Measuring with an accelerometer

Introduction

Acceleration measurement is used in many branches of the industry, to check and avoid vibration in mechanical systems or to track moving objects.

To measure acceleration, several different sensors are available, which convert acceleration into an electrical signal, which can be measured with a standard measuring instrument.

Requirements

Accelerometer TP-ACC20

The TiePie engineering Accelerometer TP-ACC20 is the ideal sensor for measuring static and dynamic accelerations in two directions. The Accelerometer TP-ACC20 is designed to work with the Handyscope HS3, Handyscope HS4 and Handyscope HS4 DIFF. By means of two BNC connectors, the Accelerometer TP-ACC20 is connected to two inputs of the measuring instrument. Power is supplied to the Accelerometer TP-ACC20 through the 25 pins D-Sub connector on the back of the measuring instrument. No additional cables or power supplies are required.

This measuring example describes an acceleration measurement performed with the Accelerometer TP-ACC20, connected to a Handyscope HS4, using the Multi Channel software.

Setting up the hardware

First the 25 pin sub-D connector of the Accelerometer TP-ACC20 must be connected to the extension connector on the rear panel of the Handyscope HS4. This connector supplies the required power to the Accelerometer TP-ACC20.

The next step is connecting the signal cables to the inputs of the Handyscope HS4. The two signal cables are color coded, red for the acceleration signal in X direction, yellow for the acceleration signal in Y direction. In this example, the X signal is connected to Ch1, the Y signal to Ch2.

The Accelerometer TP-ACC20 can measure accelerations in two axes. For initial calibration, the Accelerometer TP-ACC20 should be placed so that both axes are in their idle state, parallel to the earth's surface. To achieve this, place the Accelerometer TP-ACC20 on a stable, not moving surface, with the label facing up.

Setting up the software

Input sensitivity

When idle and not moving, the Accelerometer TP-ACC20 has an output offset of approximately 1.4 Volt. The Accelerometer TP-ACC20 has a typical output sensitivity of 420 mV per g acceleration and a range of approximately -2 .. +2 g. This would give a maximum output voltage of almost 2.5 Volt. This makes the 4 Volt range the most suitable input range for a channel. Auto ranging is not convenient when doing acceleration measurements, therefore, switch Auto ranging off, by clicking the Auto ranging button Auto ranging button on the channel toolbar of the required channels.

Calibrating the sensor

When idle and not moving, the Accelerometer TP-ACC20 has an output offset of approximately 1.4 Volt and an output sensitivity of 420 mV per g.

To calibrate the sensor, first place it on a stable, not moving surface, with the label facing up. Use the cursors in the software to measure the offset voltage on both channels.

Acceleration offset measurement

In the screen shot above, the offset value for Ch1 is 1.4374 Volt and for Ch2 is 1.4289 Volt.

TP-ACC20 sensor

To calibrate the sensitivity, a known acceleration has to be applied and the output voltage has to be measured. Since the Accelerometer TP-ACC20 is capable to measure static acceleration, it can measure the earth's gravity, being 1 g.

To measure the earth's gravity, place the sensor on a stable, not moving surface, with the X axis parallel to the earth surface and the Y axis pointing up or downwards, perpendicular to the earth surface. The Y axis, is now experiencing +1 g, measure the voltage on channel 2.

Then rotate the sensor 180 degrees around the X axis, the Y axis is now pointing down, experiencing -1 g. Measure the voltage on Ch2 again.

Repeat this with the Y axis parallel to the earth surface and the X axis pointing up and down and measure the voltages on Ch1.

As a result, six voltages are measured: +1 g, 0 g and -1 g on both axes:

  -1 g 0 g +1 g
X axis 1.0259 V 1.4374 V 1.8448 V
Y axis 1.0382 V 1.4289 V 1.8598 V

The sensitivity for a channel can now be determined with :

sensitivity = (V+1 g - V-1 g) / 2

This results in the following sensitivities: X axis = 409.45 mV/g and Y axis = 410.80 mV/g

Converting the measured values

When displaying the signal of the Accelerometer TP-ACC20 directly, the oscilloscope will display volts, not in g's or in m/s². To display the measured accelerations with unit g or m/s², the measured values need to be converted.

