Ideas for an optical ground speed sensor?

[Mr RB]

My friend who owns a large mechanical engineering workshop has built a 1000 hp drag bike. I designed the microcontroller electronics and display unit for his 1200 hp dyno and now he seems to think I can make anything...

He needs a sensor to measure ground speed of the drag bike and connect it the onboard datalogger.

Typically they use a third wheel, on a sprung lever to contact the ground. This is done because on a drag bike the back wheel will be spinning and/or grows in size at speed so it is useless for speed feedback. Likewise the front wheel probably won't touch the ground reliably until the last half of the quarter mile.

So before he builds a complex sprung lever system with a little sensor wheel on it I thought it might be good to explore the options of making some type of optical sensor or other non-contact ground speed sensor. Obviously things like GPS are not going to give the very fast accurate data needed for 100 samples/second.

I seem to remember someone modifying an optical mouse sensor to use as a surface speed sensor, does anyone have any experience with this? Or can you suggest another option?

Thanks!

 

[crutschow]

An optical mouse sensor would be my suggestion, if it can operate fast enough to detect the ground movement of a high speed bike.

You would need to modify the optics so that it would be focused on the ground. That could be a problem if the bike is moving up and down due to the front wheel coming off the ground.

The mouse sensor output is a indication of the rate of movement it detects. You would only need to monitor one axis of the two axis outputs, of course.

 

[MikeMl]

How about an aircraft airspeed indicator? Dirt simple, but reasonably accurate for speeds over ~30MPH. New indicators and Pitot tubes are a bit pricey, but I can get find used ones at about 20% of new.

Or get a pitot, and put an IC pressure sensor behind it. I could take it flying so we could calibrate it. I can get up to ~200 MPH in a dive.

 

[Pommie]

You could use a 2 axis accelerometer and integrate to give speed. You would need 2 axes, 1 in line with the bike and 1 vertical, so you can take into account any lifting of the front wheel.

Mike.

 

[Boncuk]

How about an aircraft airspeed indicator? Dirt simple, but reasonably accurate for speeds over ~30MPH. New indicators and Pitot tubes are a bit pricey, but I can get find used ones at about 20% of new.

Or get a pitot, and put an IC pressure sensor behind it. I could take it flying so we could calibrate it. I can get up to ~200 MPH in a dive.

How about wind direction and speed?

The pitot tube provides a pressure signal in comparison to the environment. Even in calm conditions the circuit requires air density and temperature compensation.

A three beam doppler radar system provides true ground speed. An LN-3 platform - salvaged from an F4 (Phantom) provides true ground speed as well (accurate to one knot).


Boncuk

 

[jbeng]

I would probably buy an inexpensive radar gun such as the one in the link below and mount it on the bike. You'll probably also have to work out how to interface to the gun to get the speed info into whatever data recording system you are using.

Bushnell Velocity Radar Gun Speed Gun 101911 FREE UPS Bushnell Radar Guns radar speed guns 101911

 

[#12]

I saw an ad at www.ramseykits.com. Can measure an average sized car at 1/8th of a mile. OK...just stack 4 cars up at the end of the race track and you can measure a quarter mile run...right?

The Bushnell device can see race car at 1500 feet. That sounds better, but the point is, it needs a target at the far end of the track...a nice, flat target? I just wonder if the part that's holding still will become part of the problem. That's because I know absolutely nothing about this kind of device. Comments will be appreciated, thank you.

 

[Rbeckett]

Roman, I would be concerned with sensor error at those speeds and acceleration curves. A sensor on a mouse is not traveling 1320 feet in 4.5 seconds and even a small error could give very inaccurate results. Also what would the sensor be calibrated to detect, the black surface is not uniform nor is the line so a consistent input from track to track would be difficuly to obtain. I think a sensor capable of detecting the starting beam and the timing beams at each end may be the short answer and a lookup table to derive the intermediate times in between. But that is not going to detect speed and acceleration changes at gear change and braking. Just a thought from an old gear head and former drag/motocross racer.
Bob

 

[crutschow]

The Bushnell device can see race car at 1500 feet. That sounds better, but the point is, it needs a target at the far end of the track...a nice, flat target? I just wonder if the part that's holding still will become part of the problem.

Radar guns typically use the doppler effect to measure speed by detecting the difference between the transmitted and received radar signal frequency. This difference is caused by any relative motion between the gun and what ever is reflecting the signal, so I don't know that it needs any particular size or shape of target. Just reflection from the roadway or any objects at the side of the raceway may be sufficient. Basically when the gun is moving, the whole rest of the world is the target.

 

[#12]

Thank you.

