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Servo Motors

  • A servomotor is an electromechanical device in which an electrical input signal determines the position of the armature of a motor with built in rotation sensor, sending information concerning the degree of rotation on its axis.
  • Servo refers to an error sensing feedback control which is used to correct the performance of a system.


  • Classification of Servo motor

    • DC servo motor
    • Brushless servo
    • AC Servo Motors:-
      • Permanent Magnet Synchronous Motors
      • Variable Reluctance servo Motor


    Features of servo motors:

    • encoder/potentiometer determines the accuracy and resolution of the servo motor

    • A servo motor has 5-10 times rated torque for short periods

    • stays cool because the current draw is proportional to load

    • maintains usable high speed torque of 90% of NL RPM

    • performs quietly at high speeds silently

    • has a resonance-free and vibration-free operation

    • High Torque to Inertia Ratio can rapidly accelerate loads

    • The servo motor can approach 90% efficiency at light loads


    Construction of DC servo motor:


    A servo motor contains a DC motor mechanically linked to a position feedback potentiometer, gear box, electronic feedback control loop circuitry and output shaft with spline so as to create linkage motor with other device/chassis.

    Construction of Servo Motor


    Functioning of DC servo motor:

    • The function of the servo is to receive a control signal that represents a desired output position of the servo shaft, and apply power to its DC motor until its shaft turns to that position. It uses the position-sensing(potentiometer) device to determine the rotational position of the shaft, so it knows which way the motor must turn to move the shaft to the commanded position.

    • The shaft typically does not rotate freely round and round like a DC motor, but rather can only turn 200 degrees or so back and forth with the help of gear set.

    • The servo has a 3 wire connection: power(Red), ground(Black or Brown), and control(Yellow, Orange, white).

    • The power source must be constantly applied; the servo has its own drive electronics that draw current from the power lead to drive the motor. Servos can operate under a range of voltages, typically from 4.8V to 6V.

    • Servos are controlled by sending them a pulse of variable width. The control wire is used to send this pulse.

    • The parameters for this pulse are that it has a minimum pulse, a maximum pulse, and a repetition rate. Given the rotation constraints of the servo, neutral is defined to be the position where the servo has exactly the same amount of potential rotation in the clockwise direction as it does in the counter clockwise direction.

    • It is important to note that different servos will have different constraints on their rotation but they all have a neutral position, and that position is always around 1.5 milliseconds (ms).

    • The angle is determined by the duration of a pulse that is applied to the control wire. This is called Pulse width Modulation.

    • The servo expects to see a pulse(positive) every 20 ms. The length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90 degree position (neutral position).
    • Functioning of Servo Motor



    • When the servo is commanded to move it will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist from moving out of that position.

    • The maximum amount of force the servo can exert is the torque rating of the servo. Servos will not hold their position forever though; the position pulse must be repeated to instruct the servo to stay in position.

    • When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and holds its output shaft some number of degrees counterclockwise from the neutral point. When the pulse is wider than 1.5 ms the opposite occurs.

    • The minimal width and the maximum width of pulse that will command the servo to turn to a valid position are functions of each servo. Different brands, and even different servos of the same brand, will have different maximum and minimums.

    • Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide.

    • Another parameter that varies from servo to servo is the turn rate(90 degree, 180 degree, 360 degree, etc). This is the time it takes from the servo to change from one position to another. The worst case turning time is when the servo is holding at the minimum rotation and it is commanded to go to maximum rotation. This can take several seconds on very high torque servos.



    Important concepts:

    Servo rating:

    The most common details available on a servo are its speed and torque rating.Nearly all servo packages are listed with brand name, model name/number, speed and torque output at 4.8 volts and 6 volts.some information about metal, plastic gears or ball bearings may also be listed.


    Servo speed:

    Servo speed is defined as the amount of time(in seconds) that a servo arm attached to the servo output shaft will move from 0 to 60 degrees.

