voltage into a meaningful position, we need to calibrate it to the servo. By reading the feedback values at two known positions, we can interpolate the expected feedback values for every position in between. The following bit of code does just that. If you call "calibrate" in your setup function, it will perform the calibration on the two points you specify. These servos operate over a range of about 0 to 180 degrees. For maximum accuracy, you should choose the minPos and maxPos calibration points based on the range of motion required in your project. #include <Servo.h> Servo myservo; // Control and feedback pins int servoPin = 9; int feedbackPin = A0; // Calibration values int minDegrees; int maxDegrees; int minFeedback; int maxFeedback; int tolerance = 2; // max feedback measurement error /* This function establishes the feedback values for 2 positions of the servo. With this information, we can interpolate feedback values for intermediate positions */ void calibrate(Servo servo, int analogPin, int minPos, int maxPos) { // Move to the minimum position and record the feedback value servo.write(minPos); minDegrees = minPos; delay(2000); // make sure it has time to get there and settle minFeedback = analogRead(analogPin); // Move to the maximum position and record the feedback value servo.write(maxPos); maxDegrees = maxPos; delay(2000); // make sure it has time to get there and settle maxFeedback = analogRead(analogPin); } ©Adafruit Industries Page 7 of 15 void setup() { myservo.attach(servoPin); calibrate(myservo, feedbackPin, 20, 160); range } // calibrate for the 20-160 degree void loop() { } Using feedback in your code Now that we have a calibrated feedback signal, we can easily convert between servo position and feedback voltages in our code. Seeking to a position The following bit of code will seek to a position and return as soon as we reach it. There is no need to add an arbitrary delay to the code because the feedback signal will tell us exactly when we get there! void Seek(Servo servo, int analogPin, int pos) { // Start the move... servo.write(pos); // Calculate the target feedback value for the final position int target = map(pos, minDegrees, maxDegrees, minFeedback, maxFeedback); // Wait until it reaches the target while(abs(analogRead(analogPin) - target) > tolerance){} // wait... } Finding out where you are Another great thing about feedback is: You don't need to write code to remember the last position command you sent to the servo (assuming it got there). If you want to find out what position your servo is in, you can simply ask it! Once you have calibrated your servo with the calibration function above, this bit of code will tell you the current position (in degrees) of your servo: int getPos(int analogPin) { return map(analogRead(analogPin), minFeedback, maxFeedback, minDegrees, maxDegrees); } ©Adafruit Industries Page 8 of 15 The ability to simply read the servo position opens up the possibility of using it as an input device as well. The next page will show you how. Servos as Input Devices Another neat feature of feedback servos is that they can be used as an input device too! The Servo Record/Play Demo lets you record a series of servo movements, then it will replay them back for you! The recorded positions are saved in EEPROM, so they will be remembered even after resetting or powering down the Arduino To run this demo, first wire up your Servo as in the Fritzing diagram below: Components used: • Arduino Uno (http://adafru.it/50) • Feedback Servo (http://adafru.it/1404) • 2x pushbuttons (http://adafru.it/1119) • LED  (http://adafru.it/299)(most any 3 or 5mm led will work) • 220 ohm resistor • Misc. jumpers (http://adafru.it/758) • Breadboard (http://adafru.it/64) ©Adafruit Industries Page 9 of 15 Next, download the example sketch from Github using this button: // Example code for recording and playing back servo motion with a // analog feedback servo // http://www.adafruit.com/products/1404 #include #include #define CALIB_MAX 512 #define CALIB_MIN 100 #define SAMPLE_DELAY 25 // in ms, 50ms seems good uint8_t uint8_t uint8_t uint8_t uint8_t recordButtonPin = 12; playButtonPin = 7; servoPin = 9; feedbackPin = A0; ledPin = 13; Servo myServo; void setup() { Serial.begin(9600); pinMode(recordButtonPin, INPUT); digitalWrite(recordButtonPin, HIGH); pinMode(playButtonPin, INPUT); digitalWrite(playButtonPin, HIGH); pinMode(ledPin, OUTPUT); Serial.println("Servo RecordPlay"); } void loop() { if (! digitalRead(recordButtonPin)) { delay(10); // wait for released while (! digitalRead(recordButtonPin)); delay(20); // OK released! recordServo(servoPin, feedbackPin, recordButtonPin); } ©Adafruit Industries Page 10 of 15 if (! digitalRead(playButtonPin)) { delay(10); // wait for released while (! digitalRead(playButtonPin)); delay(20); // OK released! playServo(servoPin, playButtonPin); } } void playServo(uint8_t servoPin, uint8_t buttonPin) { uint16_t addr = 0; Serial.println("Playing"); myServo.attach(servoPin); while (digitalRead(buttonPin)) { uint8_t x = EEPROM.read(addr); Serial.print("Read EE: "); Serial.print(x); if (x == 255) break; // map to 0-180 degrees x = map(x, 0, 254, 