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looking for a gyroscope

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bigfarmerdave

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I'm looking for a solid state gyroscope with an analog output to measure tilt. Looking online, I see lots of accelerometers, but not as many gyros. Of the gyros I'm finding, they are yaw gyros and I'm more interested in roll and pitch. Does anyone have any recommendations?
 

Sceadwian

Banned
The only difference between a yaw gyro and a roll/pitch gyro is it's orientation. Simply rotate the plane of detection to whatever axis you need.
 
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dknguyen

Well-Known Member
Most Helpful Member
The only difference between a yaw gyro and a roll/pitch gyro is it's orientation. Simply rotate the plane of detection to whatever axis you need.
It's a hassle to make two little vertical PCBs mounted to a main horizontal one. He wants to measure tilt so needs two axis so he can't just mount his PCB vertically.

MEMS Gyro | Gyroscope | Motion Plus | Processing - InvenSense
produces the only dual-axis single chip gyros I know of. The axis measured are pitch and roll (sitting on a horizontal PCB).

www.sparkfun.com carries them and so does www.digikey I believe.
SparkFun Electronics - Dual Axis Gyro - IDG1215
SparkFun Electronics - Dual Axis Gyro - IDG500
There are a number of other angular rate ranges as well, but Sparkfun only carries the most useful.

What are you using this for? If it's a static application a 3-axis accelerometer would be much better. You just produce a single unit vector from the acceleration provided by each axis and voila, there's your drift-free vector that points in the direction of gravity. None of this integration and bias drift nonsense that you get with gyros.

Either way...you do know what vectors and unit vectors are right? You are going to need to know that math to do this. It's dead easy math though. I'm just saying because I mentioned unit vectors and if you don't know what they are it might scare you off from something that's really trivial.
 
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bigfarmerdave

New Member
Actually this is an agricultural based application. I want to make a circuit that controls pull type equipment on rolling terrain. Example-> If a tractor is pulling a sprayer on a side hill, the tractor must continue to drive between the corn rows but the sprayer that it is pulling is drifting down the hillhe little corn rows. If the driver were to correct by driving the tractor further up the hill, then the tractor would be running over the corn rows in order to keep the sprayer tires between the rows. If I could sense the slope that I'm on, feed it into a micro, and PWM out a correlated frequency to control some FETS, which open or close a solenoid that control oil flow to a horizontal cylinder that would move the sprayer left or right to compensate for the side hill. Is that just absolutely as clear as mud? I looked at the data sheets of the two sparkfun gyros that you suggested and it appears that I could just feed the X and Y outputs to an A/D on a micro to find my angle couldn't I?
 

dknguyen

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You are talking about moving laterally on a slope (not up the slope, but along it so you aren't changing your height? If that is the case, wouldn't a yaw gyro work just fine since you just want to keep the sprayer pointed on the same direction regardless of which way the tractor was pointed? If it's slipping down the hill about the joint between tractor and sprayer, it would be rotating about that joint which changes the yaw.

Sounds like a cross-wind type of problem for a plane.

Yes, you would need to use an MCU and an ADC. It would have been the same with an accelerometer too.
 
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DirtyLude

Well-Known Member
dknguyen kinda touched on this, but MEM's Gyro's are tricky because of the drift. Just sitting still, the angle will slowly change on it's own without actually moving. You need to have something to correct for this drift every once in a while. Usually an accelerometer is used in conjunction in order to correct the gyro data every once in a while.
 

dknguyen

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Most Helpful Member
Yeah, how fast is this thing moving? It's not going to be accelerating very much right? Nor can the hydraulics correct the sprayer at lightning fast rates. A single 3-axis accelerometer that is low pass filtered by the right amount (to ignore vibration and what there acceleration is), might prove to be more effective. It is certainly simpler and more straightforward than fusing acclerometer and gyro readings together (that's the kind of stuff you do for planes!). You'll want to avoid the use of gyros if possible due to their integration error and bias driftwhich is difficult to correct for. They are forced to use them in planes because nothing else works. On the ground with slow moving vehicles, you have more options.

YOu could also do the old fashion method...two weight gimbals that only rotate in one axis. Put them 90 degrees to each other and read their positions with an encoder. The mechanical damping should be enough to weed out acceleration from the tractor. EVen if it's not you can still lowpass filter it just like an accelerometer.
 
