Tech Briefs Magazine - October 2021 - ST-18

Applications
AV
I in
I F
Bin
BF
Figure 7. Inductive current sensor schematic. (TE Connectivity)
VOUT
RF
A shunt current sensor is a very low
value power resistor placed in series with
the electric power line to the motor. The
voltage developed across the resistor is
proportional to the current flowing
through it. Using Ohm's law (I = E/R)
and knowing the voltage and resistance
values, the current can be calculated.
Shunts are used where very high accuracy
is required or for high frequency brushless
motors. Figure 6 shows the schematic
for a shunt sensor.
Inductive current sensors use a high
permittivity ring around the power wire.
Current through the wire induces a magnetic
field in the ring (Bin). A Hall effect
sensor embedded in the ring measures
the magnetic field, and through an
amplifier and coil wrapped around the
ring, induces a reverse magnetic field
(BF) to counteract Bin. The voltage
required to create the BF field is the output
signal. The net effect of the two magnetic
fields will appear as a null field at
the Hall sensor. The higher the current
flow in the power wire, the higher the BF
voltage must be. (See Figure 7)
Beyond the Motor
Often, equipment attached to the
motor can also be monitored for health.
Figure 8 shows an example of a milling
machine that uses a multi-fluted rotary
cutter to machine a flat surface into a
piece of work. The condition of the
rotary cutting tool is very important to
the quality of the cutting operation.
The sharper the flutes on the cutter, the
more precise the cut, and the better the
surface on the finished work. As each
flute makes a cut, the motor current
increases to provide the needed power.
Sharp flutes require less power to cut
the metal. Dull flutes require more
power.
Figure 8. Multi-flute endmill. (Andrey Armyagov/Shutterstock)
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0.1
0.2
0.3
0.4
0.5
Time (sec)
Figure 9. Milling machine motor current. (TE Connectivity)
18
Cov
www.techbriefs.com
ToC
0.6
0.7
0.8
0.9
1
Dull Flute
Sharp Flute
By monitoring the motor current for
this milling machine, it's possible to
determine which of the flutes on the cutting
tool are sharp and which are dull
and need to be sharpened.
The Contributions of Condition
Monitoring to the IIoT
From the invention of the production
line and the industrial revolution it created,
there have been numerous technical
innovations that have pushed
progress in manufacturing. Factories
have experienced average productivity
gains of 2.8% per year over the last 70
years. Technologies such as power tools,
injection molding, pick & place equipSensor
Technology, October 2021
Motor Current (amps)

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Tech Briefs Magazine - October 2021

Table of Contents for the Digital Edition of Tech Briefs Magazine - October 2021

Tech Briefs Magazine - October 2021 - Intro
Tech Briefs Magazine - October 2021 - Sponsor
Tech Briefs Magazine - October 2021 - Cov1
Tech Briefs Magazine - October 2021 - Cov2
Tech Briefs Magazine - October 2021 - 1
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Tech Briefs Magazine - October 2021 - Cov3
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Tech Briefs Magazine - October 2021 - CN-Cov1
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Tech Briefs Magazine - October 2021 - MD-Cov1
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Tech Briefs Magazine - October 2021 - MD-1
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Tech Briefs Magazine - October 2021 - MD-Cov3
Tech Briefs Magazine - October 2021 - MD-Cov4
Tech Briefs Magazine - October 2021 - ST-Cov1
Tech Briefs Magazine - October 2021 - ST-Cov2
Tech Briefs Magazine - October 2021 - ST-1
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