RPM-8000 |
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principle. By connecting to an automotive cigarette lighter adapter the RPM measurement unit RPM-8000 generates both an analogue and a TTL digital pulse train signal, proportional to engine RPM. By sensing the ripple on the DC connection, the alternator RPM and, via the linear scaling factor from the alternator drive belt, the engine RPM is derived.
From an 18 pole alternator for example, 18 sine wave signals are generated that are spaced about 200 out of phase. The alternator commutator unit performs the function of switching the highest instantaneous voltage signal to its output. The vehicle supply therefore comprises signal envelope (see below) which takes the form of a DC signal level of about 12V superimposed with an AC signal of about 50mV.
By careful filtering and amplification of the signal envelope, the
impulse form and thereby the frequency of the alternator can be sensed, and "as per
design" this is linear to its RPM. Important notes The operating principle is based on the condition that the vehicle
DC supply is switched stable from the lead acid battery to the alternator. Therefore RPM
measurement with the RPM-8000 unit is not possible during start up operations! In Diesel engine vehicles however the main noise source is from the electronic injection system and, due to the spectral content of the injection signals, is very difficult to decouple from the relatively small signal of interest from the alternator. This technical background helps to explain the different accuracies of the RPM signal which are achieved: approx. 0,5% for petrol and approx. 1% for Diesel engines. Generally it is important to note that switching on constant load accessories such as vehicles lights, windscreen heaters etc. improves the signal to noise ratio and the measurement. This does not apply however to non-constant load devices or electrical noise generators such as electric fan motors and newer technology gas discharge lamps ! Calibration / setup The RPM-8000 generates analogue and digital output signals which are proportional to the engine RPM over the entire measurement range. The calibration is limited simply to the definition of a few vehicle dependent constants which provide the relationship between the alternator frequency and engine RPM. For digital measurements this simply includes one constant multiplier, for the analogue signal the output is simply adjusted to a known reference with a trim potentiometer.As a coarse, general rule, an engine speed of @ 1000 RPM will produce an alternator frequency of @ 1kHZ ± 500Hz. The above range of alternator frequency comes from the lower engine speeds in Diesel vehicles with equivalent alternator power ratings producing alternator frequencies that are somewhat higher than in petrol engined vehicles for a given engine speed. The exact relationship for the vehicle engine constants can be set from the vehicle technical data or by comparative measurement. The actual engine RPM n has linear relationship to alternator frequency f, and is defined by dividing f by the conversion factor g (gearing) between the engine and alternator, and, the number of poles p on the alternator, giving: n = f / ( g * p * 2 ) The value for g can be taken from direct measurement of the engine crankshaft to alternator pulley dimensions (e.g. g=2.83) and p is a fixed constant (e.g. p=18) for the number of alternator poles, normally given in the technical data for the alternator.To calibrate the output voltage v a push button is used to activate the calibration mode. In this mode the RPM-8000 internally generates a 4kHz signal derived from a quartz reference. Below is an example of how the internal frequency reference (4kHz), and the parameters g and p are used in the calibration: 1.000V output = 1000rpm of engine.
The displays or digital readouts in the drivers dash panel instruments are only indicators and typically provide an accuracy of around ± 10%. Visual errors also have to be accounted for together with relatively high non-linearity. These error differences are also worth considering on the first test drives, where in most cases a comparison between the RPM-8000 and the dash panel instruments takes place. The RPM-8000 unit ensures linearity via the physical measurement principles used and precision acquisition electronics! Calibration via comparative measurement simply relies on adjustment of the potentiometer, while in operating mode, so the analogue output voltage of the RPM-8000 is checked against the reference value. How to use the RPM-8000 Application areas Generally the RPM-8000 unit is useful in all manner of applications where RPM measurement is required on internal combustion engines. It is equally suited in stationary as well as mobile applications and some examples are:
Technical D ata - RPM8000-V4
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