Industrial applications of brushless servo motor

Introduction:

Asynchronous machine with permanent magnets on the rotor is the heart of the modern actuator without a servo motor.

The motor remains in synchronism with the supply frequency, although there is a limit to the maximum torque that can be developed before the rotor is out of sync, with the breakout torque generally being between 1.5 and 4 times the continuous nominal torque.

The torque-speed curve is therefore simply a vertical line.

The industrial application of brushless servomotors has grown considerably for the following reasons:

• Reduced price of power conversion products

• Implementation of advanced control of PWM inverters

• Development of new materials with more powerful and easier to use permanent magnets

• The growing need for extremely precise position control

• The manufacture of all these components in a very compact form

They are, in principle, easy to control because the torque is generated in proportion to the current. Besides, they have high efficiency and high dynamic responses can be obtained.

Brushless servo motors are often called brushless DC servo motors because their structure is different from DC servo motors. They rectify the current by means of a transistor switching in the associated drive or amplifier, instead of a switch used in DC servo motors.

Confusedly, they are also referred to as AC Servo Motors because the synchronous type brushless servo motors (permanent magnet rotor) sense the position of the rotating magnetic field to control the three-phase current of the armature.

It is now widely recognized that brushless alternating current refers to a motor with a sinusoidal stator winding distribution designed for use with a sinusoidal or inverter supply voltage.

Brushless DC refers to a motor with a trapezoidal stator winding distribution designed for use with a supply voltage from a wave-switched or block-switched inverter. The brushless servo motor lacks a DC motor switch and has a device ( the drive, sometimes called the amplifier ) to flow current based on the position of the rotor.

In the DC motor, increasing the number of switch segments reduce the torque variation. In the brushless motor, the torque variation is reduced by making the coil three-phase and, in steady-state, controlling the current of each phase in a sine wave.

Application examples

1. Feed to a length:

   1. Barbecue making machine

   2. Welder on the fly

2. Indexing / Conveyor: Rotary indexer

3. Next: Labeling machine

4. Measurement / Dispensing: Capsule Filling Machine

5. Fly Cutoff: Rotary Tube Cut

1. Feed to length

Applications in which a continuous web, web, or strand of material is indexed lengthwise, most commonly with nip rollers or some sort of gripping arrangement.

The index stops and a process is in progress ( cutting, stamping, punching, labeling, etc. ).

Barbecue making machine

Application Type: Feed to Length
Motion: Linear
Application Description: A manufacturer used a material feed servo motor in a machine to create barbecues, shopping carts, etc. However, the feed length was irregular because the slippage between the pickup roller and the material was too frequent. Knurled pinch rollers cannot be used as they will damage the material.

The machine builder needed a more precise method of cutting the material to uniform lengths. The customer used a load-mounted encoder to provide feedback on the actual amount of material fed into the cutting head.

Objectives of the machine:

• Compensate for material sliding

• Interface with customer's control panel

• Smooth repeatable operation

• Variable-length indices

• High reliability

Motion control requirements:

• Precise position control

• Loaded encoder feedback

• Quick indexing

• Xcode language

Application solution:

Using the overall position feedback capability of the servo drive, the machine builder was able to close the position loop with the encoder mounted on load, while speed feedback was provided by the encoder mounted on the motor and the processing of the drive. Signal. The dual encoder system provides improved stability and superior performance compared to a single load-mounted encoder providing position and speed feedback.

The load-mounted encoder was coupled to friction pressure rollers located near the cutting head.
Welder on the fly

Application Type: Feed to Length
Movement: Linear
Description: In a sheet metal fabrication process, a loose part rests on a conveyor belt moving continuously at an unpredictable speed. Two spot welds should be made on each piece, 4 inches apart, the first weld 2 inches from the leading edge of the piece. A weld takes a second.

Objectives of the machine:

• Autonomous operation
• Position the welder according to the position and speed of each part
• Welding and positioning performed without stopping the conveyor
• The welding process should take 1 second

Motion control requirements

• Programmable I / O; sequence storage
• Next
• Motion profiling; complex monitoring
• High linear acceleration and speed

Application solution:

This application requires a controller capable of tracking or profiling motion based on a primary encoder position. In this application, the controller will receive speed and position data from an incremental encoder mounted on a roller located on the conveyor belt and carrying the unattached parts.

The conveyor is considered the main drive system. The secondary motor/drive system receives instructions from the controller, based on a ratio of the speed and position information provided by the primary system encoder.

The linear motor forces the welding head and is mounted on a platform suspended in the extension of the conveyor. Linear motor technology was chosen to support the welding head due to the length of the path. The linear stepper motor is not subject to the same linear speed and acceleration limitations inherent in systems converting rotary motion to linear motion.

For example, in a worm gear system, the inertia of the lead screw often exceeds the inertia of the load, and as the length of the screw increases, the inertia also increases. With linear motors, all the force generated by the motor is effectively applied directly to the load; thus, the length has no effect on the inertia of the system.

This application requires a 54-inch bed to allow tracking of conveyor speeds greater than 20 inches / s.

2. Indexing / Conveyor

Applications where a conveyor is repeatedly driven to index parts in or out of an auxiliary process.

