Electricity: motive power systems – Limitation of motor load – current – torque or force
Reissue Patent
2002-01-15
2004-04-06
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S799000, C388S903000, C388S813000
Reissue Patent
active
RE038486
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates generally to electric motor control circuits. More specifically, the invention relates to electric motor control circuits which set an upper limit on the rotation speed of electric motors and adjust the rotation speed within the set upper limit.
2. Description of Related Art
It is known to provide an electric motor control circuit which sets an upper limit on the rotation speed of an electric motor and adjusts the rotation speed within the upper limit. In this regard see U.S. Pat. No. 4,734,629, the teachings of which are incorporated herein by reference as if fully set forth, and the concept of which is explained in
FIGS. 6 and 7
.
FIG. 6
is a schematic diagram of the configuration of a known electric motor control circuit.
FIG. 7
is a schematic diagram of a part of the electric motor control circuit shown in FIG.
6
.
This known electric motor control circuit has a triac Q
7
for controlling a voltage applied to motor M from an interchange power supply
50
. A transistor Q
6
provides gate pulse signals to triac Q
7
. A tachometer-generator TG detects the rotation speed of the motor M. An upper limit rotation speed setter S
10
(see
FIG. 7
) sets the upper limit of rotation speed of the motor M. A trigger switch S
9
adjusts the rotation speed of the motor M within a range having the upper limit. A phase control IC
60
controls a gate pulse signal outputting timing from transistor Q
6
.
The signal indicative of rotation speed of the motor M detected by tachometer-generator TG is input to a terminal P
30
, and converted into a voltage signal corresponding to the rotation speed of the motor M via a frequency/voltage converter
61
(refer to FIG.
7
). A signal indicative of the upper limit of rotation speed output from setter S
10
is input to a terminal P
31
. The voltage of both signals are compared by an operational amplifier
62
built in phase control IC
60
, and a pulse signal indicative of this comparative result is input to a pulse timing setting circuit
63
. A sawtooth signal which synchronizes with the interchange power supply
50
is input to the pulse timing setting circuit
63
via a terminal P
41
, and an agreement point of the voltage of this sawtooth signal and the voltage of the pulse signal is detected.
At the detected agreement point, an operation signal is output from the pulse timing setting circuit
63
to transistor Q
6
via terminal P
39
. A gate pulse signal (trigger current) is output to triac Q
7
from transistor Q
6
, and triac Q
7
turns on, and the phase of the voltage applied to motor M is controlled.
Phase control IC
60
watches the rotation speed of motor M and controls the phase of the voltage applied to motor M to maintain the upper limit rotation speed set by setter S
10
.
The output of operational amplifier
62
is connected to trigger switch S
9
via a terminal P
34
and a transistor Q
5
. The voltage level of the output signal of operational amplifier
62
is changed by operating this trigger switch S
9
.
By operating trigger switch S
9
, the output timing of the pulse signal in the pulse timing setting circuit
63
is adjusted, and, the rotation speed of the motor M is adjusted within the range that the rotation speed doesn't exceed the upper limit rotation number.
In this known electric motor control circuit, however, trigger switch S
9
is connected to the output side of the operational amplifier
62
as shown in FIG.
7
. The output level of the operational amplifier
62
is adjusted directly by the trigger switch S
9
. Accordingly, even if the rotation speed of the motor M decreases due to a load and the decreased number is detected by operational amplifier
62
, the rotation speed of the motor M can't be increased because the output of operational amplifier
62
is lowered by trigger switch S
9
.
That is, as shown in
FIG. 8
showing a relationship between a load applied to motor M and the rotation speed of motor M controlled by the existing control circuit, if trigger switch S
9
is set to the maximum stroke and the rotation speed reaches the upper limit rotation number, the rotation speed of motor M is not slowed by a certain load because the output level of operational amplifier
62
does not increase. Whereas the rotation speed is slowed by a load until the rotation speed reaches the upper limit rotation.
Specially, in an electromotive tool such as a sander and a polisher, because required rotation speeds of the motor are different due to finishing stages and polished parts, the rotation speed of the motor is changed frequently. In the above mentioned existing circuit, unless the stroke of the trigger is made longer, the rotation speed of the motor falls when a pad for grinding is put on a grinding side. When the rotation speed of the motor is changed, an established value of the upper limit rotation speed setter S
10
must be adjusted, which is inconvenient. Moreover, it is desirable that the rotation speed of the motor can be changed by adjusting the stroke of the trigger, because delicate finishing is carried out with delicately changing the rotation speed of the motor in the grinding operation.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an electric motor control circuit which can prevent lowering of the rotation speed of a motor when a load is applied in the case where rotation speed is adjusted within a range that doesn't exceed a predetermined upper limit.
This invention provides an electric motor control circuit meeting this objective. The electric motor control circuit has: an electric motor driven by an interchange power supply, a semiconductor control element having a gate that conducts when a pulse signal is applied to the gate and which causes, in turn, a voltage supplied from the interchange power supply to be applied to the electric motor. A pulse signal output means outputs the pulse signal to the semiconductor control element. A rotation speed detection means detects the rotation speed of the electric motor. An upper limit rotation speed setting means sets an upper limit for the rotation speed of the electric motor. A rotation speed adjustment means adjusts the rotation speed of the electric motor within a range that doesn't exceed the upper limit rotation speed set by the upper limit rotation speed setting means. Comparative means compares a setting signal indicative of the upper limit rotation speed output from the upper limit rotation speed setting means with a detecting signal indicative of the rotation speed of the electric motor detected by the rotation speed detection means, and outputs a comparative signal which shows a comparative result of both signals. Output timing control means controls an output timing of the pulse signal output means based on the comparative signal output from the comparative means. The rotation speed setting means adjusts a signal level of the setting signal output from the upper limit rotation speed setting means.
The comparative means compares the setting signal which indicates the upper limit rotation speed output from the upper limit rotation speed setting means with the detecting signal which indicates the rotation speed of the electric motor detected by the rotation speed detection means, and outputs the comparative signal which shows the comparative result of both signals. The output timing control means controls the output timing of the pulse signal of the pulse signal output means based on the comparative signal output from the comparative means.
In other words, the comparative means detects the difference between a feed-back rotation speed of the motor and the upper limit rotation speed set up by the upper limit rotation speed setting means. And the output timing control means adjusts the rotation speed of the motor to maintain the upper limit rotation number, based on the detected difference.
Because the rotation speed setting means adjusts the signal level of the setting signal output from the upper limit rotation speed se
Duda Rina I.
Makita Corporation
Nappi Robert E.
Orrick Herrington & Sutcliffe LLP
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