# Measuring rotational speed for Tachometer vs Oscilloscope

• Engineering
• Special One
In summary, the conversation discusses an issue with the oscilloscope showing double the value of rotational speed compared to the tachometer. Possible explanations are discussed, such as the hall effect sensor only sensing one polarity and the motor type affecting the frequency. It is suggested to change the gain of the oscilloscope to fix the problem and to measure the period of two cycles of the signal to accurately calculate the frequency.
Special One
Homework Statement
f = 64.3 [Hz]
w = 1930 [RPM]
Relevant Equations
w=2*pi*f
Question: Why does the oscilloscope double almost the exact value of rotational speed measured by Tachometer?

Rotational speed from Tachometer = 1930 [RPM]
Frequency of 1 period = 64.3 [Hz] which means 3857.91 [RPM]

The output waveform of hall-effect sensor is attached.
Can you have any explanation please?

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We need a link to the experimental setup.

A magnet has two poles, but it appears the oscilloscope is seeing only one polarity, so that should be OK. What is the hall effect sensor sensing? How is the magnet mounted?

What type of tachometer are you using?
Maybe it is locking to a harmonic of the rotation.

What type of motor is it?
At 65 Hz, it is probably not an AC induction motor.
The distributor of a 4 stroke motor, where there might be a Hall effect sensor, rotates at half the speed of the crankshaft.

Baluncore said:
We need a link to the experimental setup.

A magnet has two poles, but it appears the oscilloscope is seeing only one polarity, so that should be OK. What is the hall effect sensor sensing? How is the magnet mounted?

What type of tachometer are you using?
Maybe it is locking to a harmonic of the rotation.

What type of motor is it?
At 65 Hz, it is probably not an AC induction motor.
The distributor of a 4 stroke motor, where there might be a Hall effect sensor, rotates at half the speed of the crankshaft.
The answer is because it puts out 2 pulses for every rotation.
So, in order fix this problem we should change the gain in oscilloscope from
+60.000,000 to +30.000,000
Now we will be getting the exact value as it is in the tachometer.

Special One said:
The answer is because it puts out 2 pulses for every rotation.
What is "it", and why does it "put out" two pulses per rotation ?

Special One said:
So, in order fix this problem we should change the gain in oscilloscope from
+60.000,000 to +30.000,000
The gain of an oscilloscope sets the vertical scale, not the sweep rate.

I would not assume that the two pulses per rev are equally spaced. So you should measure the period of two cycles of the signal with the oscilloscope, then take the reciprocal to convert that to frequency.

## What is the primary difference between a tachometer and an oscilloscope for measuring rotational speed?

A tachometer is a device specifically designed to measure the rotational speed of an object, usually in revolutions per minute (RPM). It provides a direct reading of the speed. An oscilloscope, on the other hand, is a versatile instrument used to visualize electrical signals. When measuring rotational speed with an oscilloscope, the speed is determined indirectly by analyzing the frequency of a signal generated by the rotating object.

## How do you use a tachometer to measure rotational speed?

To use a tachometer, you typically point it at or attach it to the rotating object. Contact tachometers require physical contact with the rotating part, while non-contact tachometers use a laser or optical sensor to detect the rotation. The tachometer then calculates the rotational speed and displays it on a screen, usually in RPM.

## How can you measure rotational speed with an oscilloscope?

To measure rotational speed with an oscilloscope, you need a sensor that converts the rotational motion into an electrical signal, such as a magnetic pickup or an optical encoder. This signal is then fed into the oscilloscope. By analyzing the waveform, you can determine the period of the signal and calculate the rotational speed. For example, if the signal has a frequency of 10 Hz, the rotational speed is 600 RPM (10 Hz * 60 seconds).

## Which method is more accurate for measuring rotational speed, a tachometer or an oscilloscope?

The accuracy of both methods depends on the quality and calibration of the instruments. Tachometers are generally easier to use and provide a direct reading, which can be very accurate if the device is well-calibrated. Oscilloscopes, while potentially more versatile and precise in signal analysis, require proper setup and interpretation of the waveform, which can introduce user error. For most practical purposes, a well-calibrated tachometer is sufficient and more straightforward for measuring rotational speed.

## Can an oscilloscope measure rotational speed in real-time as effectively as a tachometer?

An oscilloscope can measure rotational speed in real-time, but it usually requires more setup and interpretation compared to a tachometer. Tachometers are designed for quick and direct measurement, making them more convenient for real-time monitoring. Oscilloscopes provide more detailed signal analysis, which can be useful for diagnosing issues but may not be as straightforward for continuous real-time speed monitoring.

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