We’re looking to find the exiting velocity of a disc from a launcher. Please provide the basis for your calculation. Thanks~!
From MOE 365
Its a dynamics problem but the calculations can change based on how your device is set up. A description of your shooter should be enough to figure out the equations and what variables you’ll need to plug in though.
Agree. We were wondering if anyone actually measured the velocity of their discs when they exited the shooter/launcher. If so, what did you record and what were the shooter parameters that resulted in that specific velocity (if you are willing to share)?
Thanks.
Measuring B velocity and wheel RPM this weekend. Will post results
^This^
It would be a service to the CD community.
Here is a neat device that can help you out with your velocity numbers.
I good video analysis program is UberSense on a Ipad or Iphone
I am going to choose to answer a different question. When working on target velocities, we timed how long it took to throw the discs various distances effectively. At what speeds did the discs have ample drop (loosing altitude) and at what threshold dip the trajector appear relatively flat. We chose to do this over a 50’ distance as this would be the theoretical super shot, just to see what sort of velocities were required.
This does not give you the “muzzle” velocity, but it does give you the average velocity, and you know you should probably target faster than that at the muzzle.
If you simultaneously take times at teh 25’ mark, then you will have the average velocity for that distance as well, and you can use the 3 points to see the trend for speed (total time give total average velocty, timing at 25’ mark allows for 25’ average as well as the 25-50 foot average by subtracting the two.
I have an Arduino Sketch that you are free to use to create a Chronometer. All you need is a couple of sensors to attach that can sense when the disk passes. You may have to change the Interrupt type from “RISING” to “FALLING” depending on the sensors you use.
Just edit the distance value in the code to match the separation distance between the sensors and it will spit out the velocity in ft/sec whenever the two sensors are triggered by a passing object.
/*******************************************************************************************
* This Sketch is for a Chronometer that uses the "micros" function to capture two points in*
* time and determines how much time has elapsed between them in microseconds. By knowing *
* the distance between the to sensors, the velocity of the object can be determined. *
* Bill Kendall, Jan. 11, 2013 *
*******************************************************************************************/
#define SENSOR_DIST 6 //This value is the distance between the sensors in inches
unsigned long event1 = 0; // value in 'micros" at the time of this first trigger
unsigned long event2 = 0; // value in 'micros" at the time of this second trigger
int calculate = 0; // If set to 1, a calculation and display of the velocity will occur
unsigned long duration = 0; // Amount of elapsed time between events
unsigned long velocity = 0;
void setup (void)
{
attachInterrupt (0, ISR0, RISING); // Interupt 0 (pin 2) calls "ISR0" Thie is the first trigger
attachInterrupt (1, ISR1, RISING); // Interupt 1 (pin 3) calls "ISR1" This is the second trigger
Serial.begin(115200); // connect to the serial port
}
void ISR0 (void)
{
event1 = micros(); // record the microseconds counter
}
void ISR1 (void)
{
event2 = micros(); // record the microseconds counter
calculate = 1; // Allow the velocity calculation code to run.
}
void loop (void)
{
if (calculate != 0);
{
RunMath ();
}
}
void RunMath (void)
{
duration = (event2 - event1); // elapsed time between triggers in u seconds
velocity = ((SENSOR_DIST/duration)/12)*1000000; // Calculate velocity in feet per second
Serial.print("The projectile is moving at ");
Serial.print(velocity);
Serial.println(" ft./sec.");
calculate = 0;
}
Can you share some of your findings? I’d really be interested in knowing what a good range of ‘times’ for the 25 foot distance is, for a fairly flat trajectory.
I built a limit-switch triggered arduino timer tonight, I’ll do some testing with the team tomorrow and report my results here. We will be shooting at a fridge around 24’ 9" away using two different types of wheels. The first an 8" AM Pneumatic wheel and the second an 8" plaction wheel.
You can see a video of our shooter here; http://www.youtube.com/watch?v=ThZCOtjVJmw
I’ve “refined” my Chronometer sketch (read that as “I now have it working correctly”). After testing it, I found it wasn’t quite what I wanted, so I fixed it.
Please enjoy!
