RPM-8000
Contact and Sensor-less Automotive
RPM Measuremen

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.
The RPM-8000 transforms the impulses to a TTL signal that can be fed directly into a frequency or pulse counter. Alternatively, via an internal jumper, a synchronous signal with 128x frequency can be generated and used for a counter input.
In parallel, the analogue output produces a voltage - proportional - to - frequency signal in the range of 0,5 to 10V.

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 practice the generated alternator ripple also includes electrical noise and disturbances from other electrical devices and loads connected to the vehicle supply. In petrol engine vehicles this is mainly due to the ignition system and is relatively straightforward to eliminate.

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.

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.

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.

Using the adjustment potentiometer in the calibration mode the analog output is set for 4,711V. The RPM-8000 unit is thus set for this vehicle type with a calibration of 1000rpm = 1Volt on the analog output.Generally, calibration via comparative measurement will rarely be used simply on the ground that sensors with higher, or, at least similar accuracy are just to difficult to apply. Consider inductive pickup devices, which require for example a 50-toothed wheel to be mounted on the engine crankshaft. Tacho generators provide relatively good accuracy but produce high levels of ripple on the output signals.
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

• Start engine
• Plug RPM-8000 adapter into cigarette lighter socket
• Wait for synchronisation (max. 1s)
• Measure

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:

• Inside quality assurance in automotive production where acoustic measurements linked to engine RPM have to be applied. With ever decreasing test time available at the end of production lines sensor installation is too time consuming and impractical.

• To record vehicle speed in standard "Pass by noise" tests, in which a fixed and pre-selected gear is used. The calibration in this case, can be for example based on the engine RPM – Gear – Final drive – Wheel circumference ratios - to obtain vehicle speed or via an external Peiseler wheel.

• On vehicle proving grounds, where drivers are required to test a wide range of vehicles, the RPM-8000 is useful for fast and "fuss free" installation.

• Various applications can also be found in car, truck, bus motorcycle and development

Technical Data - RPM8000-V4