Electricity: motive power systems – Positional servo systems – With compensating features
Reexamination Certificate
1999-09-02
2001-02-06
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
With compensating features
C318S636000, C318S430000, C318S254100, C318S434000, C318S805000, C318S806000, C388S806000, C388S815000, C388S821000, C388S928100
Reexamination Certificate
active
06184645
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to control of an actuator and, more particularly, to a method and apparatus for accurately controlling the velocity of an actuator member by monitoring the back electromotive force (“EMF”) of an actuator coil, and driving the coil with a voltage.
BACKGROUND OF THE INVENTION
Conventional actuators, sometimes referred to as “motors”, have a movably supported member, and a coil. When a current is passed through the coil, a motive force is exerted on the member. A control circuit is coupled to the coil in order to controllably supply current to the coil. One example of such an arrangement is found in a hard disk drive, where the movable member of the actuator supports a read/write head adjacent a rotating magnetic disk for approximately radial movement of the head relative to the disk. There are situations in which it is desirable to move the member to one end of its path of travel at a predetermined velocity which is less than its maximum velocity. An example of such a situation is a power failure. In such a situation, it is desirable to move the member to a parking location, where it is held against potentially damaging movement which could occur if the member were not so parked. The movement of the member to the parking location is commonly referred to as a retract of the member.
When a current is applied to the coil of the actuator, the member is subjected to a force tending to accelerate the member at a rate defined by the magnitude of the current, and in a direction defined by the polarity of the current. Consequently, in order to accelerate or decelerate the member until it is moving at a desired velocity and in a desired direction, it is important to know the actual direction and velocity of the member. In this regard, it is known that the back-EMF voltage on the coil of the actuator is representative of the velocity and direction of movement of the member. Specifically, the following relationship applies to actuators:
V
M
=I
M
*R
M
+K
e
&ohgr;
where:
V
M
=voltage across actuator (motor),
I
M
=current through actuator,
R
M
=internal resistance of actuator,
K
e
=torque constant of actuator, and
&ohgr;=velocity of actuator.
The term, K
e
&ohgr;, represents the back-EMF of the actuator coil.
Apparatus have been provided that control such actuators by providing a drive current to the coil of the actuator in response to the provision of a target speed voltage signal having a voltage corresponding to the target speed of the moveable member. For example, co-pending patent application Ser. No. 09/239,188, filed on Jan. 28, 1999, and entitled “CONSTANT VELOCITY CONTROL FOR AN ACTUATOR USING SAMPLED BACK EMF CONTROL,” discloses such an apparatus. However, such apparatus does not lend itself readily to providing such control in cases where the drive transistors for the actuator are power MOSFETs external to the integrated circuit (“IC”) containing the control circuitry. In such cases, it is difficult and/or expensive to implement a current mode output. To do so would require current feedback. To process this feedback, additional circuitry would be required. This additional circuitry would add expense and would be difficult to operate at low voltages.
Accordingly, it is desired to have an apparatus that controls actuators using a voltage mode output. Such a configuration would eliminate the requirement for current sensing and should, therefore, be simpler and less expensive to build.
SUMMARY OF THE INVENTION
The present invention provides an apparatus for controlling an actuator in response to a target speed voltage signal, the actuator having a moveable member and having a coil that influences movement of the member. A first summing node is provided having a control output voltage, for summing or subtracting voltages applied thereto, as the case may be and receiving the target speed voltage signal at a first, summing input thereto. A proportional compensation unit is provided, receiving the control output voltage and providing as an output a proportionally compensated output signal. An integral compensation unit is provided, receiving the control output voltage and providing as an output an integrally compensated output signal. An inverse compensation unit is provided, receiving the target speed voltage signal as an input and providing an inversely compensated output signal having a voltage corresponding to the voltage of the target speed voltage signal multiplied by the inverse of a predetermined amplification factor. A second summing node is provided, receiving the proportionally compensated output signal, the integrally compensated output signal and the inversely compensated output signal, and having a command output signal corresponding to the sum of the input signals. An amplifier is provided, receiving the command output signal and having a drive output signal for driving the actuator. Finally, a sense unit is provided for sensing a back-EMF of the actuator and providing a voltage corresponding to the sensed back-EMF to a second, subtracting input of the first summing node, such that the control output voltage corresponds to the difference between the target speed voltage and the sensed back-EMF. The inversely compensated output signal substantially provides a voltage to the amplifier sufficient to cause the amplifier to provide the drive output signal at a voltage corresponding to the voltage of the target speed voltage signal.
These and other features of the invention will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings.
REFERENCES:
patent: 4531167 (1985-07-01), Berger
patent: 5602689 (1997-02-01), Kadlec et al.
patent: 5677809 (1997-10-01), Kadlec
patent: 6040671 (2000-03-01), Brito et al.
Brito Joao Carlos F.
Rote John K.
Whyte, Jr. Robert E.
Brady III Wade James
Martin Edgardo San
Moore J. Dennis
Nappi Robert E.
Telecky , Jr. Frederick J.
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