Title:
SYSTEM FOR CONTROLLING THE ROTATIONAL SPEED OF A MICROMOTOR FOR USE IN MEDICAL INSTRUMENTS
United States Patent 3838324
Abstract:
A system for controlling the rotational speed of a micromotor for use in medical instruments such as a dental handpiece comprises a first transistor connected in series between said motor and a source of electric energy; a second transistor Darlington-connected to said first transistor; a third transistor through which part of the base current of said second transistor flows; circuit means for supplying to said third transistor a base current from the terminal voltage of said motor; a fourth transistor to which part of the current flowing through said second transistor is applied as a base current; and a variable resistor through which both the current caused by the source voltage to flow through said fourth transistor and the current flowing through said third transistor flow, said variable resistor providing a reference voltage for setting the rotational speed of said motor to a predetermined value.
US Patent References:
Circuit arrangement for regulating the speed of miniature electric d.-c.-motors
Limley - March 1967 - 3309596
Transistorized d. c. motor speed control system
Saeli - October 1967 - 3348112
MOTOR SPEED CONTROL SYSTEMS
Kearns - January 1969 - 3422331
D.C. MOTOR CONTROL SYSTEM
Igarashi - April 1969 - 3440511
MOTOR CONTROL CIRCUIT
Wickens - February 1971 - 3566229
Circuit arrangement for regulating the speed of miniature electric d.-c.-motors
Limley - March 1967 - 3309596
Transistorized d. c. motor speed control system
Saeli - October 1967 - 3348112
MOTOR SPEED CONTROL SYSTEMS
Kearns - January 1969 - 3422331
D.C. MOTOR CONTROL SYSTEM
Igarashi - April 1969 - 3440511
MOTOR CONTROL CIRCUIT
Wickens - February 1971 - 3566229
Application Number:
05/303128
Publication Date:
09/24/1974
Filing Date:
01/02/1972
View Patent Images:
Export Citation:
Assignee:
Kabushiki Kaisha Morita Seisakusho (Kyoto, JA)
Primary Class:
Other Classes:
388/928.100
International Classes:
H02P7/288; H02P7/18; H02P5/16
Field of Search:
318/331,332,345
US Patent References:
| 3617846 | DC MOTOR PROJECTOR DRIVE SYSTEM | November 1971 | Hanberg |
Primary Examiner:
Gilheany, Bernard A.
Assistant Examiner:
Langer, Thomas
Attorney, Agent or Firm:
Christensen, O'Connor, Garrison & Havelka
Claims:
What I claim is
1. A system for controlling the rotational speed of a micromotor which micromotor is responsive to a motor voltage applied to its terminals, comprising:
2. A circuit for controlling the rotational speed of a micromotor which micromotor is responsive to a motor voltage applied to its terminals, comprising:
3. A circuit according to claim 2, wherein said first control means comprises a first transistor the emitter-collector circuit of said first transistor being connected between said one terminal of said source of direct voltage and one end of said variable resistance, the other end of said variable resistance being coupled to the other terminal of said source of direct voltage, the base of said first transistor being connected to said second control means.
4. A circuit according to claim 3, wherein the base of said first transistor is coupled to said one terminal of said source of direct voltage through a given resistance.
5. A circuit according to claim 4, wherein said comparing means is a second transistor, the collector of said second transistor being coupled to said second control means and said one terminal of said voltage source, the emitter of said second transistor being coupled to said one side of said variable resistance and the base of said second transistor being coupled to said detecting means.
6. A circuit in accordance with claim 5, wherein said second control means is a third transistor, the collector of said third transistor being coupled to said one terminal of said source of direct voltage, and the base of said first transistor, the emitter of said third transistor being coupled to said current passage means, and the base of said third transistor being coupled to said collector of said second transistor.
1. A system for controlling the rotational speed of a micromotor which micromotor is responsive to a motor voltage applied to its terminals, comprising:
2. A circuit for controlling the rotational speed of a micromotor which micromotor is responsive to a motor voltage applied to its terminals, comprising:
3. A circuit according to claim 2, wherein said first control means comprises a first transistor the emitter-collector circuit of said first transistor being connected between said one terminal of said source of direct voltage and one end of said variable resistance, the other end of said variable resistance being coupled to the other terminal of said source of direct voltage, the base of said first transistor being connected to said second control means.
4. A circuit according to claim 3, wherein the base of said first transistor is coupled to said one terminal of said source of direct voltage through a given resistance.
5. A circuit according to claim 4, wherein said comparing means is a second transistor, the collector of said second transistor being coupled to said second control means and said one terminal of said voltage source, the emitter of said second transistor being coupled to said one side of said variable resistance and the base of said second transistor being coupled to said detecting means.
