Ball drop detection - Page 1
Forum section: electronics / projects

author: etienne
author_rank: Newbie
timestamp: July 10, 2017, 02:59:54 pm
content: Hi everybody!we have a project which involves detecting a (heavy) ball being dropped by a person,and are thinking of the kind of sensor we could use to accurately detect it.Note that the ball is being dropped in a known 3D zone,so we know "where" to track motion.We'll be outside so we must handle weather changes (lighting conditions, etc...)We want to detect the ball before it hits the ground,so we cannot rely on a sensor reacting to the ball's impact.We also cannot use load cells to detect weight loss as the ball is being droppedas it would interfere with other components of the project(we have a fixed structure and we do not want to modify it to insert load cells)So we are left (as far as i know) with the following options:- PIR (likely not fast enough and unable to accurately discriminate between the "volume of interest" and the outside world)- Sonar (likely not fast enough and unable to accurately discriminate between the "volume of interest" and the outside world)- IR Distance (this one may work, we'd have to check the sensor update rate though)- Camera (using a computer vision toolkit, pretty confident it would work but maybe overkill)- Laser tripwire web (pretty confident it would work but lots of hardware for a dense enough web)Is there any options we could include here?Which one makes most sense in your opinion?Thank you!

author: Nusa
author_rank: Super Contributor
timestamp: July 10, 2017, 03:05:50 pm
content: Are you allowed to have an accelerometer and radio link inside the ball?

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 10, 2017, 03:16:29 pm
content: Quote from: etienne on July 10, 2017, 02:59:54 pmHeavy. OK. One that falls fast.How big is the ball? Is it contrast-y to the background?How fast do you need to sense it?(It's Wimbledon at the moment... so Hawkeye recommended... which is CVhttp://www.pyimagesearch.com/2015/09/14/ball-tracking-with-opencv/)What sort of distances are involved? The larger the field of play the further away the observer will be and therefore the angular resolution becomes harder to resolve...PSI've been intending to build a tennis ball collecting robot for ages so I'm quite interested in this.

author: etienne
author_rank: Newbie
timestamp: July 10, 2017, 06:06:25 pm
content: Thanks,@Nusa, unfortunately we aren't allowed to put electronics inside the ball, otherwise yes, an accelerometer would be a good idea!@NivagSwerdna, the ball is like a bowling ball, so the sensor needs to be pretty fast in order to "see" it cross its field,the volume we work with is like a cylinder whose diameter is approximately 2 meters,our sensor will likely end up somewhere at half the height of the cylinder, on the cylinder surfaceAt the moment i guess we could pick the color we want for the ball, including a color that helps for tracking!Thanks for the ball tracking link!

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 10, 2017, 06:08:12 pm
content: What are the dimensions of the 3D zone?There's another option, magnetic sensing

author: etienne
author_rank: Newbie
timestamp: July 10, 2017, 06:10:43 pm
content: @MasterTech : think of a cylinder, diameter 2 meters, height around 3-5 meters

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 10, 2017, 06:13:30 pm
content: Quote from: etienne on July 10, 2017, 06:10:43 pmWell, I did a 3d magnetometer sensing prototype that detects a magnet in 3d space with 4 magnetometers. Of course the longer the distance the bigger the magnet needed.It only reached prototype level but the concept works

author: suicidaleggroll
author_rank: Super Contributor
timestamp: July 10, 2017, 06:25:22 pm
content: You mentioned the laser tripwire web, but were concerned that it would take a lot of hardware. How about a LIDAR instead, and use a mirror setup to reflect it around the chamber to create your web for you. When the ball crosses through one or more of the beams, the measured distance will change. eg:https://www.sparkfun.com/products/14032?gclid=CNCh5ZGr_9QCFQSUaQodtuQBGQ

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: July 10, 2017, 07:26:49 pm
content: doppler RADAR?, speed guns, kits readily available, ignore background below delta-f thresholdcould do similar with SONAR - use doppler freq shift to give "contrast"

author: fcb
author_rank: Super Contributor
timestamp: July 10, 2017, 07:48:14 pm
content: Radar would probably work if the ball is at all radar reflective. Very fast.Modules are quite cheap (rfbeam.ch, etc..)

author: Dubbie
author_rank: Unknown
timestamp: July 10, 2017, 08:23:04 pm
content: How many times are you going to run this? If only a handful of times, a mesh of fine magnet wire might work. Could be made on a wooden frame in a few mins. Would be extremely reliable and would only need the bare minimum of electronics to process.

author: TK
author_rank: Super Contributor
timestamp: July 10, 2017, 09:07:19 pm
content: What about a cheap used microsoft kinect with a Raspberry Pi? I am sure it will be much less expensive than a LIDAR.

author: Avacee
author_rank: Unknown
timestamp: July 10, 2017, 09:23:13 pm
content: The Pixy has a wonderful demonstration of tracking a ball ..http://charmedlabs.com/default/pixy-cmucam5/And can be used with arduino and pi's really easily if you just need something to send a trigger event.