  • Note: Lower is the time(seconds), fasted the servo can move an attached wheel or arm.

  • servo speed is measured by the amount of time(seconds) it takes a 1 inch servo arm to sweep left or right through a 60 degree arc at either 4.8 volts or 6 volts. A servo rated at 0.22 seconds/60 degrees takes 0.22 seconds to sweep a 60 degree arc. some of the fastest servos moves in the 0.06 to 0.09 second range. in some servos faster speed result in lower torque.


    Servo torque:

    Servo torque is defined as ounce-inch(oz-in), the total push/pull power a servo can apply on 1 inch servo arm when moving. servos have certain amount of torque(strength) that is generally proportional to their size.servos comes is all kinds of size, strengths and weight. torque is a measurement of force over a distance.servo torque is measured by the amount of weight(in ounces) that a servo can hold at 1 inch out on the servo output arm in horizontal plane at either 4.8 volts or 6 volts. servo XYZ=100 oz/in says that servo XYZ is capable of holding 100 ounces using a 1 inch output arm without excessive deflection at 6.0 volts at input. It can also be represented in Kg-cm by dividing it by 13.9 Pulpit rock






    Servo Power:

    Servo power is defined as the amount of DC voltage needed to operate a servo without damage.At the higher voltage servo tends to be faster and sometimes stronger, but can heat up faster when stalled or in hold position with stress forces against the servo output shaft. some servo controllers require a separate power source from the control source to deliver the higher 6.0 volts.The current drain(Amps required) depends on the torque being put out by the servo motor and can be in excess of one amp if the servo is stalled under load. It is best to calculate one amp per servo when figuring power supply needs for most servos.


    Stall Torque, Stall Current, Current Drain:

    One the rated characteristic is stall torque. This is the torque required to stop the motor from rotating. You normally would want to design using only the operating torque value, but there are occasions when you want to know how far you can push your motor. If you are designing a wheeled robot, good torque means good acceleration. If you have 2 motors on your robot, make sure the stall torque on each is enough to lift the weight of your entire robot times your wheel radius. Always favor torque over velocity. your torque ratings can change depending on the voltage applied. If you need a little more torque then 20% above the rated motor voltage value is fairly safe to apply. This is less efficient, and that you should heat sink your motor.



    Velocity:

    The servo turn rate, or transit time, is used for determining servo rotational velocity. This is the amount of time it takes for the servo to move a set amount, usually 60 degrees. For example, suppose you have a servo with a turn rate of 0.17sec/60 degrees at no load. This means it would take nearly half a second to rotate an entire 180 degrees. More if the servo is under a load. This information is very important if high servo response speed is a requirement of your application. It is also useful for determining the maximum forward velocity of your robot if your servo is modified for full rotation. The worst case turning time is when the servo is at the minimum rotation angle and is then commanded to go to maximum rotation angle, all while under load. This can take several seconds on a very high torque servo.


    Efficiency and Noise:

    Due to noise and control circuitry requirements, servos are less efficient than uncontrolled DC motors. The control circuitry typically drains 5-8mA just on idle. Secondly, noise can draw more than triple current during a hold position (not moving), and almost double current during rotation.Noise is often a major source of servo inefficiency and therefore should be avoided.

    Note:

  • Do your servo ever jitter or vibrate?

  • This is because your servo is rapidly jumping between two different angles due to interference.

  • What causes interference?

  • The signal wire is no different than a long antenna, capable of accepting unwanted foreign signals and sending them straight to your servo as a command. A common interference source is usually from other nearby servos and/or servo wiring. to prevent this problem, keep your signal wire short, meaning do not add say 3+ feet of extension cables to your servo. If you have many servo wires going through one area, and it isn't feasible to keep them apart, then twist them together, this may reduces cross interference. You can also buy something called a servo booster extension which buffers and amplifies the signal.


    Applications:

    • Robotic arm
    • RC(remote controlled) car, airplane and boat.
    • Industrial printing machines








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