How to measure low velocity
 

ADXRS150's resolution is 0.004 degree/s but when I rotate the table by hand slow enough (still can observe by eyes) the gyro seems not to reaction. So how can I do?

Re: How to measure low velocity
 

IF your trying to integrate and track a heading this is a problem. Another solution may be to use a digital compass. They have their problems too. They are slow. < 100 samples per second and can be influenced by hard and soft iron fields. Honeywell and PNI are some sources.

Re: How to measure low velocity
 

If I remember correctly (someone feel free to correct me if I'm wrong), even though it's actual resolution is that small... the values you get on the IFI controller will not be that accurate.

The Analog Inputs work from 0 to 1023. This covers a 5 volt range. Therefore, 1024 possible values / 5 volts = ~205 values per volt. I believe with the actual ADXRS150, it only works with + 1.5 volts around the 2.5 volt center. So a positive 1.5 volt means 150 degree/second rate.

So, that your 205 values per volt * 1.5 volts(at 150 degrees) ~= 308 values over the center (511) of the range. Now, (150 degrees / 308 values) = ~0.49 degrees/sec rate. So, by using the IFI controller, your accuracy is 0.49 degrees/sec. You won't see any change until you get it going at that rate.

I hope this makes sense, and I haven't gotten much sleep... so if I just completly messed up those calculations or put bad values or something, someone please correct me.

Re: How to measure low velocity
 

How are you measuring the signal? Are you using and oscilloscope, or are you using the FIRST controller to integrate a heading?

If you are using the FIRST controller, the resolution is less than said. The ADXRS150 measures approximately +/-150 deg/s, which means it measures a total range of 300 deg/s. The FRC has a 10-bit A/D which means you have 1024 A/D counts. Therefore, your resolution is 300/1024 = 0.293 deg/s.

Re: How to measure low velocity
 

Al this sounds about right.... To combat this, what's the feasibility of fabricating/buying a fairly precise amp to spread the voltage range to 0-5V? Possibly greater if you're unlikely to be turning faster than 150deg/s.

Re: How to measure low velocity
 

I don't know about the amp part... but I do know that you probably want to keep it at 150deg/s. Yea, in a normal turn you may never go over 80deg/s and you'll be fine... but say, for example, a robot hits you and it jolts quickly. You now are off by a few degrees if you use a smaller range cause it went over your maximum view. Then with movements over a few feet you can definitly be far enough off from whatever task you are trying to accomplish that it isn't performed properly. However, I guess this can vary depending on how easily the robot is spun. Amplifying it to go from 0-5v could be useful though, but I think in nearly all cases you don't want to make the spectrum wider and have to worry about a jolt turning you further than the sensor measures.

Re: How to measure low velocity
 

Since the original poster isn't on a team, I'm unsure how much help our advice is going to be anyways. The 10-bit A/D resolution might not apply to him. At any rate, now that I think about it, 150 deg/s might be a bit slow for our robots turning at full speed. Though if your tracking only matters in autonomous, then you might not have to worry about it and could expand the range further and restrict the turning speed.

Other options more complicated than an op-amp, in order of increasing difficulty would be an off-board 16-bit A/D tied to a serial driver and clocking circuitry to send acceleration readings through the TTL serial port, or an entire off-board processor with better A/Ds to keep track of the heading.

Re: How to measure low velocity
 

I'm using ADC 10bit of ATMEGA8535. The resolution is 1lsb = 1/1023 corresponding to (1/1023)*5 V = 0.0049V. ADXRS150's sensitivity is 12.5 mV/deg/s so 0.0049V means 0.0049/0.0125=0.392 deg/s =141.12 deg/hour.
That's means if gyro can measure all angular rate from -150 deg/s to +150 deg/s , my ADC can detect the rate at 141.12 deg/hour.
But when I rotate the table faster than that (It takes only about 5 minutes/cycle) the value I get is constant: 480. That is the value at zero point. It shows that ADXRS150 can't detect this slow rate. How to do with this prob?

Re: How to measure low velocity
 

0.392 degrees per second is 1411 degrees per hour. That's a bit more than 15 minutes per revolution, so your 5 minutes per revolution theoretically should be detected as about 2 or 3 counts.

Looking at it another way, 5 minutes per revolution is a little bit more than 1 degree per second. That's a little bit more than 12.5 millivolts, which again should be read as about 2 or 3 counts.

The people responding in this discussion forum are generally concerned with time frames measured in tens of seconds. I don't think many of us will have given a lot of thought to trying to measure angular rate over more than a minute or two.

Re: How to measure low velocity
 

Id be really surprized if you can turn your robot so slowly that the rate sensor reads zero.

Turning a lazy susan by hand on a table top... ok... how does that relate to anything your robot will do?

Re: How to measure low velocity
 

Ken-

The posters lack of a team number, and location in Vietnam leads me to belive he isn't using the gyro on a robot, FIRST or otherwise.

