Landracing Forum
Tech Information => Technical Discussion => Topic started by: ggl205 on February 21, 2020, 11:08:10 AM
-
Over the decades, LSR racers have come up with a variety of devices to determine if their cars are aerodynamically lifting to the point of loosing control. Unless you have wind tunnel data, the only other way to determine if front wheels are still on the ground, was to see if the car still responds to driver input. Of course, this may be too late and a spin or crash may ensue.
Off and on, I have given some thought to this problem. Everything from mechanical devices to simple electrical solutions which all met with impracticality or outright failure. After watching one of Danny Tompson's in-car runs depicting what I was told is an operating lift light. Once again, interest in a workable and reliable lift light began anew.
I mentioned this problem to a friend who took it upon himself to develop a very practical and economical device that has many features other than measuring lift. It is all based on a laser that shoots photons to the ground, bounce back, are collected and time they take to go round trip is converted to distance. Run data is saved and can be used to determine lift at any point in the run. Data can also be downloaded into Google Maps for a visual on where any serious lift may have occurred. This is the general idea but there is much more capability in this device but don?t want to take up too much space in the introduction. Time will tell if this device works in a salt environment but steps are being taken to shield everything much like you would with other electronics in the car.
Photos attached (left to right) are of the light box. Green indicates satellite acquisition and red indicates when both lower and upper limits have been reached. Center box is the computer and box on the right is the laser.
John
-
I like it John.
I actually noodled on using a laser distance gauge (the diy type) but they get inaccurate at close range.
I look forward to hearing how your device works in practice but I fear your forward speed will mean that by the time the photons are taking the return trip from the salt they will have been left behind and missing the sensor altogether lol8
-
John:
The laser chosen has a user definable range. Chose one in the range you need and they are very accurate. As to speed these photons travel; I asked the same question and was told the speed spec for photons. The short answer is they travel at the speed of light so no way will I go faster than that (but one could dream).
John
-
One feature that will be added is to automatically start data acquisition at 30 mph and shut it off at 30 mph. This eliminates the driver forgetting to turn the bloody thing on!
John
-
Let's assume (and we know what assumptions do!) that the device works by measuring the time it takes for a pulse of light to travel from the laser to the salt and back to a sensor, e.g., a simple time/distance measurement.
Speed of light in a vacuum is about 30,000,000,000 cm/sec. Let's say the laser and sensor are mounted 10 cm (about 4") off the ground. The path length to the salt and back is therefore 20 cm, and the light (pulse) will require 0.667 nanoseconds (6.67E-10 sec) to traverse the 20 cm path.
Let's further say that the high limit is set to 20 cm, meaning the car has lifted 10 cm (about 4"). The light pulse will now require twice as much time as before, or 1.33 ns (1.33E-9 s), to traverse the 40 cm path (20 down, 20 back).
In order to detect this change, the controller will need to be able to resolve the difference between 0.667 ns and 1.33 ns, a difference of 0.66 ns. Accurate measurements usually require a resolution 10x better than what you are trying to measure, meaning that the controller might have to be able to resolve 0.066 ns. Furthermore, the jitter in the timing path must be substantially less than that amount (this is the really hard part).
To put this in perspective, 0.066 ns is the equivalent to about 15 gigahertz. Pretty demanding, and impressive if it can be made to work. Perhaps the device uses a more sophisticated measurement approach (Doppler?).
Meanwhile, if mounted to a car moving at 700 MPH (about 313 m/s), the car will have moved (313 m * 1.33E-9 sec) = 4.16E-7 meters, or about 0.016 thousands of an inch, while the laser pulse is meandering its way to the salt and back.
Good stuff. Curious to learn how the device actually works.
-
TD, I have forwarded your comments and questions to the developer. He can address the more technical aspects of the device. When he responds, I will post it here. For reference, my ground clearance will be between 2?-2.5? with total travel in compression of .5?. Rebound is rubber bumper limited to .5? from static ride height.
