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Selection of the proper AI-Tek magnetic sensor may
require the calculation of sensor output voltage to
assure proper operation in your specific application.
To assist in this area, let us consider the following
typical application: Requirement is speed display
with overspeed and underspeed control as well as 4-20
mA signal to a PLC. Speed range is 0-3600 RPM with
low speed set point at 300 RPM, available shaft diameter
for mounting a gear is 2.000 in. and a .030 in. air
gap is ideal.
You have selected
a TACHTROL 3, P/N T77310-11, with a 60T cast iron,
split gear, P/N G79870-202-0301, and you are considering
to use sensor P/N 70085- 1010-001. The question is
if the sensor has enough output voltage at 300 RPM.
We can list the following parameters:
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| a.
TACHTROL 3: |
Load impedance - 2000 ohms |
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Sensitivity - 200 mV rms |
| b.
Split gear: |
Outside dia. - 5.166 in. |
| |
D.P. - 12 |
| |
No. of Teeth - 60 |
| c.
Sensor: |
Standard output voltage
- 40V (P-P) min. |
| |
Guarantee Point - 3.4V P-P
min. |
| |
D.C. Resistance - 130 ohms
max. |
| |
Typical inductance - 33 mH
ref. |
|
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Step
1: Calculate surface
speed of gear: |
| SS = |
RPM x Outside Dia. x  |
= |
300 x 5.166 x 3.14 |
SS = 81 IPS |
| |
60 |
|
60 |
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Step
2.: Determine
Peak-to-Peak output voltage:
Referring to the performance curves
of sensor P/N 70085-1010-001 the min. output voltage
is approx. 0.3 V (P-P) at 81 IPS and 0.030 in. gap.
It is a fact that output voltage vs. surface speed
is a near linear function; therefore, another method
of determining output voltage is to set up a ratio
using the guarantee point: |
| 3.4V (P-P) |
= |
E |
|
E = 3.4 |
x |
81 |
= |
.55 V (P-P) |
| 500 IPS |
|
81 |
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500 |
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Step
3: Correction
for pitch:
For a 0.106 in. pole piece dia.
and a 12 D.P. gear the correction factor from Table
A is 1.41.
EC = .55 x 1.41 = .78 V
(P-P) |
|
Step
4: Converting
to rms voltage:
Simply divide by 3, a method which is close enough.
(If the peak-to-peak output voltage is a sine function,
the divisor is 2 times the square root of 2 or 2.83).
EC = .78 ÷ 3 = .26
V rms |
|
Step
5: Correction
for load:
The .26V or 260 mV rms sensor output voltage will
be divided across the impedance of the load and sensor.
The load impedance is 2000 ohms resistive. The impedance
of the sensor has a resistive and inductive element.
At low frequencies the inductive element is very small
and can therefore be disregarded, leaving the max.
DC resistance of 130 ohms for consideration.
The load correction
factor (fL) can be expressed as:
|
| (fL) = |
Z (load) |
= |
2000 |
= .94 |
| |
Z (load) + Z (sensor) |
|
2130 |
|
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EC = .94 x 260 = 244 mV
rms
The final adjusted
value is 244 mV rms.
As stated earlier,
the sensitivity or threshold of the Tachtrol·3
is 200 mV rms at the stated conditions, the selection
of P/N 70085-1010-001 is acceptable.
If the final
value of EC had been slightly less than 200 mV, a
reduction of the air gap (from .030" to .025")
would boost the output above 200 mV.
If it should
be determined that the required sensor cannot be selected
from the catalog models, the best procedure is to
compile a list of all your requirements and contact
your area distributor to assist you in the selection
of the correct sensor. |
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