Do-it-yourself servomotor 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 gripping device powered by a mini servo 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 option better as it only took 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 using 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 the 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 purchase 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. For positioning the power units of CNC machines, stepper motors and servo motors (servo drives) are usually used.

Video (click to play).

Stepper motors are cheaper. However, servo drives offer a wide range of benefits, including high performance and positioning accuracy. So what should you choose?

Image - DIY Servo Motor Repair

A stepper motor is a brushless DC synchronous motor that has multiple stator windings. When current is applied to one of the windings, the rotor turns and then is fixed in a certain position. Sequential excitation of the windings through a stepper motor controller allows the rotor to rotate 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 turn strictly at a certain angle.

· High torque at low and zero speeds;

· Quick start, stop and reverse;

· Work under high load without the risk of failure;

· The only wear mechanism affecting the service life is bearings;

· Possibility of resonance;

· Constant power consumption regardless of the load;

· Drop in torque at high speeds;

· Lack of feedback during positioning;

· Poor repairability.

Image - DIY Servo Motor Repair

A servo motor (servo motor) is an electric motor with negative feedback control, which allows you to precisely control the parameters of movement in order to achieve the required speed or to obtain the desired angle of rotation. The servomotor includes the electric motor itself, the feedback sensor, the 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 about the position of the rotor, and determine its positioning by external signs. On the other hand, the operation of a servo motor 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 with small dimensions;

· Fast acceleration and deceleration;

· Continuous and uninterrupted position tracking;

· Low noise level, absence of vibrations and resonance;

· Wide range of rotation speed;

· Stable work in a wide range of speeds;

· Low weight and compact design;

· Low power consumption at low loads.

· Demanding for periodic maintenance (for example, with 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, you 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 the installation of 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 not only used in aeromodelling and robotics, they can also be used in household appliances. Small size, high performance, as well as easy control of the servo motor make them the most suitable for remote control of various devices.

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

But if we want to control the servo motor "manually", for example, using a potentiometer, we need an impulse control generator.

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

This circuit allows manual control of servomotors by supplying 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 microcircuit (IC1), which consists of 4 NAND gates. A generator is created on the elements IC1A and IC1B, at the output of which pulses with a frequency of 50 Hz are formed. These pulses activate the RS flip-flop, which consists of gates IC1C and IC1D.

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Image - DIY Servo Motor Repair

With each pulse coming from the generator, the IC1D output is set to "0" and the capacitor C2 is discharged through the resistor R2 and the potentiometer P1. If the voltage across the capacitor C2 drops to a certain level, then the RC circuit transfers 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 by 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 2 ms, then the rotation angle is up to 120 °. The components in the circuit are selected so that the output pulse is in the range of 0.6 to 2 ms, and therefore the angle of installation is 120 °. The S3003 servo motor from Futaby has a sufficiently large torque, and the current consumption can range from tens to hundreds of mA, depending on the mechanical load.

Image - DIY Servo Motor Repair

Image - DIY Servo Motor Repair

The servo motor 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 cope with the assembly of the device.

Valve motors are synchronous brushless (brushless) machines. On the rotor there are permanent magnets made of rare earth metals, on the stator there is an armature winding. The stator windings are switched 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 trapezoidal or sinusoidal.

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

Unlike synchronous machines of continuous rotation, stepper motors have pronounced poles on the stator, on which the coils of the control windings are located - their commutation 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 located so that at one step the magnetic resistance is less along the longitudinal axis of the motor, and at the other - along the transverse one. If you discretely excite the stator windings with direct current in a certain sequence, then the rotor with each commutation will turn by one step, equal to the pitch of the teeth on the rotor.

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

Servo systems use the principle subordinate management: the current loop is subordinate to the speed loop, which in turn is subordinate to the position loop (see automatic control theory). The innermost loop, the current loop, is tuned first, followed by the speed loop, and last, the position loop.

Current loop always implemented in the servo.

Velocity loop (as well as the 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, or an external one.

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

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

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

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

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

  • 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 encoder for feed axis coordinates)
  • Servo Drive
  • Servo controller (Motion Controller)
  • Operator interface (HMI).

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