There's some devils in the details here. How accurate do you need this to be? Do you just need a decent idea of the current speed, or are you going to be trying to use this in order to track the vehicle's position over time? What considerations are there when the vehicle is turning or maneuvering such that one part of the vehicle is traveling at potentially a considerably different speed that another?

If you are going to use the ground, what does the ground look like? How uniform and consistent is it? How smooth? How reflective?

How far off the ground would your sensor be located? How much might that distance change, either as the vehicle moves (rocks, bounces, whatever) or as the ground varies (small up/down features or objects/holes)?

Fair enough, I want the speed measurement to be fairly accurate, to possibly accuracy of +/-0.1 m/s. The device is hanging over above plants which is estimated around 2 meters. The ground is uneven due to plants being underneath. Turning and maneuvering is not important imo since if i know the movement speed I can calculate the rest. Feel like I'm getting to a point where I might aswell just add a small motor with a constant speed and a on and off switch.

 

The moving object is a distribution system for fertilizer in my greenhouse. I want to input the amout that has to be distributed per square meter and have everything else automated. So for instance if it is hanging still it doesn't distribute and when it is tugged on or pushed it measures it's movement and starts spreading out the amount put in by me.

Don't they make all kinds of fertilizer spreaders that already do this mechanically? Some mechanism (varies a lot) is driven by the wheels of the spreader and there is usually some control that can be adjusted to essentially control how much fertilizer is dispensed per wheel rotation. When you stop, the wheels aren't turning and nothing gets dispensed.

 

hi jordiggie,
Is this project a college assignment.?
Moderation.

 

Don't they make all kinds of fertilizer spreaders that already do this mechanically? Some mechanism (varies a lot) is driven by the wheels of the spreader and there is usually some control that can be adjusted to essentially control how much fertilizer is dispensed per wheel rotation. When you stop, the wheels aren't turning and nothing gets dispensed.

Yeah I have thought about attaching a wheel to a arm or bar on the roof and measuring the movement like this. I just figured there might be a more advanced option, but it seems like all the other options are a little more complicated then anticipated.

 

hi jordiggie,
Is this project a college assignment.?
Moderation.

No it's just a hobby project in my down time. I have looked in to buying pre constructed mechanisms but I had a spare Pi laying around and started with making the distributing system which works, just doesn't stop when stationary. Project might seem silly but it's just grown from being bored over time.

 

Fair enough, I want the speed measurement to be fairly accurate, to possibly accuracy of +/-0.1 m/s. The device is hanging over above plants which is estimated around 2 meters. The ground is uneven due to plants being underneath. Turning and maneuvering is not important imo since if i know the movement speed I can calculate the rest. Feel like I'm getting to a point where I might aswell just add a small motor with a constant speed and a on and off switch.

You had mentioned that you considered using an opto on a wheel. What would this wheel have been running against? What is this hanging from? How much can it swing? Presumably it shouldn't be dispensing if it is swinging back and forth, since that would be going over the same area? So how would you detect that it's swinging and when it stops swinging and is now moving over virgin territory?

 

Are your plants in pots or flats?
How are you watering now?
Are you using liquid fertilizer added to the water?
Have you considered using drip irrigation?

 

You had mentioned that you considered using an opto on a wheel. What would this wheel have been running against? What is this hanging from? How much can it swing? Presumably it shouldn't be dispensing if it is swinging back and forth, since that would be going over the same area? So how would you detect that it's swinging and when it stops swinging and is now moving over virgin territory?

Right now it is just hanging on a bar that i've installed in the green house. I could attach the wheel against the bar and measure it like that. Swinging could be an issue yes. Could add an accelerometer to detect in the z direction to make sure that when it's swinging and not moving it will just remain closed.

 

Are your plants in pots or flats?
How are you watering now?
Are you using liquid fertilizer added to the water?
Have you considered using drip irrigation?

At the moment they are granules being dispenced by a servomotor and the watering is automated. In the future I could switch to liquid fertilizer added to the water but this is what I purchased at the time and have a significant amount of it left.

 

Now that we know that you are dispensing granular fertilizer from an overhead hopper, I would take a different approach.
Since your objective is to deliver a measured amount of fertilizer per square meter I would use an ultrasonic range finder and measure position and dispense the fertilizer accordingly. You can also use position information to determine velocity.

 

Now that we know that you are dispensing granular fertilizer from an overhead hopper, I would take a different approach.
Since your objective is to deliver a measured amount of fertilizer per square meter I would use an ultrasonic range finder and measure position and dispense the fertilizer accordingly. You can also use position information to determine velocity.

Do you have a recommendation for a range finder that can always sense a greenhouse wall? Most "consumer"/DIY ultrasonic range finders are only good for about 2.5 meters (if that).

 

Do you have a recommendation for a range finder that can always sense a greenhouse wall? Most "consumer"/DIY ultrasonic range finders are only good for about 2.5 meters (if that).

Firstly we need to know the size and layout of the greenhouse.
I can make a guess of what is growing in the greenhouse.

 

Firstly we need to know the size and layout of the greenhouse.
I can make a guess of what is growing in the greenhouse.

Do you have a recommendation for ANY ultrasonic device that can sense more than 3 meters? (Regardless of what is growing in the greenhouse)? Just asking as a polite way of saying, don't think ultrasound is a viable suggestion

 

Firstly we need to know the size and layout of the greenhouse.
I can make a guess of what is growing in the greenhouse.

Size is around 80m2

 

I'm contemplating using a accelerometer and calculating the acceleration to velocity. Would mean that it's only usable from stand still which should be fine. I have also read a bit about them and am worried about the accuracy since it says that the deviation increases over time while relying on the subsequent measurements meaning the result over time is going to divert a lot.

 

Do you have a recommendation for ANY ultrasonic device that can sense more than 3 meters? (Regardless of what is growing in the greenhouse)? Just asking as a polite way of saying, don't think ultrasound is a viable suggestion

Ultrasonic range finders max out at 6m. For a large greenhouse you can use LiDAR.

https://www.sparkfun.com/distance_sensor_comparison_guide

 

Ultrasonic range finders max out at 6m. For a large greenhouse you can use LiDAR.

https://www.sparkfun.com/distance_sensor_comparison_guide

The maximum lenght is 13 meters so this sadly isn't an option. Feel like I'd be best off installing bars at the top and just running a electrical motor at a constant speed..

Didn't expect measuring speed would be this tough without GPS or wheel based powering mechanism

 

If you don't want to use LiDAR you can still use ultrasonics by using multiple reflectors.

 

If you don't want to use LiDAR you can still use ultrasonics by using multiple reflectors.

How does Lidar work with glass? I'm guessing not so well since a laser range finder doesn't work with glass. Look up the basics of Lidar - the L is for Light.

 

If you don't want to use LiDAR you can still use ultrasonics by using multiple reflectors.

How will "multiple reflectors" work with ultrasound. Could you post a quick drawing of that?