Do-it-yourself servo motor repair

In detail: do-it-yourself servo motor repair from a real master for the site my.housecope.com.

I recently made a robot arm, and now I decided to add a mini servo-powered gripper to it. I decided to make two variations to see how it would work better with a straight or round gear. I liked the round gear version better, as it took only 2 hours to make, and the gap between the gears was very small.

First, I cut out the parts on a milling machine:

I assembled the parts with 2x10mm screws.

And here's how the mini servo attaches to the gripper:

How the servo gripper works:

And now, when everything is assembled and the mechanical part is also almost ready, I just have to finish the electronic part of the work! I chose an Arduino to control my robot, and made a circuit (it's on the right) to connect the Arduino to the servo.

The circuit is actually very simple, it just sends signals to and from the Arduino. There is also a connector for an infrared receiver and some connectors for a power supply and 4 connections to the rest of the (unused) Arduino pins. Thus, another switch or sensor can be connected.

And here is how the manipulator arm moves:

The acquisition by the enterprise of a CNC milling machine for the manufacture of facades from MDF raises the question of the need to overpay for certain mechanisms and power units installed on expensive and high-tech equipment. To position the power units of CNC machines, as a rule, stepper motors and servo motors (servo drives) are used.

Video (click to play).

Stepper motors are cheaper. However, servo drives have a wide range of advantages, including high performance and positioning accuracy. So what to choose?

Image - Do-it-yourself servo motor repair

A stepper motor is a brushless DC synchronous motor with multiple stator windings. When current is applied to one of the windings, the rotor rotates and then is fixed in a certain position. Sequential excitation of the windings through the stepper motor control controller allows you to rotate the rotor at a given angle.

Stepper motors are widely used in industry, as they have high reliability and long service life. The main advantage of stepper motors is positioning accuracy. When current is applied to the windings, the rotor will rotate strictly at a certain angle.

· High torque at low and zero speeds;

·Quick start, stop and reverse;

· Work under high loading without risk of failure;

· The only wear mechanism that affects the duration of operation is bearings;

· Possibility of occurrence of a resonance;

· Constant power consumption regardless of the load;

Loss of torque at high speeds;

· Lack of feedback when positioning;

· Poor repairability.

Image - Do-it-yourself servo motor repair

A servomotor (servo drive) is an electric motor controlled through negative feedback, which allows you to precisely control the movement parameters in order to achieve the required speed or obtain the desired angle of rotation. The composition of the servomotor includes the electric motor itself, a feedback sensor, a power supply and control unit.

The design features of electric motors for a servo drive are not much different from conventional electric motors with a stator and a rotor, operating on direct and alternating current, with and without brushes.A special role here is played by a feedback sensor, which can be installed both directly in the engine itself and transmit data on the position of the rotor, as well as determine its positioning by external signs. On the other hand, the operation of a servomotor is unthinkable without a power supply and control unit (aka inverter or servo amplifier), which converts the voltage and frequency of the current supplied to the electric motor, thereby controlling its action.

· High power at the small sizes;

· Fast acceleration and deceleration;

· Continuous and uninterrupted position tracking;

· Low noise level, lack of vibrations and a resonance;

· Wide range of speed of rotation;

· Stable operation in a wide range of speeds;

· Small weight and compact design;

· Low consumption of the electric power at small loadings.

· Demanding for periodic maintenance (for example, with the replacement of brushes);

The complexity of the device (the presence of a sensor, power supply and control unit) and the logic of its operation.

When comparing the characteristics of a servo drive and a stepper motor, one should pay attention, first of all, to their performance and cost.

For the production of MDF facades in a small enterprise working with small volumes, I think there is no need to overpay for installing expensive servo motors on a CNC milling machine. On the other hand, if an enterprise seeks to reach the maximum possible production volumes, then it makes no sense to cheapen on low-performance stepper motors for CNC.

Servo motors are used not only in aircraft modeling and robotics, they can also be used in household devices. Small size, high performance, and simple servomotor control make them the most suitable for remote control of various devices.

The combined use of servomotors with radio modules for receiving and transmitting does not create any difficulties, it is enough on the receiver side to simply connect the appropriate connector to the servomotor, containing the supply voltage and the control signal, and the job is done.

But if we want to control the servomotor "manually", for example, with a potentiometer, we need a pulse control generator.

Below is a fairly simple oscillator circuit based on the 74HC00 integrated circuit.

