Accelerometers, Gyroscopes and IMUs

What does each do, and what is the diference between them ?

 

The first measures acceleration, the second measures rotation about one or more axis, the third combines the first two.

 

Thanks,

This sensor looks like it has everything integrated: http://www.robotshop.ca/home/produc...s-imu/spark-fun-6dof-imu-sensor-adxrs300.html

I saw that I have to use MAX232 to comunicate with it, using the RS232 port, but what do I do if I want more than one sensor using the RS232 port ? what do i do then ?

 

Set up another port and MAX232. Remember that we have absolutely no idea about your application or the kind of controller/processor you plan on using. Makes it hard to give sensible answers.

 

What I mean is that the computer has only one RS232 port/jack...

If I use MAX232, that will mean that there is no other RS232 to connect another sensor.

Lets say I want to connect 2 of http://www.robotshop.ca/home/produc...s-imu/spark-fun-6dof-imu-sensor-adxrs300.html to the computer, using the MAX232. How would I connect 2 MAX232 to one RS232 port ?

 

perhaps some sort of bus network, like CAN ??

 

You likely have multiple USB ports. Use a USB to RS232 converter. Note that you may need to try a couple to find one that works the way you need it. Some of them are a bit flaky. I would buy one at a local electronics store with the option to return it if it doesn't work.

 

(back to Gyroscopes...)

So what is the diference between these:

Dual Axis 500°/s Gyroscope Combo
Triple Axis 300°/s Gyroscope Combo
Triple Axis 150°/s Gyroscope Combo
Triple Axis 75°/s Gyroscope Combo

I know what the Axis is... x, y, z, but what is " °/s " ? Is it the precision ? or speed ?

 

That is degrees per second. They are rate of change limited. Those specs are for the maximum rate of change they can accurately respond to.

 

That is basically the speed, I guess...

So, let's say if I use this sensor "Triple Axis 300°/s Gyroscope Combo" and the object using this sensor spins and makes 600° rotation in one second, since it's doing 300°/s, then it means that it will report every 2° something like this:
0°
2°
4°
6°
....
....
600°

All that it will report in one second, Right ?

I guess I should just buy the thing, and see for myself...

 

I suggest that you not exceed the °/S rating of the device. Along with the virtual certainty that it's output will not be accurate, there is also a real possibility that the device may be damaged or destroyed.

 

What I mean is that the computer has only one RS232 port/jack... If I use MAX232, that will mean that there is no other RS232 to connect another sensor. Lets say I want to connect 2 of http://www.robotshop.ca/home/produc...s-imu/spark-fun-6dof-imu-sensor-adxrs300.html to the computer, using the MAX232. How would I connect 2 MAX232 to one RS232 port ?

I'd suggest considering a RS232 <=> RS485 converter. It allows you to connect many serial devices to one COM port.
See: http://www.rs485.com/pfaq.html for more details.
The software for dealing with this is more complicated. How is your program going to distinguish the data coming in from the two different IMU's on the same COM port? Probably easier to deal with 2 COM ports - 1 for each sensor.

 

That is basically the speed, I guess...

So, let's say if I use this sensor "Triple Axis 300°/s Gyroscope Combo" and the object using this sensor spins and makes 600° rotation in one second, since it's doing 300°/s, then it means that it will report every 2° something like this:
0°
2°
4°
6°
....
....
600°

All that it will report in one second, Right ?

I guess I should just buy the thing, and see for myself...

Nope, if the device rotates at a constant rate of the 600 degrees per second, it will constantly try to report 600 degrees per second output no matter how many times you read it per second. However, it will likely only show about 300 degrees per second since it is railed.

If you go to analog devices and read the datasheet for their gyro rate sensors, it is all explained. Basically, a gyro rate sensor gives out an analog voltage corresponding to the rotation rate in degrees per second. The device you are buying samples this analog voltage and converts it to a number.

If you have a 300 degree/second device, then the analog voltage reaches its voltage rail at that rate, so any rate above that simply rails. The device is not damaged, but the output is railed. The exact same sensor is used on the 600 degree/second device, but an external resistor is added into the circuit to turn the gain down, so it will rail at a larger rotation rate. However, this means that you get less resolution as well. It is a typical resolution vs. dynamic range tradeoff.

 

Thank you very much, now I understand this much better...

But to make sure I did understand it:

Basically what I have to do is think how much my robot will rotate. If it does not rotate too much, then I should get a low degree/second Gyroscope, because that will give me more precision, and if it rotates a lot, then I should get a high degree/second Gyroscope, because that will not rail off...

Knowing how my robot will be, I don't think it will rotate a lot, and I do want a lot of precision when it does rotate, so considering the new info, I will get a low degree/second Gyroscope, like this one:
http://www.robotshop.ca/home/produc...s-imu/spark-fun-6dof-imu-sensor-adxrs401.html

 

PS...

Does the Accelerometer work the same way ? i.e. Lower Gs Accelerometer has more precision than higher Gs Accelerometer ?

In that case, I should not get a 6G Accelerometer...

 

You need to consider something else here. If you are trying to detect exactly which direction your robot is pointing, you will have to add up (integrate) the gyro output over time. Unfortunately, this will cause drift error due to uncorrected errors. These cannot be completely removed because part of it is due to random noise from the sensors. So, you need an dead-reckoning sensor type measurement for rotation as well. Generally, these small IMUs use electronic compasses. Since these are usually slow, the gyrosensor handles the quick and accurate rotation sense, and its error is corrected by the less frequent compass reading.

 

PS...

Does the Accelerometer work the same way ? i.e. Lower Gs Accelerometer has more precision than higher Gs Accelerometer ?

In that case, I should not get a 6G Accelerometer...

Any sensor will generally work the same. The only way to get more is to have adjustable settings for different ranges. If you notice your bench power supply or scope clicking sometimes when you switch the range, that is a relay that is switching in a different circuit that gives the best performance for that range.