0, 180); Serial.print(" -> "); Serial.println(x); myServo.write(x); delay(SAMPLE_DELAY); addr++; if (addr == 512) break; } Serial.println("Done"); myServo.detach(); delay(250); } void recordServo(uint8_t servoPin, uint8_t analogPin, uint8_t buttonPin) { uint16_t addr = 0; Serial.println("Recording"); digitalWrite(ledPin, HIGH); pinMode(analogPin, INPUT); while (digitalRead(buttonPin)) { uint16_t a = analogRead(analogPin); Serial.print("Read analog: "); Serial.print(a); if (a < CALIB_MIN) a = CALIB_MIN; if (a > CALIB_MAX) a = CALIB_MAX; a = map(a, CALIB_MIN, CALIB_MAX, 0, 254); Serial.print(" -> "); Serial.println(a); EEPROM.write(addr, a); addr++; if (addr == 512) break; delay(SAMPLE_DELAY); } if (addr != 512) EEPROM.write(addr, 255); digitalWrite(ledPin, LOW); Serial.println("Done"); delay(250); } ©Adafruit Industries Page 11 of 15 To run the Servo Record/Play Demo Sketch: 1. Upload servo_recordplay to the arduino 2. press the top button to start recording. (The LED should light up.) 3. Press the top button once more to stop recording. 4. Press the bottom button to replay. 5. You can press the green button as many times as you want. 6. To record a new sequence, go back to step 2. Watch the video below to see it in operation: You can record up to 512 samples (about 12.8 seconds worth). When you reach the limit, the led will go out and recording will stop automatically. Using With CircuitPython All of the general discussion from the previous sections still apply. Here we simply provide CircuitPython (https://adafru.it/cpy-welcome) versions of the Arduino examples. For the basics on using servos with CircuitPython, checkout the information in the Essentials guide: CircuitPython Servo https://adafru.it/DOf Wiring The example codes are based on the wiring shown below. • RED wire to USB • BLACK wire to GND • ORANGE signal wire to A1 • WHITE feedback wire to A5 ©Adafruit Industries Page 12 of 15 Reading the feedback The basic mechanism is the same as before - simply connect the feedback to an analog input and read the value. To learn more about reading analog inputs, see the Essentials guide: CircuitPython Analog In https://adafru.it/Bep All that is needed is to setup an analog input and get its value. FEEDBACK_PIN = board.A5 feedback = AnalogIn(FEEDBACK_PIN) position = feedback.value The examples that follow will show this in more detail. Calibrating The Feedback You can use the program below to help determine the feedback values that correspond to your servo's range of motion. If you want to calibrate over a difference range of motion, you can change these lines at the top: ©Adafruit Industries Page 13 of 15 # Calibration setup ANGLE_MIN = 0 ANGLE_MAX = 180 When the code runs, it will print out the analog reading values that correspond to the min/max range of motion. Write these values down, you'll need them to calibrate the other programs. # Example code for calibrating analog feedback values to servo range import time import board import pwmio from adafruit_motor import servo from analogio import AnalogIn # Pin setup SERVO_PIN = board.A1 FEEDBACK_PIN = board.A5 # Calibration setup ANGLE_MIN = 0 ANGLE_MAX = 180 # Setup servo pwm = pwmio.PWMOut(SERVO_PIN, duty_cycle=2 ** 15, frequency=50) servo = servo.Servo(pwm) servo.angle = None # Setup feedback feedback = AnalogIn(FEEDBACK_PIN) print("Servo feedback calibration.") # Move to MIN angle print("Moving to {}...".format(ANGLE_MIN), end="") servo.angle = ANGLE_MIN time.sleep(2) print("Done.") feedback_min = feedback.value # Move to MAX angle print("Moving to {}...".format(ANGLE_MAX), end="") servo.angle = ANGLE_MAX time.sleep(2) print("Done.") feedback_max = feedback.value # Print results print("Feedback MIN = {}".format(feedback_min)) print("Feedback MAX = {}".format(feedback_max)) # Deactivate servo servo.angle = None Finding and Seeking This example shows how to find the current position and use that to "seek" to a specific angle. Be sure to run the calibration program first and change these lines at the top of the code with your servo's values. ©Adafruit Industries Page 14 of 15 # Calibration setup CALIB_MIN = 18112 CALIB_MAX = 49408 If you calibrated over a different range of angles, also change those lines to match. # Example code for using analog feedback value to seek a position import board import pwmio from simpleio import map_range from adafruit_motor import servo from analogio import AnalogIn # Demo angles angles = [0, 180, 0, 45, 180] # Pin setup SERVO_PIN = board.A1 FEEDBACK_PIN = board.A5 # Calibration setup CALIB_MIN = 18112 CALIB_MAX = 49408 ANGLE_MIN = 0 ANGLE_MAX = 180 # Setup servo pwm = pwmio.PWMOut(SERVO_PIN, duty_cycle=2 ** 15, frequency=50) servo = servo.Servo(pwm) servo.angle = None # Setup feedback feedback = AnalogIn(FEEDBACK_PIN) def get_position(): return map_range(feedback.value, CALIB_MIN, CALIB_MAX, ANGLE_MIN, ANGLE_MAX) def seek_position(position, tolerance=2): servo.angle = position while abs(get_position() - position) > tolerance: pass print("Servo feedback seek example.") for angle in angles: print("Moving to {}...".format(angle), end="") seek_position(angle) print("Done.") ©Adafruit Industries Page 15 of 15