John
-
Thanks John. Note I'm not saying it cannot work, it may be entirely feasible given modern microelectronics. I don't have a laser, LIDAR, or radar background but I did spend a lot of time working on precision time and frequency distribution in (Internet) packet routers. A nanosecond here, a nanosecond there, and pretty soon your precision frequency and/or time stamp ain't so accurate. Your friend probably has it all figured out and if so good on him, certainly the packaging looks good!
Thanks, best
Tim
-
Tim, one of the great things about posting here are the helpful comments offered. So many people with varied and experienced backgrounds make for valuable information. All comments are welcome.
John
-
Hi,
I'm not sure the ride height sensor works the way it's been presented. At Racepak the lazer distance sensors we sold to the drag racers are not based on flight time but where the returned light beam strikes on a angled sensor built inside the sensor. This forms a triangle and the distance is resolved from that info. The way we did it with the Turbinator was to use analog linear position sensors mounted on the suspension. 0-5 volts was equal to full travel of the sensor which can be ordered in one inch increments. The system recorded at a accuracy of one part in 1024 or for a 4 inch travel sensor was a resolution of .004 in.
Ron
-
Ron, you are right. The triangulation method works very well at short ranges. Conversely, the time-of-flight measurement system is optimum at long range. I worked on some distance measurement systems for detonating a warhead at a precise distance from a target using the triangulation method. It was simple & accurate.
-
If you are concerned with knowing about lift before getting to the point of no return, would measuring the down/up force on the body or wheel rather than distance to a potentially uneven track surface be any more accurate? If you are concerned with the point where the wheel is no longer in contact with the surface, what about a simple electrical contact that is closed when the suspension reaches (or gets close to) full extension? - assuming this is a moving suspension.
-
Doc, the distance measurement gives a valuable data point on the lift/downforce on the body at high speed. Kalman filtering will smooth out the uneven surface effects.
-
I was present for the demo and asked questions.... the guy is pretty sharp... I think we can come up with a good test plan... and yes, the idea is to use the data to discover it a car is lifting as it goes faster.... or if it is compressing... The test fixture had a little slider and worked well in a static test... but I agree a good dynamic test is the logical next step. Should be easy since John has a couple of working units.
-
I was just thinking (and that can be dangerous) shouldn't there be a sensor at each end of the car. We're not really worried if the front end lifts as long as the rear lifts an equivalent amount. The problem comes when the front lift starts to exceed the rear. Excessive lift in the rear can also cause problems. I guess both ends should be measured and the rear should exceed the front within certain parameters. With this theory I'm assuming there should be a certain small amount of rake in the chassis.
Pete
-
Pete:
There will be chassis rake. Plan calls for between 2?-2.5? ride height at front and 3.5?-4? at rear.
I agree with a rear sensor and it was discussed. One concern was salt dust generated by air flow and how it would affect the sensor. Proof of concept is more important so front sensor only for now.
A dynamic test was conducted on Chuck?s (the developer) road car. Seemed to work well on a street car but the great white is a different story altogether.
I asked Chuck to get more technical on this thread. He understands all of what has been discussed so far and will soon respond. If it helps, Chuck is working with LIDAR technology.
John
-
As Ron pointed out, I'm pretty sure linear and rotary potentiometers have been regularly used to measure various displacements (steering angle, suspension loading, throttle position) on race cars.
Great stuff here, thanks! I'll go back to lurking now 1drink
Tim
-
Tim:
You are correct. I use an LVDT to measure throttle position and originally thought about using them to measure axle movement. But LVDT?s are expensive and you need four of them. Chuck?s idea to use LIDAR tech to measure lift and compression could also be used to measure axle movement. We talked about that but once again, I did not want to overdrive my headlights here. If the lift sensor works, it will be applied elsewhere on the car.
