Tài liệu Drive Sizing Considerations pptx

13

Drive Sizing Considerations

13.1 INTRODUCTION

In the application of industrial servo drives to machines it is important that

the drive be large enough to meet all the load torque requirements and be

stable. It is necessary to make sure that the servo drive torque rating is large

enough to meet the load thrust and acceleration requirements plus the

machine friction losses; an organized method to accomplish this is referred

to as ‘‘sizing the drive.’’

There are many drive-sizing software programs available from

commercial servo drive suppliers. It is of critical importance that machine

designers size the servo drive in an organized engineering manner. Either

manual or computer software sizing must be used to avoid an unacceptable

servo-drive performance. It is also a requirement that the software drive￾sizing programs be documented and interactive with the user. Machine

design engineers may not have a feedback control background; thus it is

important that the programs have complete documentation (sometimes

referred to as remark statements). To be of value to the user, the software

programs should be interactive with enough description of what is

happening during the drive-sizing process.

The criteria of the drive sizing are different for hydraulic and electric

drive sizing. Hydraulic servo drives usually have more than ample torque to

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved

meet the load requirements. However, the hydraulic fluid has compressi￾bility, which is like a spring inside the servo loop and can cause a minor loop

instability. This hydraulic spring is represented as a hydraulic resonance.

Any hydraulic servo drive with a large volume of hydraulic oil compressed

between the actuator and the servo valve has a potential for an unstable

minor loop servo drive. Hydraulic servo valve drives that use long-travel

piston actuators have a great potential for servo instability. Likewise,

hydraulic servo pump drives with large volumes of hydraulic fluid between

the pump and actuator are prone to servo instability. The hydraulic

resonance is an indicator of what to expect for stability. An industrial index

of performance (I.P.) for the hydraulic resonance is that the resonance

should be 200 rad/sec or greater. The hydraulic resonance can be calculated

from

oh ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

26b6D2

m

Vc6JT

s

ðrad=secÞ (13.1-1)

where:

Dm ¼ motor displacement (in:

3

=rad)

Vc ¼ oil under compression (in:

3

)

JT ¼ total inertia at the motor (lb-in.-sec2

)

b ¼ bulk modulus of oil ¼ 16105 lb=in:

2

In general, hydraulic servo drives also have the added complexity of oil

contamination, leakage, and changes in viscosity with temperature.

Electric drives do not have the hydraulic medium problems of leakage,

compressibility, etc. However, electric drives do have some problems

obtaining sufficient torque for load requirements. These torque limitations

stem mostly from the amplifier. DC silicon controlled rectifier (SCR)

amplifiers have the problem of excessive phase lag due to the circuit

transport lag. Transistor design amplifiers have limits on available current.

In general, electric servo drives have more than adequate capability for

torque requirements and performance for industrial feed drive applications.

Section 13.2 consists of a manual hydraulic servo drive-sizing form.

This drive sizing will give assurance that the correct size hydraulic servo

motor and hydraulic pressure will be used to meet the torque or thrust

requirements for a machine feed drive. The hydraulic resonance is of

particular importance. To have a stable drive the hydraulic resonance

should be at least 200 rad/sec or higher. If this requirement is not met, the

drive should be resized with a different ratio, lead, motor, etc.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved