Application Stories: Product Testing

Internal combustion engines need continuous, long-term, testing to ensure their safety and suitability. Round-the-clock monitoring, with automatic shut down should a danger level be crossed, is a common task for an unattended data acquisition system.

Durability and Performance Testing

The test procedure runs the engine slowly for part of every hour, and at full speed for the rest of the hour. The timings simulate the typical use of the engine: a tractor engine might be idly ticking over for 55 minutes and at full speed for 5 minutes for example. This is called the duty cycle and measures durability (tickover) and performance (full speed).

Making the Measurements

During the length of the test, which is often several months, a range of measurements are taken. These include fuel consumption, fuel temperature, top hose temperature, oil pressure, torque, exhaust back pressure, engine speed and smoke test.

Transducers convert signals from the engine into a form the data acquisition hardware can understand - normally voltage, sometimes current. For example, to measure pressure, an air or oil hose runs 3 or 4 metres from the engine to a pressure transducer. The transducer has a diaphragm that converts the pressure into an electrical signal we can measure.

Temperatures are often measured with K-type thermocouples. These are robust and cheap, and sufficiently accurate for the task.

Speed is measured in revolutions per minute by a tachometer. This produces either a voltage output, or a pulse output. You need a counter interface module to handle the pulse output.

You wire the transducers to the data acquisition interface. This is often remote from the computer and connected to it over an RS485 cable. RS485 allows long distances between the test site and the PC (around 1000 metres) - protecting the PC from unsuitable environments. It also lets you distribute several data acquisition devices around the site, connecting them all to one cable and computer. It is quite a slow communication link (handling around 35 samples per second), but for product testing such as this high speeds are not needed.

Software Control

How often measurements are taken from the engine, and what happens if a measurement exceeds its danger level, is controlled by Windmill software. If, say, 6 engines are being tested by one PC, the software typically logs around 200 measurements every minute. Once an hour the software tells the engines to increase speed to maximum revolutions. It does this by sending control signals down analogue output channels. After a specified time it tells the engine to return to tickover speed. The software provides permanent records of how key parameters have changed, highlighting any long term drifts. Importantly, the software shuts the engine down should, say, oil pressure be lost - saving the engine from irrevocable damage.

Advantages of a Computerised System

A computerised data acquisition system brings considerable advantages to product testing. As the tests run continually it saves a great deal of time. It also relieves people from monitoring the test, saving many months of labour. It is consistently accurate - all readings taken at the same time with no transcription errors. It can automatically generate comprehensive reports, on any computer on the network that you choose.