/*******************************************************************************************
* This Sketch is for a Chronometer that uses the "micros" function to capture two points in*
* time and determines how much time has elapsed between them in microseconds. By knowing *
* the distance between the to sensors, the velocity of the object can be determined. *
* Bill Kendall, Jan. 11, 2013 *
*******************************************************************************************/
float SENSOR_DIST = 6; //This value is the distance between the sensors in inches
float distance;
unsigned long event1 = 0; // value in 'micros" at the time of this first trigger
unsigned long event2 = 0; // value in 'micros" at the time of this second trigger
int firstint = 0; //Set to 1 if Interrupt 0 has occurred
int calculate = 0; // If set to 1, a calculation and display of the velocity will occur
unsigned long duration = 0; // Amount of elapsed time between events
float velocity = 0;
void setup (void)
{
attachInterrupt (0, ISR0, FALLING); // Interupt 0 (pin 2) calls "ISR0" Thie is the first trigger
attachInterrupt (1, ISR1, FALLING); // Interupt 1 (pin 3) calls "ISR1" This is the second trigger
distance = (SENSOR_DIST/12.0); // Convert sensor spacing to feet
Serial.begin(115200); // connect to the serial port
}
void ISR0 (void)
{
event1 = micros(); // record the microseconds counter
firstint = 1;
}
void ISR1 (void)
{
event2 = micros(); // record the microseconds counter
calculate = 1;
}
void loop (void)
{
if (calculate == 1 && firstint == 1)
{
RunMath ();
}
delay (500);
}
void RunMath (void)
{
duration = (event2 - event1); // elapsed time between triggers in u seconds
velocity = (((distance)*1000000)/(duration)); // Calculate velocity in feet per second
Serial.print("The projectile is moving at ");
Serial.print (velocity,2);
Serial.println(" ft./sec.");
calculate = 0;
firstint = 0;
}
With our latest shooter setup, we measured frisbee exit velocity very roughly with a 60fps camera as well as launch surface speed (under no load) with an optical tachometer. When making 3pters from 30" directly above the auto line, we measured an exit speed of roughly 2400 ft/min (+/- a few hundred), with the launch surface going about 6000 ft/min under no load (one-sided with no noticable slip, so you can see there is some spin down in our implementation, but it spun back up within a second). These numbers are lower than we had previously been estimating, but hey that’s why we test.
Would you please describe your latest shooter setup, to give some context to the numbers you posted.
with the launch surface going about 6000 ft/min under no load
How do you know it was 6000 ft/min, i.e., how did you measure that?
There is a nice software package for the I phone. “Vernier Physics.”
Take a video of the frisbee launching past some landmarks of known separation, mark those spots, and you can get the time in between.
We powered a 360mm circumference pulley off of a CIM at 12V, and measured about 5200rpm under no load (the tach actually pegged out, so we measured at 3V, 6V, and 9V and extrapolated). This gives a tangential velocity of 360mm*5200rpm/25.4/12=6100 ft/min.
We had about 12in of contact with the belt.
To measure exit velocity, we took several 60fps videos of the launch, and measured a 4ft travel out of the shooter in 6 or 7 frames, averaging about 2400 ft/min. The low frame count of course makes the uncertainty on this figure substantial (about 300ft/min per measurement, 100ft/min over 10 measurements).
This is a great open source package to track movement of objects and such:
http://www.cabrillo.edu/~dbrown/tracker/
We used it last year with good success. It’s very easy to use and works great.
-Mike
questions:
-
could you post the tach values at 3, 6, & 9 please
-
was this measurement done with the belt in place and tensioned?
We had about 12in of contact with the belt.
What did you use for backing support for the belt? i.e. Steel, Aluminum, Teflon, etc etc
3V - 800rpm
6V - 2300rpm
9V - 3500rpm
with belt in place, tight, and backed by delrin
Check out this spreadsheet that I made to roughly calculate the resulting linear and angular velocity of a disc based on the shooter wheel size and speed.
Let me know if you have any questions about it.
FRC Shooter Speed.xlsx (113 KB)
FRC Shooter Speed.xlsx (113 KB)
That extrapolates to more like 4400 rpm, not 5200. No?
*