Tachometer Frequently Asked Questions

Measurement is possible, although the measurement range will vary. The measurable range will vary depending on the following conditions:

Influence of measurement distance

As a simple example, let's consider halving the size of the reflective mark. Simply put, halving the size of the reflective mark means halving the amount of reflected light, which reduces the light-receiving sensitivity. Therefore, to obtain the same amount of light as a standard-sized (12 x 12 mm) reflective mark, the only option is to shorten the measurement distance. If we let S be the standard reflective area and Lmax be the standard maximum measurement distance, then as shown in the following diagram:

This is the result.
In other words, the maximum measurement distance needs to be approximately 70% of what it would be when using a reflective mark of the specified size.

Effect of rotational speed

As shown in the following diagram, when the reflection mark becomes smaller, the reflection time shortens even when rotating at the same speed. The light receiving amplifier will not respond unless this reflection mark is present for a specified time (minimum receivable response time).

As shown in the following figure, if the rotational speed is n (r/min) and the distance from the center of rotation to the reflection mark is r, then the time Δt during which a reflection mark of width Δd is effectively reflecting is:

The minimum time Δt (Δt min, minimum receivable response time) varies depending on the sensor and equipment, and is shown in the table below as a guideline.

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

The maximum measurable rotational speed can be expressed using the response time Δt min:

Furthermore, since the time when the reflected light is OFF (the part without the reflective mark) is also required to be at least the same, the formula for representing the maximum rotation speed is:

Therefore, when the reflection mark (Δd) is halved, the maximum rotation speed () will be halved. The above is purely theoretical; in reality, you need to allow for a little more leeway.

 

To increase the maximum measurable rotational speed

It is also clear that increasing Δd in the above equation will increase the maximum rotation speed. In other words, by attaching two or three reflective marks in a continuous line without any gaps, the maximum rotation speed can be doubled or tripled.