Frequently Asked Questions about the Tachometer

The measurement range varies, but it is measurable. The measurable range changes based on the following conditions.

The influence of measurement distance.

As an easy-to-understand example, let's consider halving a reflective mark. Simply put, halving the reflective mark means that the amount of reflected light is also halved, which results in a decrease in light sensitivity. Therefore, to obtain the same amount of light as a standard size (12×12 mm) reflective mark, the measurement distance must be shortened. Let S be the standard size reflective area and Lmax be the standard maximum measurement distance, as shown in the next diagram.

It will be.
In other words, the maximum measurement distance needs to be brought closer to about 70% compared to when using the specified size reflective marks.

the influence of rotational speed

As shown in the next figure, when the reflection mark becomes smaller, the reflection time decreases even if it is rotating at the same speed. If this reflection mark is not present for a specified duration (the minimum response time), the light receiving amplifier will not respond.

As shown in the next figure, if the rotation speed is n (r/min) and the distance from the center of rotation to the reflective mark is r, the effective reflection time Δt for the Δd width of the reflective mark is;

The minimum time Δt = Δt min (the minimum response time that can be received) varies by sensor or device, and as a guideline, it is as shown in the table below.

model name	Minimum response time Δt min (ms)
HT-4100	0.2
HT-5100/5200	0.2
LG-9	0.6
LG-930	0.5
FG-1300	0.1
PP-940	0.05

Expressing the maximum measurable rotational speed in terms of response time Δt min;

Furthermore, the time with the reflected light OFF (the part without the reflection mark) is also required to be at least of a similar duration, so the formula representing the maximum rotational speed is;

Therefore, when the reflection mark (Δd) is halved, the maximum rotation speed (It will be halved. The above is merely a theoretical calculation, and in reality, it is necessary to allow for a bit more margin.

 

To increase the maximum rotational speed of measurement.

To increase the maximum rotational speed, it is clear that we can make Δd larger as per the above equation. In other words, by attaching two or three reflective marks without leaving any gaps, the maximum rotational speed can be doubled or tripled.