 

[crutschow]

Roman, I would be concerned with sensor error at those speeds and acceleration curves. A sensor on a mouse is not traveling 1320 feet in 4.5 seconds and even a small error could give very inaccurate results. Also what would the sensor be calibrated to detect, the black surface is not uniform nor is the line so a consistent input from track to track would be difficuly to obtain.

A mouse sensor does not need to be calibrated to any particular surface. If you've used such a mouse you will see that it reacts with the same speed sensitivity to just about any surface that has a reflected roughness. The mouse senses the moving pattern, independent of the texture or uniformity in the sensed surface.

 

[Rbeckett]

Roman, What about dragging a piezo crystal or some kind of friction measuring device and recording the output and then translating the output to a usable form from a table? You would capture the raw data and the be able to apply several different logarithms for speed acceleration and have pretty good resolution too. You have way more experience, but I think it would be less drag than a spring loaded wheel and possibly more accuarte too.
Bob

 

[Rbeckett]

A mouse sensor does not need to be calibrated to any particular surface. If you've used such a mouse you will see that it reacts with the same speed sensitivity to just about any surface that has a reflected roughness. The mouse senses the moving pattern, independent of the texture or uniformity in the sensed surface.

Exactly why I asked, I have a logitec optical track ball and it fails to see the ball move sometimes and others it doesnt react to input the same as last time and in drag racing they are using .001 second event timing regularly so sensor error is a big deal at very small error rates. When I was a kid and doing the drag race deal it was all about consistency and that was what all the measurements were about. Reaction time, 60 foot time, shift time, hook up time and tons more, The better you got the more they added to the equation. When they started talking about tuning clucth engagement to RPM/distance/wheel revolution I knew I was out of my league and I should just go fast and stop from then on. Way too much technology for me at the time. Wasnt smart enough to unnerstand most of it.
Bob

 

[crutschow]

Roman, What about dragging a piezo crystal or some kind of friction measuring device and recording the output and then translating the output to a usable form from a table?

There would be no particularly correlation between the friction measured and the speed. You need something that measures velocity, not friction.

 

[Rbeckett]

I was thinking something like a piezo that generated a changing frequency or some sensor that used "friction" or touch across a surface to detect movement and make a signal that changes frequency in a very predictable manner to create a way to measure changes in speed and acceleration. Then relate the frequency to feet per second or increase in feet per second to get acceleration and speed lookup tables or a logarithims to get usable data. The sampling speed would have to be very fast and amplifiers would be needed for such a small high frequency data stream. I have a tough time with the electronic correct description, but did a fair bit of drag racing back in the day and was offering some possible issues or difficulties in gathering usable usefull data. Just some thoughts guys...
Bob

 

[Mr RB]

Wow thanks for all the responses.

MikeMl- re airspeed pitot tube; probably not usable because of the massive air pressure pulses from the unsilenced exhaust. With 1000hp and nitro methanol etc just standing in the vicinity of the engine tugs your shirt around all over the place! Then there would be windage from the blower belt and spinning back tyre. Also the ground wind on the track will be gusty due to the concrete barriers etc and hard to compensate out.

Pommie- re accelerometer, the datalogger has one in that does an ok job, barely, the trouble is the intial forward launch is usually accompanied by bucking twsiting etc and intermittant traction of the tyre it's not much use for the resolution needed (ie 100 samples per second preferred) as short term data is so extreme and noisy.

Jbeng- A radar gun is an interesting idea. It would need to be able to get a doppler reading from the ground itself, at high sample rate. I'm not sure how practical it would be or how reliable in such a high vibration environment.

Crutschow- re the mouse sensor it still seems the better option. The problem looks to be focus, there's no part of the bike at a stable distance from the ground at the launch. And any type of sprung lever to keep the sensor at the right ground height may as well just read the sprung wheel rpm.

Rbecket- Very interesting idea re the contact audio sensor. I think it would work ok with processing as the noise generated would be proportional to surface speed. But practically it's a bust as the track has different surfaces including a solid layer of melted rubber and again you would need to keep it in contact with the track... The track itself is reasonably well setup with laser sensors and automatic printouts of the time and some launch times etc. It's just for critical tuning of the clutch and tyre setup etc.

At this point I think I will tell him to make a sprung lever with a wheel on it, and we can use a hall sensor against a fine cog rotor on the wheel to get fast feedback. It's clumsy and large but he can unbolt it easy enough when not doing tuning runs.

I'll also look into Pommies idea, maybe a custom accelerometer can be made with hardware or math damping to do a pretty good job. The instantaneous ground speed is probably less important than the actual forward thrust anyway for the data he needs to set up the bike.

What a shame there's not an optical sensor that you can just point at the ground from 6" to 18" distance and get a good speed reading.