Rotary indexer

Type of application: Indexing conveyor
Movement: Rotary
Description of application: An engineer for a pharmaceutical company is designing a machine for filling vials and wants to replace an old Geneva mechanism. A micro-step motor allows smooth movement and prevents overflows. The index wheel is aluminum and measures 0.250 inches thick and 7.5 inches in diameter.

Solving the equation for the inertia of a full cylinder indicates that the wheel has 119.3 oz-in 2. Holes in the index wheel reduce inertia to 94 oz-in 2. The vials have negligible mass and can be ignored for motor sizing purposes. The table contains 12 vials (separated by 30 °) which must be indexed in 0.5 seconds and remained for one second. The throttle torque is calculated to be 8.2 oz-in at 1.33 rpm 2.

A triangular displacement profile will give a maximum speed of 0.33 rpm.

The torque required is less than 100 oz-in. However, a low load-to-rotor inertia ratio was required to gently move the vials and fill them.

Machine requirements:

• Smooth movement
• PLC control
• Variable index length

Motion control requirements:

• Smooth movement
• Sequence selection
• I / O for sequence selection
• Programmable acceleration and deceleration

Application solution:

The index distance can be changed by the engineer who controls the machine with a PLC. The travel parameters are changing and can therefore be set via the BCD inputs. The indexer can be "buried" in the machine and activated with a remote START input.

3. Continuation

Labeling machine

Applications requiring movement coordination should be paired with an external speed or position sensor.

Type of application: Next
Movement: Linear
Description of application: Bottles on a conveyor pass through a labeling mechanism that applies a label to the bottle. The bottle spacing on the conveyor is not set and the conveyor can slow down, speed up or stop at any time.

Machine requirements:

• Apply labels accurately to moving bottles

• Provide a variable conveyor speed

• Provide an inconsistent distance between the bottles

• Pull the label strip through the dispenser

• Consistent and uniform labeling at all speeds

Motion control requirements:

• Synchronization on the conveyor axis

• Electronic gearbox function

• Check-in check

• High torque to overcome high friction

• High resolution

• Open-loop stepper if possible

Application solution:

A motion controller that can accept inputs from an encoder mounted on the conveyor and referencing all speeds and distances from the label roll to the encoder is required for this application. A servo system is also needed to provide the torque and speed to overcome the friction of the dispensing head and the inertia of the large label roll.

A photosensor connected to a programmable input of the controller monitors the position of the bottles on the conveyor.

The controller commands the label motor to accelerate to line speed as the first edge of the label contacts the bottle. The label motor moves at line speed until the full label is applied, then decelerates to a stop and waits for the next bottle.

4. Dosage / Distribution

Applications where it is necessary to control movement and/or speed to dose or dispense a precise amount of material.

Capsule filling machine

Application Type: Dosing / Dispensing
Movement: Linear
Application Description: The design requires a machine to dispense radioactive fluid into capsules. Once the fluid is dispensed, it is inspected and the data is stored on a PC. It is necessary to increase the flow rate without introducing an overflow.

Machine requirements:

• Increase flow
• No release of radioactive fluid
• Automate two axes
• PC compatible system control
• Low-cost solution
• Smooth, repeatable movement

Motion control requirements:

• Quick and precise shots
• Multi-axis controller
• PC bus-based motion control card
• Open-loop stepper if possible
• High-resolution motor/drive (micro-step)

Application solution:

The multi-axis indexer is selected to control and synchronize the two axes of motion on a card residing in the IBM PC computer.

An additional feature is the full I / O required to activate the filling process. The horizontal axis carrying the capsule tray is driven by a linear motor. The simple mechanical construction of the motor facilitates application and guarantees a long service life without maintenance. The vertical axis raises and lowers the filling head and is driven by a micro-step motor and screw.

A linear motor was also considered for this axis, but the filler head would have fallen onto the platter with a loss of motor power. The friction of the lead screw and the residual torque of the stepper motor prevent this event.

5. In-flight cut-off

Applications where a strip of material is cut while moving the material. Typically, the cutter moves at an angle to the web and at a speed proportional to the web.

Rotary pipe cutter

Application Type: Flight Cutoff
Movement: Linear
Application Description: A metal tube feeds from a coil and must be cut into predetermined lengths. A rotating blade mechanism is used to cut the tube, and the blade mechanism must rotate around the tube several times to complete the cut.

The throughput of this machine should be maximized so that the tube cannot be stopped while cutting. Therefore, to make a clean cut on the tube, the blade must move with the tube during the cut.

Machine requirements:

• Autonomous operation
• Move the cutting mechanism with the tube to make the cut without stopping
• A simple user interface to define different tube lengths
• High cutting precision

Motion control requirements:

• Programmable I / O
• Storage program
• Next position
• High acceleration and speed

Application solution:

A single-axis servo controller/drive was chosen to solve this application. An external encoder monitors the output of the tube and sends this information back to the slave system.

The servo system follows the length of the tube which passes in front of the cutting blade. Once the correct amount of material has been fed past the blade, the servo accelerates the cutter to tube speed, sends an output to start the cutting machine, and then exactly follows the tube speed.