Post by swanrail on Jan 30, 2008 0:02:33 GMT 1
Servos consist of 4 basic parts:
1. The motor, designed to be used intermittently, but often used continuously on modified servos. Has 3 leads, a common, one for forward and one for reverse operation.
2. The gear box. As the servo arm is often only moving short distances, and this is not very efficient for motors, gearing is used so that the motor can turn many times for a part revolution of the servo arm.
3. Potentiometer (variable resistor) is attached to the final gear, and its slider reflects the position of the servo arm. It is used to generate a feedback voltage to the comparator.
4. Electronic circuit. This consists of an integrator circuit, which will take the input pulses from the receiver and turn them into a DC voltage dependant on the width of the pulse. There is also a voltage comparator, which has two inputs and two outputs. One of the inputs is fed from the integrator, the other fed from the central arm of the potentiomenter. If both voltages are equal, there is no output on either of the two output terminals. However, if, say, A goes higher than B (wider input pulse) then output C will generate a voltage which is attached to a motor input so turning the motor. Likewise, if A is less than B (narrower input pulse) then output D will generate a voltage causing the motor to go the other way.
Transmitter:
This generates pulses the width of which is dependant on the joystick. At the neutral (centre) position, the pulse is approx. 1.5 msec wide and is repeated every 20 msec. Moving the joystick fully in one direction will increase the pulse width to approx. 2.0 msec, and moving it fully the other way will reduce the pulse width to approx. 1.0msec. Intermediate will have values between these two extremes.
In the neutral position , the voltage at A is (say) 0.86 volts (my servo). The potentiometer in the servo has 1.3 volts one end and 1.1 volts the other with 0.86volts in its central postion. Thus there is no output on C or D and nothing moves.
Fully in one position of the joystick will raise the input to 1.3 volts, and thus output C (as an example) will generate a voltage driving the servo motor which will turn the servo arm and the potentiometer until the centre arm of the pot reaches 1.3 volts when output C falls to zero and the motor stops.
Releasing the joystick will let it spring back to the centre, now A has 0.86 volts whilst B still has 1.3 volts. This will make the comparator produce an output voltage on D, driving the motor the opposite way until it reaches the .86 volts on the pot when the motor will stop in its neutral postion.
Similarly, the same effect happens if the joystick is moved the other way.
Note that the final gear has two mechanical stops fitted to prevent the servo over running with possible damage to equipment fitted to the servo arm.
Modifications
Some people want the servo to run continously in either direction. This is achieved by disconnecting the potentiometer physically from the final gear so that it does not move, maintaining the central voltage of .86volts so that the comparator can work properly and gluing the arm in postion so that it cannot move.
The overrun stops must then be removed. In use, the joystick can be used to move the servo arm either clockwise or anticlockwise continously until the joystick goes back to neutral when the motor will stop. This is useful for robotics.
Markus has modified his servos so that when modified as above, two microswitches are attached in their normally open postion between the pot centre arm and the two outside terminals respectively.
When the joystick is moved, say, fully left, the motor will run until the servo arm mechanism operates the microswitch causing the input B to the comparator to rise to max volts (in my case 1.3 volts for one direction). The motor will then attempt to briefly run in the opposite direction causing the input B to drop back to its normal .86 volts.(as the microswitch reopens). The joystick should then be released, so that input A is also at .86 volts, causing the motor to stop, and the servo arm to stay at its new postion.
Likewise, for intermediate postions, the input A voltage will depend on the width of the pulse which in turn depends on how much the joystick is moved. Here, the motor will turn as either output C or D is energised (depending on clockwise or anticlockwise movements), but input B stays at the 0.86. As soon as the joystick is released, then the motor will stop leaving the servo arm in its turned postion.
Note that all the above voltages are dependant on the make of servo, but should be approx correct.
Also note that the servo can be used to continously rotate suitable for radar aerials for instance, but here the simplest way is to take two of the motor leads (neutral and one other depending on clockwise or anticlockwise rotation) and connect the supply voltage directly to it (may need a resistor to lower voltage to suit motor). Make sure that the physical stops are also removed, else catastrophe!!!