This circuit allows manual control of servomotors by applying control pulses with a width of 0.6 to 2 ms. The scheme can be used, for example, to rotate small antennas, outdoor spotlights, CCTV cameras, etc.

The basis of the circuit is the 74HC00 (IC1) chip, which is 4 NAND gates. An oscillator was created on the elements IC1A and IC1B, at the output of which pulses are formed with a frequency of 50 Hz. These pulses activate the RS flip-flop, which consists of logic elements IC1C and IC1D.

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Image - Do-it-yourself servo motor repair

With each pulse coming from the generator, the output of IC1D is set to "0" and the capacitor C2 is discharged through the resistor R2 and the potentiometer P1. If the voltage on the capacitor C2 drops to a certain level, then the RC circuit switches the element to the opposite state. Thus, at the output we get rectangular pulses with a period of 20 ms. The pulse width is set with potentiometer P1.

For example, the Futaba S3003 servo drive changes the angle of rotation of the shaft by 90 degrees due to control pulses with a duration of 1 to 2 ms. If we change the pulse width from 0.6 to 2ms, then the rotation angle will be up to 120°. The components in the circuit are chosen in such a way that the output pulse is in the range of 0.6 to 2 ms, and therefore the installation angle is 120°. Futaby's S3003 servo motor has a sufficiently large torque, and the current consumption can be from tens to hundreds of mA, depending on the mechanical load.

Image - Do-it-yourself servo motor repair

Image - Do-it-yourself servo motor repair

The servomotor control circuit is assembled on a double-sided printed circuit board measuring 29 x 36 mm.Installation is very simple, so even a novice radio amateur can easily handle the assembly of the device.

Valve motors are synchronous brushless (brushless) machines. On the rotor are permanent magnets made of rare earth metals, on the stator there is an armature winding. The switching of the stator windings is carried out by semiconductor power switches (transistors) so that the stator magnetic field vector is always perpendicular to the rotor magnetic field vector - for this, a rotor position sensor (Hall sensor or encoder) is used. The phase current is controlled by PWM modulation and can be either trapezoidal or sinusoidal.

The flat rotor of the linear motor is made of rare earth permanent magnets. According to the principle of operation, it is similar to a valve motor.

Unlike continuous rotation synchronous machines, stepper motors have pronounced poles on the stator, on which the control winding coils are located - their switching is performed by an external drive.

Consider the principle of operation of a reactive stepper motor, in which teeth are located on the stator poles, and the rotor is made of soft magnetic steel and also has teeth. The teeth on the stator are arranged so that at one step the magnetic resistance is less along the longitudinal axis of the motor, and at the other - along the transverse axis. If the stator windings are discretely excited in a certain sequence with direct current, then the rotor will turn one step at each switching, equal to the pitch of the teeth on the rotor.

Some models of frequency converters can work with both standard asynchronous motors and servo motors. That is, the main difference between servo drives is not in the power part, but in the control algorithm and calculation speed. Since the program uses information about the position of the rotor, the servo drive has an interface for connecting an encoder mounted on the motor shaft.

Servo systems use the principle subordinate control: the current loop is subordinate to the speed loop, which in turn is subordinate to the position loop (see automatic control theory). First, the innermost loop, the current loop, is set up, then the speed loop, and the last one is the position loop.

current loop always implemented in the servo.

speed loop (as well as a speed sensor) is also always present in the servo system, it can be implemented both on the basis of a servo controller built into the drive, and external.

Position loop used for precise positioning (for example, feed axes in CNC machines).

If there are no backlashes in the kinematic connections between the executive body (coordinate table) and the motor shaft, then the coordinate is indirectly recalculated by the value of the rotary encoder. If there are backlashes, then an additional position sensor (which is connected to the servo controller) is installed on the executive body for direct measurement of the coordinate.

That is, depending on the configuration of the speed and position loops, the appropriate servo controller and servo drive are selected (not every servo controller can implement a position loop!).

  • Positioning
  • Interpolation
  • Synchronization, electronic gear (Gear)
  • Precise maintenance of rotation speed (machine spindle)
  • Electronic cam (Cam)
  • Programmable logic controller.

In general, a servo system (Motion Control System) can consist of the following devices:

  • Servo motor (Servo Motor) with a circular speed feedback sensor (it can also act as a rotor position sensor)
  • Servo Gear
  • Actuator position sensor (e.g. linear feed axis coordinate sensor)
  • Servo Drive
  • Servo controller (Motion Controller)
  • Operator Interface (HMI).

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