kopia lustrzana https://github.com/micropython/micropython
147 wiersze
5.7 KiB
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147 wiersze
5.7 KiB
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Controlling hobby servo motors
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There are 4 dedicated connection points on the pyboard for connecting up
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hobby servo motors (see eg
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[Wikipedia](http://en.wikipedia.org/wiki/Servo_%28radio_control%29)).
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These motors have 3 wires: ground, power and signal. On the pyboard you
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can connect them in the bottom right corner, with the signal pin on the
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far right. Pins X1, X2, X3 and X4 are the 4 dedicated servo signal pins.
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.. image:: img/pyboard_servo.jpg
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In this picture there are male-male double adaptors to connect the servos
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to the header pins on the pyboard.
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The ground wire on a servo is usually the darkest coloured one, either
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black or dark brown. The power wire will most likely be red.
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The power pin for the servos (labelled VIN) is connected directly to the
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input power source of the pyboard. When powered via USB, VIN is powered
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through a diode by the 5V USB power line. Connect to USB, the pyboard can
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power at least 4 small to medium sized servo motors.
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If using a battery to power the pyboard and run servo motors, make sure it
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is not greater than 6V, since this is the maximum voltage most servo motors
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can take. (Some motors take only up to 4.8V, so check what type you are
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using.)
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Creating a Servo object
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-----------------------
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Plug in a servo to position 1 (the one with pin X1) and create a servo object
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using::
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>>> servo1 = pyb.Servo(1)
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To change the angle of the servo use the ``angle`` method::
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>>> servo1.angle(45)
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>>> servo1.angle(-60)
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The angle here is measured in degrees, and ranges from about -90 to +90,
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depending on the motor. Calling ``angle`` without parameters will return
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the current angle::
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>>> servo1.angle()
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-60
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Note that for some angles, the returned angle is not exactly the same as
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the angle you set, due to rounding errors in setting the pulse width.
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You can pass a second parameter to the ``angle`` method, which specifies how
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long to take (in milliseconds) to reach the desired angle. For example, to
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take 1 second (1000 milliseconds) to go from the current position to 50 degrees,
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use ::
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>>> servo1.angle(50, 1000)
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This command will return straight away and the servo will continue to move
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to the desired angle, and stop when it gets there. You can use this feature
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as a speed control, or to synchronise 2 or more servo motors. If we have
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another servo motor (``servo2 = pyb.Servo(2)``) then we can do ::
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>>> servo1.angle(-45, 2000); servo2.angle(60, 2000)
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This will move the servos together, making them both take 2 seconds to
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reach their final angles.
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Note: the semicolon between the 2 expressions above is used so that they
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are executed one after the other when you press enter at the REPL prompt.
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In a script you don't need to do this, you can just write them one line
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after the other.
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Continuous rotation servos
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--------------------------
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So far we have been using standard servos that move to a specific angle
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and stay at that angle. These servo motors are useful to create joints
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of a robot, or things like pan-tilt mechanisms. Internally, the motor
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has a variable resistor (potentiometer) which measures the current angle
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and applies power to the motor proportional to how far it is from the
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desired angle. The desired angle is set by the width of a high-pulse on
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the servo signal wire. A pulse width of 1500 microsecond corresponds
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to the centre position (0 degrees). The pulses are sent at 50 Hz, ie
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50 pulses per second.
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You can also get **continuous rotation** servo motors which turn
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continuously clockwise or counterclockwise. The direction and speed of
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rotation is set by the pulse width on the signal wire. A pulse width
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of 1500 microseconds corresponds to a stopped motor. A pulse width
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smaller or larger than this means rotate one way or the other, at a
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given speed.
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On the pyboard, the servo object for a continuous rotation motor is
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the same as before. In fact, using ``angle`` you can set the speed. But
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to make it easier to understand what is intended, there is another method
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called ``speed`` which sets the speed::
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>>> servo1.speed(30)
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``speed`` has the same functionality as ``angle``: you can get the speed,
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set it, and set it with a time to reach the final speed. ::
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>>> servo1.speed()
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30
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>>> servo1.speed(-20)
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>>> servo1.speed(0, 2000)
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The final command above will set the motor to stop, but take 2 seconds
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to do it. This is essentially a control over the acceleration of the
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continuous servo.
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A servo speed of 100 (or -100) is considered maximum speed, but actually
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you can go a bit faster than that, depending on the particular motor.
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The only difference between the ``angle`` and ``speed`` methods (apart from
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the name) is the way the input numbers (angle or speed) are converted to
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a pulse width.
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Calibration
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-----------
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The conversion from angle or speed to pulse width is done by the servo
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object using its calibration values. To get the current calibration,
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use ::
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>>> servo1.calibration()
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(640, 2420, 1500, 2470, 2200)
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There are 5 numbers here, which have meaning:
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1. Minimum pulse width; the smallest pulse width that the servo accepts.
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2. Maximum pulse width; the largest pulse width that the servo accepts.
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3. Centre pulse width; the pulse width that puts the servo at 0 degrees
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or 0 speed.
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4. The pulse width corresponding to 90 degrees. This sets the conversion
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in the method ``angle`` of angle to pulse width.
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5. The pulse width corresponding to a speed of 100. This sets the conversion
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in the method ``speed`` of speed to pulse width.
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You can recalibrate the servo (change its default values) by using::
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>>> servo1.calibration(700, 2400, 1510, 2500, 2000)
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Of course, you would change the above values to suit your particular
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servo motor.
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