1. The motor, designed to be used intermittently, but often used continuously on modified servos. Has 3 leads, a common, one for forward and one for reverse operation.
2. The gear box. As the servo arm is often only moving short distances, and this is not very efficient for motors, gearing is used so that the motor can turn many times for a part revolution of the servo arm.
3. Potentiometer (variable resistor) is attached to the final gear, and its slider reflects the position of the servo arm. It is used to generate a feedback voltage to the comparator.
4. Electronic circuit. This consists of an integrator circuit, which will take the input pulses from the receiver and turn them into a DC voltage dependant on the width of the pulse. There is also a voltage comparator, which has two inputs and two outputs. One of the inputs is fed from the integrator, the other fed from the central arm of the potentiomenter. If both voltages are equal, there is no output on either of the two output terminals. However, if, say, A goes higher than B (wider input pulse) then output C will generate a voltage which is attached to a motor input so turning the motor. Likewise, if A is less than B (narrower input pulse) then output D will generate a voltage causing the motor to go the other way.
Transmitter:
This generates pulses the width of which is dependant on the joystick. At the neutral (centre) position, the pulse is approx. 1.5 msec wide and is repeated every 20 msec. Moving the joystick fully in one direction will increase the pulse width to approx. 2.0 msec, and moving it fully the other way will reduce the pulse width to approx. 1.0msec. Intermediate will have values between these two extremes.
In the neutral position , the voltage at A is (say) 0.86 volts (my servo). The potentiometer in the servo has 1.3 volts one end and 1.1 volts the other with 0.86volts in its central postion. Thus there is no output on C or D and nothing moves.
Fully in one position of the joystick will raise the input to 1.3 volts, and thus output C (as an example) will generate a voltage driving the servo motor which will turn the servo arm and the potentiometer until the centre arm of the pot reaches 1.3 volts when output C falls to zero and the motor stops.
Releasing the joystick will let it spring back to the centre, now A has 0.86 volts whilst B still has 1.3 volts. This will make the comparator produce an output voltage on D, driving the motor the opposite way until it reaches the .86 volts on the pot when the motor will stop in its neutral postion.
Similarly, the same effect happens if the joystick is moved the other way.
Note that the final gear has two mechanical stops fitted to prevent the servo over running with possible damage to equipment fitted to the servo arm.
Modifications
Some people want the servo to run continously in either direction. This is achieved by disconnecting the potentiometer physically from the final gear so that it does not move, maintaining the central voltage of .86volts so that the comparator can work properly and gluing the arm in postion so that it cannot move.
The overrun stops must then be removed. In use, the joystick can be used to move the servo arm either clockwise or anticlockwise continously until the joystick goes back to neutral when the motor will stop. This is useful for robotics.
Markus has modified his servos so that when modified as above, two microswitches are attached in their normally open postion between the pot centre arm and the two outside terminals respectively.
When the joystick is moved, say, fully left, the motor will run until the servo arm mechanism operates the microswitch causing the input B to the comparator to rise to max volts (in my case 1.3 volts for one direction). The motor will then attempt to briefly run in the opposite direction causing the input B to drop back to its normal .86 volts.(as the microswitch reopens). The joystick should then be released, so that input A is also at .86 volts, causing the motor to stop, and the servo arm to stay at its new postion.
Likewise, for intermediate postions, the input A voltage will depend on the width of the pulse which in turn depends on how much the joystick is moved. Here, the motor will turn as either output C or D is energised (depending on clockwise or anticlockwise movements), but input B stays at the 0.86. As soon as the joystick is released, then the motor will stop leaving the servo arm in its turned postion.
Note that all the above voltages are dependant on the make of servo, but should be approx correct.
Also note that the servo can be used to continously rotate suitable for radar aerials for instance, but here the simplest way is to take two of the motor leads (neutral and one other depending on clockwise or anticlockwise rotation) and connect the supply voltage directly to it (may need a resistor to lower voltage to suit motor). Make sure that the physical stops are also removed, else catastrophe!!!