John
-
We recommend using string or linear pots. They work great. We had linears on the front end and a string pot on the rear end. Both wired to the Racepak. We also had a rod that was mounted to the front axle and extend thru the body so the driver could see any lift. New car will have them as we wouldn't consider running without them. Racepak also sells a laser ride height sensor for cars without suspension.
POPS
7800 A/BFS
-
A few more thoughts on suspension travel sensors: I read one posting talking about the cost of the linear sensors. Of course cost is relative but one can build with a little work travel sensors from throttle position sensors (TPS) TPS sensors sell in the 30-50 dollar range and then with a little machining can be made to work. Reading the data and interpreting the data will give some insight on what is happening. At Racepak we setup the suspension travel channels to record at 1,000 Hz. At that rate one can see the front suspension move and then followed by the appropriate time delay based on velocity and wheel base the rear senors will tell you if the car has gone over a bump. Looking at the front suspension data signature after a bump I believe one could tell by the suspension "hang time" if you were getting close to the edge. I must say after looking at hundreds of graphs this is something that I have never seen but then I've never seen a car with suspension travel sensors blow over. I'm basing this on driving hydroplanes and what the feeling is before your in trouble. The ride gets eerily smooth. So I'm quite sure the data from a car that was close to the edge would look different on how it responded to bumps than one that wasn't.
-
Couldn't resist this . With the front bias of front wheel drive lift isn't a concern .
-
Us bike guys are using those little go pro cameras to do things like watch how the suspension works. It is far less mental than sensors.
-
Hi all
We use GM TPS sensors re purposed as suspension sensors, they work great and are cheap. We also use EFI with a MegaSquirt MS3 Pro Ultimate controller/data system. It has the ability to give an output when the readings go outside a preset range which will turn on a light.
ted
-
Datadoc/n49racer:
Aside from the lift light, plans are to gather axle data next. I like your idea for using TPS?s for this purpose. I assume you have data from your car that has been treated for analysis. Would it be possible to see a portion of it to get an idea if it will serve my purpose? Anything would be greatly appreciated.
John
-
Hi GGL205,
I'm trying to attached a screen print showing shock travel at both the front and rear. Not sure if this was what you where looking for or not. For some reason when I scanned in the screen print it was upside down and I couldn't seem to save it right side up. I'm not even sure that the file will attach to this reply. On top of that I can't seem to get the Preview function to work. I'll hit post and hope for the best.
-
I can see it and yes, it's upside down. As advertised. :cheers:
Wayno
-
Datadoc, it came through loud and clear. Thank you
-
How about?
Mike
-
I?ll try post a screen shot of our front and rear data
Ted
-
Thanks, guys. This has really been helpful.
John
-
Mike:
Looking at your graph, am I seeing damper motion for a beam axle? Right shock moves opposite to left shock.
John
-
Here's a screen shot showing our front and rear TPS suspension sensors. At first we were worried because it looks like is rising throughout the run. When we investigated we found we had the sensors set up wrong. They were reading backwards. So with smoothing we found that the front and rear were settling. The max on the rear was actually just over 1/4 inch, the front even less.
-
Does that software allow you to apply some low-pass filtering to your data?
-
We found the filters need to be put on prior to the record. We are investigating now. If the data guy can do it, I repost
Ted
-
You might try DSP (digital signal processing).
-
Hi I've attached a new file only this time it is shown with a moving averaging filter applied to the two shock travel sensors output (front and rear) along with zeroing the data points at the start of the run. This time I hope the screen print will show up right side up.
-
Here is some shots with smoothing.
In the upper left x-y graph, Front reading increases by 20 ADC steps @ 0.0033?/step = 0.066? lift (upward is lift on front reading)
In upper right x-y graph, Rear reading increases by about 100 ADC steps @0.0042?/step = 0.42? drop (upward is drop for rear reading)
Something we did notice is that the front lifts at the end of the run at throttle cut and chute deployment. I was thinking...what if the front was just at the limit of lift and the chute was deployed., which is what most people would probably do.