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3v0

Coop Build Coordinator
Forum Supporter
The sprayer must maintain the wheels to within a few inches* left or right of the optimum path. You can expect changes in orientation or elevation at any time.

The only thing you know is that the sprayer was tracking correctly on flat ground. A field on a hill side is going to be a problem, no flat ground.

You can not depend on the angle between the tractor and the sprayer because as the hill side gets steeper the tractor will not be point in the direction of the corn rows. It will point uphill such that it travels up the hill exactly as much as it is sliding down the hill. They cancel out.**

I like the idea of using a vision system to know where the corn rows are but it would increase the complexity.

Without a vision system you may have provide an operator interface on the tractor to set initial alignment.

Is the sprayer two or four wheeled?

* allowable drift will depend on the crop spacing.
** unless you use a tractor with steering on both axles and turn both sets of wheels uphill. This is known as crab steering.
 

bigfarmerdave

New Member
here is a bit more info

Ok guys, Some comments and questions thus far. First, I should have mentioned that the tractors all have RTK GPS Autopilot systems so they steer themselves down the center of the rows very well. The sprayer however does not have any sort of GPS steering to it and would be economically unfeasible to do add that feature.

The tractor is not articulating in the middle like the big 4 wheel drives but rather steers similar to your car, with tie rods and each front wheel pivots left/right. This sprayer is pulled behind the tractor between 8-11mph. Terrain can be bumpy at times and we are on 30" row spacing. Our poor mans solution has been to mount a switch in the cab that controls a laterally mounted cylinder that moves the sprayer hitch left and right. When you hold the switch to the left, it puts power to the right solenoid and pushes the hitch to the right. When you hold the switch to the right, it puts power to the left solenoid and pushes the hitch to the right. You are correct, hydraulics are not lightning fast but have been sufficient to handle this task. I'm just looking for a more automated way to do this.

I could go the old school route and do this a variety of ways including dknguyens suggestion but this would be more fun! :)

dknguyen, I understand what your saying about using a yaw gyro to measure the angle between the tractor and sprayer at the pivot point where the two meet, however this would be very difficult and fairly inconsistent as that angle would change as the tractor tries to aim up hill to compensate for sliding down as well.

I'm getting the message that gyros have a certain amount of drift to them, meaning that while they sit perfectly still, the angle that they think they're measuring can actually increase or decrease? You'd have to zero it out periodically when you know your on a flat surface? That doesn't sound like any fun to deal with. I guess I don't understand how I could use accelerometers to measure the side hill slope that I'm on though? Can you explain please?
 

dknguyen

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Most Helpful Member
3-axis accelerometers give the acceleration measured in 3 directions. THis gives you the vector of gravity. VEctors, by definition have a direction and a magnitude. YOu are only interested in the direction. By lowpass filtering it you can make it dampen most of the vehicle's acceleration and vibration out of the reading leaving only gravity. (Notice I am not paying attention to the magnitude since there are scaling errors involved unless you calibrate the accelerometer).

I, J, and K are like 3 separate variables representing a direction.
http://en.wikipedia.org/wiki/File:3D_Vector.svg
http://en.wikipedia.org/wiki/Euclidean_vector
 
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Sceadwian

Banned
Accelerometers in this type of situations are going to be virtually useless. Drift occurs no matter how much you prevent it unless you have an absolute position reference, which if you have then you don't need the accelerometers in the first place. The only thing they'd be reliable for is tilt (slope) sensing. Why not interface to a GPS directly, and rig that up to your steering system? Almost all over the counter GPS systems have serial output ports.
 

user_88

Member
Error Driven Servomotor

If the plant rows are a constant distance apart, you might be able to establish an error measurement with respect to the centerline.

This centerline error signal could be used as the input to a hydraulic servomotor which would control left and right motion of the hitch point, and steer the sprayer chassis as it followed the tractor.
... A rack and pinion arrangement might be appropriate for controlling the hitch motion.

If you have a picture or sketch of the actual tractor and sprayer, it might be possible to determine what sort of sensor and guidance arrangement would work in this application.

Even if it is difficult to obtain an error value for the row centerline distance, it may be possible to measure the angular deviation between the tractor and sprayer chassis centerlines.
 
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