# Issue with acceleration plot when accelerometer is mounted inside a running wheel

I am using MEMS 3 axis accelerometer to measure the acceleration along all the 3 axis when the accelerometer is mounted inside the vehicle tyre. The testing was carried out for different speeds.

When the accelerometer mounted wheel is in operation(runned assuming the wheel is balanced),the graphs plotted extracting the data from the accelerometer at different speed are as shown below. ]2]

Here the issue is with acceleration along the z axis. As the speed is increased the z graph is lifted upwards and when it reaches 2g the graph is clipped off. As per prediction the acceleration along both x and z axis has to remain same with some phase shift (this is because when wheel is rotating there will be variations only along 2 axis, in this case it is x and z. the variation along y axis shows the vibration along lateral axis or y axis only). Can anyone help me understand why is this happening? Kindly suggest if my prediction is wrong too.

• Why what is happening? It's not very clear what part of this you need help with. As far as I can tell you've been explaining the situation and don't mention any specific issue besides something vague about the z-axis. – JMac Aug 30 '17 at 10:36
• As the speed increases the z graph is lifted upside. This is the issue. – user141909 Aug 30 '17 at 10:57
• As you can see in the graphs attached, the acceleration along z axis ( the green one) ranges from about 0.25 to 2 in second graph, the first graph is plotted from the test where the speed is gradually increased. As the increase in speed the z vanishes at 2g. that is when the acceleration tends to increase beyond 2g, the graph stops there. but my question is why this z increasing with the speed. – user141909 Aug 30 '17 at 11:09
• How is the accelerometer mounted? Does it rotate with the wheel? In that case the 'z' acceleration would actually be radial acceleration, which would explain why it is of constant sign. – Eddy Aug 30 '17 at 11:25
• yes. accelerometer is mounted in such a way that z represents radial acceleration. how would you relate this to the issue? – user141909 Aug 30 '17 at 12:18

I would assume that the z axis is centripetal/radial acceleration. The equation is $$a = \frac{v^2}{r}$$ this should remain fairly constant at lower speeds and as seen does not really change. If your accelerometer is positioned so that the radius changes, you may experience bumps in the z axis (as seen).