author: cdev
author_rank: Super Contributor
timestamp: July 10, 2017, 09:27:16 pm
content: No shape is easier to recognize from all angles as a sphere, because its always going to be a circle. So video and Open CV or similar or even something simpler may be your best bet.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 10, 2017, 09:29:27 pm
content: Quote from: evb149 on July 10, 2017, 07:10:55 pm+1Why is sensing the ball important? How soon after it starts accelerating in free fall do you have to know it is moving? If the human wobbles it up and down is that to be sensed too? Why can't you sense it falling through a particular level and work backwards? Is the intent to hit the falling ball?

author: etienne
author_rank: Newbie
timestamp: July 10, 2017, 09:49:47 pm
content: thanks everybody, lots of good advice here@evb149 you're right, some information is missing here,we plan to install the device outside,and it is supposed to work for an indeterminate number of times (ie: we'd like to avoid a limit on the number of ball drops before maintenance is needed)we will however have staff around when the system is operating,and in case of really harsh conditions (significant rain / snow, ... ) the system would simply be turned down then rebooted when the weather gets better,The system should not be destroyed by bad weather but is not supposed to operate under it.We won't be able to place sensors on the ball, and would also like to avoid painting it or anything that might degrade quickly as the ball gets usedThe volume the ball falls into (cylinder) won't be crossed by anything else than the ball, so we do not have to discriminate among objects within the cylinder volume (apart from say, a dead leaf or an insect)Quote from: NivagSwerdna on July 10, 2017, 09:29:27 pmThe whole thing is essentially built for fun so we do not have a practical purpose here,Detecting that the ball has crossed a given section of the cylinder is enough, we do not care about the exact drop moment or the ball trajectory,We won't aim for the kind of accuracy we'd need to hit a falling ball@MasterTech nice experiment!

author: Nusa
author_rank: Super Contributor
timestamp: July 10, 2017, 10:43:19 pm
content: For a laser tripwire web you need 1 laser, 1 laser sensor, and a bunch of small mirrors or prisms of a suitable or adjustable angle. Initial planning and setup of the mirrors is the only hard part. Weather/light shades/tubes over all the optics would probably be sufficient to avoid most environmental factors. Maintenance would be cleaning the optics as required. Cleaning could be automated, but probably not worth it for a fun project.

author: mycroft
author_rank: Regular Contributor
timestamp: July 11, 2017, 12:07:41 am
content: Maybe winding a coil and detecting the change of impedance or resonant frequency when crossed by the ball, supposed metallic.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 11, 2017, 11:58:11 am
content: Quote from: Nusa on July 10, 2017, 10:43:19 pmWhat sort of sensor? Assuming the ball is somewhat reflective the amount of light scattered/reflected back to the sensor will be really really small? If the ball is spherical then the reflections from the surface will mostly reflect away from the sensor?

author: Nusa
author_rank: Super Contributor
timestamp: July 11, 2017, 12:14:40 pm
content: Quote from: NivagSwerdna on July 11, 2017, 11:58:11 amFlip your logic. The stable state is the laser beam hitting the sensor with nothing in the way. When the ball is dropped and blocks the beam from hitting the sensor is when you know the ball is present.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 11, 2017, 12:21:50 pm
content: Quote from: Nusa on July 11, 2017, 12:14:40 pmAh. And the amount of change is then a function of the blocked area versus the detection area... so the wider the field of detection the smaller the decrease in received amplitude. (I'm assuming a net of light rather than a beam since a beam doesn't provide much detection area)

author: Nusa
author_rank: Super Contributor
timestamp: July 11, 2017, 12:39:33 pm
content: Quote from: NivagSwerdna on July 11, 2017, 12:21:50 pmActually my suggestion was to bounce a single beam off reflectors to create as large a detection web as is needed. If any part of the laser web is blocked the ball is detected, which satisfies the stated goal of the exercise. I like simple when it gets the job done.If you read above, evb149 had a different approach to how to use lasers.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 11, 2017, 12:52:11 pm
content: Quote from: Nusa on July 11, 2017, 12:39:33 pmQuoteAh. yes. Multiple sensors. I can see how that might work.Fun project

author: larsdenmark
author_rank: Regular Contributor
timestamp: July 16, 2017, 11:30:20 am
content: Use a laser tripwire web to get exact information on when the ball start to block the web and when it stops blocking it.If the ball drops vertically and you know the diameter of the ball you can use this information to calculate when it hits the floor. If you don't know the diameter you need an extra tripwire web.If the ball fall in an arc you may need to use Newton's law to calculate the position and velocity with accuracy.In any event you'd need fast hardware to get useful results in time before the ball hits the floor.If you go for using LIDAR or RADAR then a Kalman filter can be used to increase the accuracy.