6. A circuit in accordance with claim 5, wherein said second control means is a third transistor, the collector of said third transistor being coupled to said one terminal of said source of direct voltage, and the base of said first transistor, the emitter of said third transistor being coupled to said current passage means, and the base of said third transistor being coupled to said collector of said second transistor.
Description:
This invention relates to a system for controlling the rotational speed of a micromotor for use in various medical instruments.
As is well known, micromotors are widely used as a drive for various instruments for cutting, grinding, forming, abraising and drilling of bones and tissues. Since the micromotor is very compact in size, it can be easily encased in an instrument having the shape of the elongated cylindrical body of a fountain pen, so that the doctor can easily manipulate the instrument in treatment. In dentistry the handpiece is an example of an instrument which uses a micromotor as a power source. The handpiece is provided at its outer end with a cutting tool which is rotated for performing the required treatment on the tooth. Since the handpiece is held by the hand like a pen, it must have such a shape that it can be easily gripped or held by the hand.
To make the handpiece compact, a micromotor is used as a drive for rotating the cutting tool. However, for such specific use in medical instruments the micromotor must be such that its rotational speed can be freely changed as occasions demand. In some cases, when the motor is being rotated under no load, it must be abruptly stopped. Also it is required that under loaded conditions the motor should be run at a constant speed to ensure accuracy in cutting or other operations and efficiency of the treatment.
To control the rotational speed of the micromotor there have been proposed various methods or systems. For example, one method uses a constant voltage circuit; another depends on detection of the counter electromotive force and a third utilizes the generated voltage of a generator. However, in the first method in which a constant voltage circuit is used, under a heavy-load, low-speed condition, that is when a low voltage is being applied to the motor, the rotational speed cannot be kept constant but is greatly decreased. In the second method, while the counter electromotive force is being detected, the input voltage cannot be applied to the motor. Therefore, the input voltage must be applied by means of a chopper, so that the input voltage necessarily becomes higher than the counter electromotive force and the motor is alternately speeded up and down by the variation of the applied voltage. This causes vibration to the instrument, making it unsuitable for use in accurate cutting or forming operation. In the third method in which a generator is used, if the generator is attached to the micromotor, the casing of the instrument becomes the larger or longer, so that it requires a greater muscular force of the operator with a resulting increase in his or her fatigue. In addition, the control circuit becomes complicated, and since the various elements of the control circuit are enclosed in a box remote from the motor in the handpiece so as to be remotely operated by the hand or foot of the operator, the connecting cables or wires become thicker, thereby obstructing free and efficient performance of the doctor in treatment.
Accordingly, it is one object of the invention to provide a system for controlling the rotational speed of a motor, which is capable of running the motor at a constant speed regardless of variation of the load thereon.
The invention will be explained in detail with reference to the accompanying Drawing illustrating one preferred embodiment thereof.
In the Drawing there is shown a motor M which is connected to direct voltage source terminals P through a transisotr TR1. The transistor TR1 directly controls the power applied to the motor M. A transistor TR2 is connected in Darlington configuration to the base of the transistor TR1. The base current of the transistor TR2 is provided by the current from the terminals P flowing through a resistor R1. Part of this current is applied to the collector of a transistor TR3. The transistor TR3 is controlled by the base current which is determined by a current caused by the voltage applied to the motor to flow through a fixed resistor R3 and a variable resistor VR1. A current flowing through a pre-fixed resistor VR2 and a fixed resistor R2 is applied to the base of a transistor TR4, the collector current of which flows through the previously mentioned variable resistor VR1.
As will be understood from the above description, the transistor TR1 controls the supply of power to the motor, the transistor TR2 functions as an amplifier, the transistor TR3 functions as a voltage comparator and the transistor TR4, as a current amplifier. The variable resistor VR1 provides a reference voltage for setting the number of rotation of the motor; the resistor R3 detects the voltage applied to the motor and the resistor VR2 detects the current applied to the motor.
In operation, suppose that a direct current voltage is applied between the terminals P. A base current is applied to the transistor TR2 through the resistor R1, whereupon the transistor TR2 fires so that the transistor TR1 receives a base current through the resistor VR2 and fires, whereupon the voltage applied to the terminals P is impressed on the motor M. At the same time, a base current is applied through the resistors VR2 and R2 to fire the transistor TR4, the collector current of which then flows through the resistor VR1, thereby inducing across the resistor a reference voltage ER1 in accordance with the resistance value of the resistor. The voltage ER1 charges a condenser C.
