How to measure low velocity

[Dave Garnett]

The first thing you need to do is establish wether the sensor will actually give a valid reading at these low levels - the specs do not give any values for 'hysteresis' which might be an issue.

So, set up an op amp with some gain (eg x 20) and look at the amplified readings - don't worry about the reduced range at this point. This should show you if the sensor actually registers the motion. [Watch out for noise and drift - alternate readings for stationary, clockwise and counterclock] You could do all this with a quality voltage meter instead, of course.

If the sensor actually works at these levels, then you could implement a dual channel readback - in parallel with your normal +- 150 deg channel, add a channel with some gain centered about the null position. In operation this channel will saturate at high turn rates, but you have the readings from the other channel. Of course you will need a strategy to select which reading to use, as well as thinking about compensating for zero offset and drift.

good luck !

[nobtiba]

Some body tell me to change the ADC with higher resolution e.g 12 bits or 14 bits...This may help me to know if having a small change in input voltage from zero point voltage which means having a very slow rotation.
But I wonder if anybody use 10 bit ADC and still measure angular exactly.My purpose is that if the system (with ADXRS on it) rotate, I have to control it to turn back to the starting point exactly.
Is changing ADC is a good idea, because it's expensive to have high resolution ADC and waste time to re write program for another one.

[Chris Hibner]

I don't think you should have a problem with the 10-bit A/D.

How do you define "exactly" to the starting position? What is your measurement tool to determine how close to the starting position you came? What is the resolution of this tool? You will not need to be more precise than you can measure with an independent tool.

[nobtiba]

Let take an imagine experiment: if you have a laser pointer mounted on a rotation board. At starting point, the laser pointer lights a point on the wall (or any target). Mark this point on the wall. Now turn an alpha angle. All I have to do is turn the table back so that the pointer lights exactly in the marked point on the wall.
In fact, robot or airplane also have the same mission when they need to keep straight direction. So how they to do that with this ADXRS150 gyro ?

[Chris Hibner]

Originally Posted by nobtiba

Let take an imagine experiment: if you have a laser pointer mounted on a rotation board. At starting point, the laser pointer lights a point on the wall (or any target). Mark this point on the wall. Now turn an alpha angle. All I have to do is turn the table back so that the pointer lights exactly in the marked point on the wall.
In fact, robot or airplane also have the same mission when they need to keep straight direction. So how they to do that with this ADXRS150 gyro ?

First, about the laser: You should have no trouble getting the laser pointer back to your point using the ADXR150, a 10-bit A/D, and a PID control loop. We have been able to have great accuracy with the ADXR300 with a 10-bit A/D. The key is to have a fast-enough sample rate (< 1 ms).


2nd, about the airplane:

Airplanes do not use solid-state angular rate sensors - they use mechanical gyroscopes with sensors to measure the angle of the gymbals. These gyros have anti-precession mechanisms built in, but in the case of precession, there is a pilot "trim" knob in which the pilot can remove the precession of the gyro.

Also, airplanes will not use a gyro when precision navigation is required. They use a navigation system to provide the heading input (and therefore the bank input) to the autopilot. The navigation system tells the auto-pilot if it needs to turn right or left and how much to turn. The auto-pilot then banks accordingly. For enroute navigation, the autopilot can use GPS (if the airplane is equipped) or VOR inputs. For approaches, the autopilot can switch over to an ILS localizer or it can stick with the GPS if the GPS is approach certified.

So, to answer the airplane question: if the airplane needs to hit a precise point from far away, it will use ground or satellite navigation, not a gyro. (However, before the days of GPS, you could use inertial navigation systems (gyros and accelerometers) to get across the ocean until you could pick up ground-based navigation systems.)

[nobtiba]

Originally Posted by Chris Hibner

First, about the laser: You should have no trouble getting the laser pointer back to your point using the ADXR150, a 10-bit A/D, and a PID control loop. We have been able to have great accuracy with the ADXR300 with a 10-bit A/D. The key is to have a fast-enough sample rate (< 1 ms).

To have fast enough sample rate, what I have to change ? And how to get the pointer back to the point? Is this using angular value? If this is, so how to take the integration ?
Can you tell me more detail, please !

[nobtiba]

Now I'm using an op amp integrator (integration circuit using LM324) to integrate angular rate. The output of the integrator (angular, in theory) is fed into ADC0 channel.
When I fed square wave into input, the out put is triangular. That's seems good. But when I fed the rate out (the output of gyro) into input, the out put observed in oscilloscope is zero when gyro not moved. When gyro move, the out put observed in oscilloscope sometime increase suddenly.
The capacitor and resistor is 0.001uF and 100kOhm (reference: http://www.physics.ucdavis.edu/Class.../Lab02_rev.pdf)
Will this integration circuit is suit for my purpose?