At least I don't think there are any teams out of Vietnam.

Maybe with some more information on the application, we could be of more help, nobtiba?

-Andy A.

Re: How to measure low velocity
 

If you need to measure a small rotational velocity then maybe the ADXRS401 is a more suitable part. Its measurement range is +-75 degrees per second.

Re: How to measure low velocity
 

That is generally the only difference when you have a manufacturer that sells similar sensors with different output ranges: the measurement cell is usually the same and they just change an amplifier stage on their ASIC.

For a one-off application, this would be an okay thing to do if you need to really optimize your range and you can't buy an off-the-shelf part to fit your needs. For a mass production application, you wouldn't want to do this - not only from a cost standpoint, but the amplifier adds another error source and then it will probably become impossible to meet your performance objectives with the error stack up. For a one-off you're going to calibrate anyway so that shouldn't be an issue.

Re: How to measure low velocity
 

Oh, many universities in VietNam won many prizes in ROBOCON (robot contest of ASIA). I'm in Coleage of Technology (Ha Noi) team. If I can use gyro to measure angular exactly I can use it for stabilization. My duty is using ADXRS150 to do this.
ADXRS300 or the others only different in upper level (75, 150, 300 deg/s) but what about their bellow level?
I think if the sensor can't detect low rate it will lead to my following problems (in fact now):
1. Can't measure angular exactly
2. With the same real angular but measure very different values in different times
3. The same value positive and negative but measure different values (that means if you turn it clockwise then turn to starting point it never be zero)
So I wonder any body used ADXRS and measure angular exactly enough can tell me what to do?

Re: How to measure low velocity
 

In the datasheet of ADXRS, it says that we can setting bandwidth by changing the external capacitors: f(out)=1/(2*pi*R(out)*C(out)). Do you think it can effect gyro's capacity of detecting low rate? If not, what is the function of setting the bandwidth?

Re: How to measure low velocity
 

This capacitor should not change the ability of the sensor to detect a low rate.

The purpose of this capacitor is to change the corner frequency of the low-pass filter on the sensor. You want to have a low enough corner frequency to avoid aliasing of your signal when you digitize the signal. Too much filtering and there will be a problem with a phase lag in the signal.

If you're not sure how to select the capacitor, search the internet on "nyquist frequency".

Re: How to measure low velocity
 

I believe if you look through the data sheet carefully, you will find that the capacitor and bandwidth is an attempt to quiet down some noisey data. By bandwidth limiting the high frequency energy, you should get a more accurate output.
It seems that I remember someone (a thread perhaps) that discussed rapidly taking samples and averaging over 10 or fifteen ( or more) samples to look for trends. By knowing where you were before you may be able to find out where you are now.

Re: How to measure low velocity
 

This is the whitepaper you are thinking of: http://www.chiefdelphi.com/forums/pa...le&paperid=234

Re: How to measure low velocity
 

Thank you very much for your very useful information. In this pdf file it says that: "if your're integrating, you don't need to filter-the effect of improved resolution will be taken care of by the integration". Now I need integrate to change angular rate to angular, so I needn't average the samples?
In Kevin's code (gyro.h, gyro.c) the time lapse between samples decided by programmer with a timer. If this interval (time) shorter, is the angular get more exactly?

Re: How to measure low velocity
 

You won't need to average before the integral because the integral acts like a very large average.

If the sample period (interval between samples) is shorter, the integral will be more exact. However, the sample period must be consistent from one sample to another.

Re: How to measure low velocity
 

What I meant was another routine looking to see if there was movement and check to see if it was valid data before the integration process.

Re: How to measure low velocity
 

So I think all I have to do now are the following steps:
1. Programming the ADC in uC to get samples in free running mode
2. Since the ADC is 10 bits but a data frame to transmit to pc is 8 bits, then I have to devide it to a frame of 3 bits and another of 7bits, then send them to pc
3.In pc, my program written in turbo C receive these bytes through COM1, merge them to get 10 bits value (as origin value after ADC).Then change them to voltage and then to corresponding angular rate.(we call rate value)
4.Also in this program (in pc) I use the timer of uC in PC to count the interval between two samples.
5.Calculate: angular=angular +[(rate value i)+(rate value (i+1))]*(time interval), and loop this routine
<in the beginning angular =0>
I'm sorry if some of these steps seem to suit in programming forum, but I think if I say all things in detail, it may help you to see where is my problems.
Especialy, I don't know how to do in step 4

Re: How to measure low velocity
 

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 !

Re: How to measure low velocity
 

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.

Re: How to measure low velocity
 

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.

Re: How to measure low velocity
 

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 ?

Re: How to measure low velocity
 

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.)

Re: How to measure low velocity
 

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 !

Re: How to measure low velocity
 

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?