Under the condition, suppose that the terminal voltage of the motor M has become higher than the sum of the reference voltage ER1 and the voltage drop in the base forward direction of the transistor TR3. Then, more of the base current of the transistor TR2 provided through the resistor R1 than before flows into the collector of the transistor TR3, so that the base current of the transistor TR2 decreases, with a resulting decrease in the collector current of the transistor TR1 and consequently in the voltage applied to the motor M.
On the contrary, when the voltage applied to the motor has dropped, the collector current of the transistor TR3 decreases, so that the base current of the transistor TR2 increases, with a resulting increase in the collector current of the transistor TR1 and consequently, in the voltage applied to the motor M.
In the above manner, the voltage applied to the motor M is kept at a constant level as determined by the reference voltage ER1 set by the variable resistor VR1, so that the motor can be run at a constant speed regardless of variation of the load thereon.
Since the voltage applied to the motor while it is running depends on the reference voltage ER1, if the resistance value of the resistor VR1 is changed to change the reference voltage ER1, the rotational speed of the motor can be changed. In other words, it is possible to control the rotational speed of the motor M by means of the variable resistor VR1.
Since the current through the motor without any load thereon does not depend upon the voltage but is kept substantially constant, the collector current of the transistor TR4 is kept substantially constant, so that it is possible to have a desired level of the reference voltage ER1 by changing the resistance of the resistor VR1. Of course, the base current of the transistor TR3 as well as the collector current of the transistor TR4 flow through the resistor VR1. However, if the resistances of the resistors VR2, R1 and R2 and the direct current amplification factors of the transistors TR3 and TR4 are so selected that the base current of the transistor TR3 becomes far smaller than the collector current of the transistor TR4, the reference voltage ER1 will practically depends on the collector current of the transistor TR4 alone.
Suppose that with the resistor VR1 set to a certain resistance value, the rotational speed of the motor M has been reduced by a load connected thereto. Naturally, the current through the motor M increases, so that the collector current of the transistors TR1 and TR2 increases. This causes the base and collector currents of the transistor TR4 to increase, so that the voltage across the variable resistor VR1 becomes greater than the reference level ER1 that was under the no-load condition. As a result, the voltage applied to the motor M increases, thereby increasing the rotational speed thereof to be maintained at the required constant speed.
Thus, in accordance with the invention, the rotational speed of a micromotor can be kept constant regardless of variation of the load thereon, and the speed can easily be controlled by simply adjusting a variable resistor. The torque characteristic of the motor is greatly improved, so that the motor can be used as a reliable drive which ensures constant speed running of the rotating tool in medical instruments.
As is well known, micromotors are widely used as a drive for various instruments for cutting, grinding, forming, abraising and drilling of bones and tissues. Since the micromotor is very compact in size, it can be easily encased in an instrument having the shape of the elongated cylindrical body of a fountain pen, so that the doctor can easily manipulate the instrument in treatment. In dentistry the handpiece is an example of an instrument which uses a micromotor as a power source. The handpiece is provided at its outer end with a cutting tool which is rotated for performing the required treatment on the tooth. Since the handpiece is held by the hand like a pen, it must have such a shape that it can be easily gripped or held by the hand.
To make the handpiece compact, a micromotor is used as a drive for rotating the cutting tool. However, for such specific use in medical instruments the micromotor must be such that its rotational speed can be freely changed as occasions demand. In some cases, when the motor is being rotated under no load, it must be abruptly stopped. Also it is required that under loaded conditions the motor should be run at a constant speed to ensure accuracy in cutting or other operations and efficiency of the treatment.
To control the rotational speed of the micromotor there have been proposed various methods or systems. For example, one method uses a constant voltage circuit; another depends on detection of the counter electromotive force and a third utilizes the generated voltage of a generator. However, in the first method in which a constant voltage circuit is used, under a heavy-load, low-speed condition, that is when a low voltage is being applied to the motor, the rotational speed cannot be kept constant but is greatly decreased. In the second method, while the counter electromotive force is being detected, the input voltage cannot be applied to the motor. Therefore, the input voltage must be applied by means of a chopper, so that the input voltage necessarily becomes higher than the counter electromotive force and the motor is alternately speeded up and down by the variation of the applied voltage. This causes vibration to the instrument, making it unsuitable for use in accurate cutting or forming operation. In the third method in which a generator is used, if the generator is attached to the micromotor, the casing of the instrument becomes the larger or longer, so that it requires a greater muscular force of the operator with a resulting increase in his or her fatigue. In addition, the control circuit becomes complicated, and since the various elements of the control circuit are enclosed in a box remote from the motor in the handpiece so as to be remotely operated by the hand or foot of the operator, the connecting cables or wires become thicker, thereby obstructing free and efficient performance of the doctor in treatment.