To convert the measured values, a Gain/Offset I/O is used for each channel of the Accelerometer TP-ACC20. Create them by right-clicking on I/O's in the object tree and selecting Gain/Offset.

Since all operations for both channels are similar, this example will only describe the settings for the X axis, Ch1.

Set the gain

As determined before, the Accelerometer TP-ACC20 has an X axis sensitivity of 409.45 mV/g. To display the measured values directly in g's, they have to be multiplied by 1 / 0.40945 = 2.4423. Right-click the X axis Gain/Offset I/O and select Gain -> User defined... There is no need to calculate the proper value for the gain, the Gain/Offset I/O can do this by itself, so simply enter "1 / 0.40945", "1/409.45m" or "1/409m45".

Enter gain

Set the offset

To compensate for the offset of the Accelerometer TP-ACC20, right-click the Gain/Offset I/O and select Input offset -> User defined... Then enter the measured offset value, multiplied by -1. In this example, the offset for the X axis is 1.4374 Volt, so a value of "-1.4374" has to be entered.

Enter offset

Set the unit of measure

Since gravity can be represented as g, the unit of measure should be given as g as well. To change this, right-click the Gain/Offset I/O and select Set Unit... and enter "g".

Enter unit of measure

Set the alias

To simplify recognition of the various signals that can be displayed, it is possible to assign a name (alias) to an I/O. Right-click the Gain/Offset I/O and select Set alias... and enter "X axis".

Enter alias

Connect the I/O

The X axis Gain/Offset I/O is now ready for use. Drag input channel Ch1 of the instrument on the X axis Gain/Offset I/O and then drag the Gain/Offset I/O in the graph. The new graph shows the acceleration in one axis, directly in g. The corresponding live channel of the instrument can now be deleted from the graph. For more information, see "Creating and connecting objects".

Set the output range

To customize the visible range of the converted accelerometer data, right-click the Gain/Offset I/O and select Set output range... to set the visible range to e.g. +4 .. -4 g.

Enter output range

Measuring in m/s²

The software is currently setup to display the measured accelerations in g. To display the measured accelerations in m/s², a few adjustments have to be made in the settings of the Gain/Offset I/O.

The gain has to be set differently. Assuming that 1 g equals 9.81 m/s², the gain of the X axis Gain/Offset I/O has to be set to "9.81 * (1 / 0.40945)" in the above example. The units have to be changed from "g" to "m/s²".

Ready to measure

The TiePie engineering Multi Channel software is now ready to measure both the tilt and the accelerations that the Accelerometer is experiencing.

The measurement shown below, was made with an Accelerometer TP-ACC20, sliding on a horizontal surface and bouncing against an obstacle. The measurement data and settings are saved in a Multi Channel software TPS file:

File name download
Acceleration.tps Disk

This setfile can be opened with both the full version and the demo version of the Multi Channel software.

Acceleration measurement

Measuring speed

When the acceleration of an object is known, as well as the initial speed of the object, it is very easy to determine the speed of the object, using an accelerometer. All one has to do is measure the acceleration in m/s² and integrate the measured data to get the speed in m/s. The Multi Channel software has an Integrate I/O that can be used for this task.

Create an Integrate I/O by right-clicking I/O's in the object tree and select Integrate. The alias of the Integrate I/O can be set to e.g. "X speed". Set the unit to "m/s". Then drag the X axis Gain/Offset I/O onto the Integrate I/O and drag the Integrate I/O on the graph.

The graph now displays the speed of the object, in m/s.

Measuring displacement

When the speed of an object is known, the displacement of that object can be determined very easy by integrating the speed.

Create another Integrate I/O, name it "X position" and set the unit to "m". Now drag the X speed Integrate I/O on the X position Integrate I/O and drag the X position Integrate I/O on the graph.

The graph now displays the relative position, or displacement, of the object, in m. The image below shows a measurement with an Accelerometer TP-ACC20, which was shifted over a table, for about 15 cm.

Acceleration measurement