FV Converter Frequently Asked Questions

When a reflective sticker is attached to the shaft and the signal is detected by a photoelectric detector such as the LG-916 or LG-930, the number of pulses per second is small. Depending on the rotational speed to be measured, this can become extremely low relative to the frequency range of the FV conversion circuit, resulting in the output voltage described in the question.

<Explanation>

The TM-2130 and FV-1100 utilize an FV conversion circuit (frequency/voltage conversion circuit) to convert the input signal into a voltage proportional to its frequency.

The diagram below shows a block diagram of the FV conversion circuit.

An integrating circuit charges when the input pulse is high and discharges when it is low. As the input pulse frequency increases, the number of charging cycles increases, and the overall charging time lengthens, resulting in a higher output voltage. Conversely, as the frequency decreases, the discharge time becomes longer than the charging time, and the output voltage decreases.

The faster the charging and discharging speed, the smaller the time constant, and the larger the difference in charging and discharging voltage per pulse (called ripple). Also, this ripple is negligible at high input frequencies, but becomes larger at low frequencies.

Photoelectric detectors like the LG-916 and LG-930 only produce a signal for each reflective sticker attached per rotation, so the input frequency tends to be low, leading to the situation described in the question. You need to calculate the input frequency from the operating rotation speed range and check whether it can be used at 10% or more of the frequency range of the FV converter.

The TM-2130 catalog lists the amount of output voltage ripple when the frequency is at 1% of the frequency range. The table below shows an example.

- Accuracy as stated in the catalog

Voltage output 0.1% FS
Current output 0.7% FS

The accuracy is worst at 20 kHz, so the resolution plus the analog circuit accuracy at this frequency is listed.

The input frequency can be modified to a maximum of 50 kHz. The same considerations apply to resolution in this case as well.