Drive Sizing Considerations

INTRODUCTIONIn 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...
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Drive Sizing Considerations 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 toCopyright 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 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 26b6D2 m oh ¼ ðrad=secÞ (13.1-1) Vc 6JT 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 ...