Servo Driver Selection Considering Factors

- Nov 15, 2017-

Servo System Selection Considering Factors

There are many factors that we need to consider when choose the right servo products for a particular application.In order to achieve the best performance of the entire servo system,there are 9 aspects need to be considered.

1.The choice of motor specifications

When the motor is start,there are 3 most important standards: speed, torque and rotary inertia.

The first two criteria are obvious,and generally,can be calculated by using the "sizing" software that provided by the manufacturer.

When use these programs,enter the type of motion required in the application and the software automatically calculates the required torque and speed based on the type of load and drive(gearbox, belt, rack and pinion, ball screw, etc.).While rotary inertia is less intuitive,it is equally important.Since there are most coupling are attached with its connecting mechanical transmission,it is paramount importance that the motor rotary inertia be compared to the load reflex inertial not too large.

As the proportion of both increases,it will lead to a significant increase in the difficulty of adjustment.As regulation algorithms evolve,microprocessor speed increases and the permissible ratio increases,but the traditional 10: 1 load-motor inertia ratio is a safe choice.

2.Load connection

Should the motor be connected directly? Some servo devices allow the load to be connected directly to the motor's rotor. With minimum compliance and low acceleration / deceleration rates,an inertia ratio in excess of 100: 1 can be achieved.The selection software provided by the manufacturer is the best tool to adjust these design guidelines to limit the system's inertia ratio to an appropriate level.

3.Energy renewable use

Another factor that servo systems need to consider in many applications is the ability to handle the reusable energy generated by motors and loads.Servo system involved here is the amplifier.When the motor torque is in one direction and the motor is moving in the opposite direction,the motor will send regenerative energy back to the amplifier.This is because the basic design of the motor is the same as generator.

The energy transmitted to the motor windings,will produce electromagnetic fields,so that the motor rotor and permanent magnet will follow to rotate.Similarly,when the motor rotor rotates,the permanent magnets energize the windings.There are many different ways of driving to use renewable energy.Some small drivers use bus capacitors to absorb these energies,and the larger driver consumes this heat with internal resistance heating.The selection software provided by the manufacturer usually gives the energy regeneration process.

4.Effective circuit regeneration

For some large systems are generally equipped with converters,so that the energy can be transmitted back to the system power supply circuit.Another factor which needs to be considered when design a servo system is the amount of regenerative energy that the drive can handle if the design allows the inertia mismatch between the motor and the load.

External power resistors are typically connected to the amplifier when handle excess renewable energy applications.As gravity

becomes a factor of concern,and need to rapidly reduce large inertia loads,these applications are generally vertically oriented.

Obviously,in addition to regenerative functions,the amplifier also needs to provide the motor with proper voltage and current to achieve desired speed and torque.Choosing the right power requirements for motors and amplifiers is just the beginning.

5.Control system requirements

Next,we need to handle the control needs.Let's go back to the motor first.According to definition,the servo system always needs a feedback device.At present,the feedback device is generally a high-resolution decoder or resolver.To ensure that the required positioning accuracy is met,the feedback device must have the proper resolution for repeatability and accuracy.The amplifier must also be compatible with the type of signal coming from the feedback device mounted on the motor.

6.Positioning accuracy

Traditionally,the decoder passes position and speed information on both channels via a pulse train and sends it back to the

amplifier.However,since the accuracy of these decoders has been greatly improved,some manufacturers have begun to use serial

decoders with higher transfer rates rather than pulse sequences to deliver these packets of information.These serial port decoders deliver signals of higher resolution for higher noise immunity and other types of signals.These additional information,includes the motor temperature and even the motor part number.

In servo systems,when the motor and amplifier are supplied by the same manufacturer,the ability of the feedback device to recognize and deliver the motor to the amplifier which automatically enables it to set internal parameters for optimal operation and motor protection.

7.Communication connection performance

When the motor and amplifier with drive and feedback devices match each other and have the resolution required for the 

application,can begin to consider the command signal between the amplifier and the motion controller.For single-axis applications,it is common to embed or attach the controller to one side of the amplifier.Some applications integrate it into a higher-level control system.The programmable logic controller (PLC) or programmable automation controller (PAC) can control the entire unit or line and transfer these information to single axis motion controller through digital input / output (I / O) or communication protocols such as EtherNet / IP or Modbus TCP / IP.

8.Control interface selection

In multi-axis applications,controllers are generally independent.Traditionally,the controller issued the command is generally +10 V signal,representing the speed or torque.Most manufacturers now offer web-based solutions.These network-based solutions require less wiring and can handle higher resolution feedback and can collect more diagnostic information from the amplifier.There are many motion networks available,each of which has its advantages and disadvantages.

9.The choice of motion control network

Many of the newer motion control networks are based on Ethernet hardware and take full advantage of their ever-increasing rates and incremental costs.However,you can not assume that it must be compatible with the relevant controller or other part of the network because of the RJ-45 connector on the amplifier.

The protocols that run on these networks will determine the compatibility of the system.EtherCAT,Mechatrolink III and Powerlink are several high-speed deterministic networks that are based on the Ethernet protocol and are suitable for motion control.

Network-based servo systems are typically configured with Ethernet ports that can use common industry protocols such as Modbus TCP / IP and EtherNet / IP to report diagnostic and product information to the monitoring network.

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