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Digital Servo Control Systems With Dynamic Modeling Technique for Integrating Power Amplifier

IP.com Disclosure Number: IPCOM000035187D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 31K

Publishing Venue

IBM

Related People

Workman, M: AUTHOR

Abstract

Some discrete time control systems, such as a disk file head positioning system as described in U.S. Patent 4,679,103, contain integrating power amplifiers. In the state estimator of such systems, a model of the integrating power amplifier must be built. Because all power amplifiers saturate, the power amplifier model can be very complex. In addition to requiring excessive computation, the model of the saturation characteristics depend on parameters which vary greatly over temperature and from unit to unit. Large parametric variations in the model can yield inaccurate state estimates. The invention described herein simplifies the required power amplifier model for the state estimator while providing accurate state estimates during the track seek operation when amplifier saturation occurs.

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Digital Servo Control Systems With Dynamic Modeling Technique for Integrating Power Amplifier

Some discrete time control systems, such as a disk file head positioning system as described in U.S. Patent 4,679,103, contain integrating power amplifiers. In the state estimator of such systems, a model of the integrating power amplifier must be built. Because all power amplifiers saturate, the power amplifier model can be very complex. In addition to requiring excessive computation, the model of the saturation characteristics depend on parameters which vary greatly over temperature and from unit to unit. Large parametric variations in the model can yield inaccurate state estimates. The invention described herein simplifies the required power amplifier model for the state estimator while providing accurate state estimates during the track seek operation when amplifier saturation occurs. Integrating Power Amplifier With Saturation

The figure shows the schematic of an integrating power amplifier which drives a current I(t) into the coil of a voice coil motor, such as is used in disk file actuators. The Laplace transform of the relationship between the control signal input U and the output I is given by Equation (1): Aol/(Rc+Rt)(Tc*s+1))(T1*s+1) I(s) = U(s) (1) 1 + Aol*s*Rt*T1/(Rc+Rt)(Tc*s+1)(T1*s+1) where Aol = Open loop gain of power amplifier Rc = Armature coil resistance Rt = Current sense resistance Lc = Armature inductance Tc = Coil time constant (Lc/Rc) Tl = Desired Integrator gain factor (1/R1 C1) epa = Power amplifier output voltage For large values of the open loop gain constant Aol, and assuming the the power amplifier is not saturated, this expression can be simplified as follows: 1 U(s) I(s) = * (2) Tl s Rt

Equation (2) describes an integrator with gain (1/(T1*Rt*S)). With this simple relationship, it is easy to model the behavior of the current I(t) in the coil between samples. However, if the amplifier saturates, i.e., if the absolute value of epa=esat where esat is the maximum output voltage of the amplifier, the integrator model is no longer valid. In a sense, what has changed is that the...