RPM and speed measurement in cars via CAN “on-board diagnostics” interface with analog and pulse outputs
RPM-8000-OBD2 offers a convenient solution for automotive RPM measurement without an additional sensor. The information will be read directly from the CAN-OBD2- interface (ISO 15765 CAN, 11/29 Bit ID, 250/500 kBaud). The instrument is simply connected to the standard OBD2 connector and the RPM or speed is shown directly on the LCD display. You even have the choice to output the data as an analog voltage signal (0-5 V) or as a digital pulse sequence (TTL).
The RPM measuring range of the analog output is 0.5 Volt per 1000 RPM. The standard TTL frequency output of 16 Hz per engine revolution can be scaled with a frequency divider of 1:1, 1:2, 1:4, 1:8 or 1:16.
The speed measuring range of the analog output is 0.020 V per km/h (0-5 V = 0-250 km/h) The TTL frequency output is 16 Hz/km/h (max. 250 km/h).
Sensorless automotive RPM measurement with LCD display and easy calibration
RPM-8000-PRO offers a convenient solution for automotive RPM measurement without an additional sensor: The practical instrument is simply connected via a standard jack to the cigarette lighter socket and the RPM is shown directly on the LCD display. You even have the choice to output the data as an analog voltage signal (1 Volt per 1000 rev/min) or as a digital pulse sequence (TTL). The smart measuring principle is based on an analysis of the ripple balance of the supply system, characteristic for all automotive alternators. The frequency of this ripple amounts to about 100 to 120 pulses per engine revolution and therefore delivers good dynamic measuring results.
Though the frequency is proportional to the engine RPM, it also depends on the gear transmission ratio between crankshaft and alternator and on the number of the alternator´s poles and phases. This makes a calibration of the input signal to the actual measured RPM necessary. The latest version of the RPM-8000-PRO now offers two elegant options:
Internal calibration: The internal calibration assumes a steady engine RPM of 2000 RPM, which can be monitored by the automotive tachometer. By pushing the “Cal. Int” button the calibration starts, whereby the instrument assigns the actual measured pulse frequency to speed “2000” and outputs the actual engine RPM to the LCD display accordingly.
External calibration (option): A laser RPM detector “RPM-LASER-CAL” is needed for this option. A reflector tag is mounted on the crankshaft and scanned by laser beam. The TTL output of the laser instrument is connected via cable to the “Ext. Cal In.” input of the RPM-8000-PRO and delivers one pulse per revolution. By comparing the pulse sequence with the measured pulse frequency, the instrument calculates automatically the relationship between ripple balance and speed. Identical displays on laser and RPM-8000-PRO signal successful calibration (also shown by LEDs).
The measuring range of the analog output can be varied on demand via jumpers (e.g. to 0.5, 1.0 or 2.0 Volt per 1000 rev/min). The TTL output can also be scaled with a frequency factor of 1:8 or 1:16 – and therefore adapted to the resolution ability of attached measuring systems. For testing purposes, the number of pulses per revolution is indicated by pressing the yellow TTL button, independently of the digital output divider (TTL Div). The instrument saves the calculated calibration data in non-volatile memory until the next calibration.