Accordingly, it is one object of the invention to provide a system for controlling the rotational speed of a motor, which is capable of running the motor at a constant speed regardless of variation of the load thereon.
The invention will be explained in detail with reference to the accompanying Drawing illustrating one preferred embodiment thereof.
In the Drawing there is shown a motor M which is connected to direct voltage source terminals P through a transisotr TR1. The transistor TR1 directly controls the power applied to the motor M. A transistor TR2 is connected in Darlington configuration to the base of the transistor TR1. The base current of the transistor TR2 is provided by the current from the terminals P flowing through a resistor R1. Part of this current is applied to the collector of a transistor TR3. The transistor TR3 is controlled by the base current which is determined by a current caused by the voltage applied to the motor to flow through a fixed resistor R3 and a variable resistor VR1. A current flowing through a pre-fixed resistor VR2 and a fixed resistor R2 is applied to the base of a transistor TR4, the collector current of which flows through the previously mentioned variable resistor VR1.
As will be understood from the above description, the transistor TR1 controls the supply of power to the motor, the transistor TR2 functions as an amplifier, the transistor TR3 functions as a voltage comparator and the transistor TR4, as a current amplifier. The variable resistor VR1 provides a reference voltage for setting the number of rotation of the motor; the resistor R3 detects the voltage applied to the motor and the resistor VR2 detects the current applied to the motor.
In operation, suppose that a direct current voltage is applied between the terminals P. A base current is applied to the transistor TR2 through the resistor R1, whereupon the transistor TR2 fires so that the transistor TR1 receives a base current through the resistor VR2 and fires, whereupon the voltage applied to the terminals P is impressed on the motor M. At the same time, a base current is applied through the resistors VR2 and R2 to fire the transistor TR4, the collector current of which then flows through the resistor VR1, thereby inducing across the resistor a reference voltage ER1 in accordance with the resistance value of the resistor. The voltage ER1 charges a condenser C.
Under the condition, suppose that the terminal voltage of the motor M has become higher than the sum of the reference voltage ER1 and the voltage drop in the base forward direction of the transistor TR3. Then, more of the base current of the transistor TR2 provided through the resistor R1 than before flows into the collector of the transistor TR3, so that the base current of the transistor TR2 decreases, with a resulting decrease in the collector current of the transistor TR1 and consequently in the voltage applied to the motor M.
On the contrary, when the voltage applied to the motor has dropped, the collector current of the transistor TR3 decreases, so that the base current of the transistor TR2 increases, with a resulting increase in the collector current of the transistor TR1 and consequently, in the voltage applied to the motor M.
In the above manner, the voltage applied to the motor M is kept at a constant level as determined by the reference voltage ER1 set by the variable resistor VR1, so that the motor can be run at a constant speed regardless of variation of the load thereon.
Since the voltage applied to the motor while it is running depends on the reference voltage ER1, if the resistance value of the resistor VR1 is changed to change the reference voltage ER1, the rotational speed of the motor can be changed. In other words, it is possible to control the rotational speed of the motor M by means of the variable resistor VR1.
Since the current through the motor without any load thereon does not depend upon the voltage but is kept substantially constant, the collector current of the transistor TR4 is kept substantially constant, so that it is possible to have a desired level of the reference voltage ER1 by changing the resistance of the resistor VR1. Of course, the base current of the transistor TR3 as well as the collector current of the transistor TR4 flow through the resistor VR1. However, if the resistances of the resistors VR2, R1 and R2 and the direct current amplification factors of the transistors TR3 and TR4 are so selected that the base current of the transistor TR3 becomes far smaller than the collector current of the transistor TR4, the reference voltage ER1 will practically depends on the collector current of the transistor TR4 alone.
Suppose that with the resistor VR1 set to a certain resistance value, the rotational speed of the motor M has been reduced by a load connected thereto. Naturally, the current through the motor M increases, so that the collector current of the transistors TR1 and TR2 increases. This causes the base and collector currents of the transistor TR4 to increase, so that the voltage across the variable resistor VR1 becomes greater than the reference level ER1 that was under the no-load condition. As a result, the voltage applied to the motor M increases, thereby increasing the rotational speed thereof to be maintained at the required constant speed.
Thus, in accordance with the invention, the rotational speed of a micromotor can be kept constant regardless of variation of the load thereon, and the speed can easily be controlled by simply adjusting a variable resistor. The torque characteristic of the motor is greatly improved, so that the motor can be used as a reliable drive which ensures constant speed running of the rotating tool in medical instruments.