# Trig Lookup Table

I’ve seen the trig lookup table function posted by Kevin Watson (this one: http://kevin.org/frc/frc_trig.zip ). I’m just wondering if anybody has an idea on how to implement an inverse cosine function?

My real problem is controlling 2 angles which have to be calculated. I have all the calculations, but they are quite involved (lots of inverse cos, sqrt, and squaring). I’ve ruled out a lookup table for the various combinations of these two angles. There are two independent variables that are controlled by the robot driver, and two calculated angles (Im pretty sure the lookup table for that would be way too huge).

Thus I return to my original question: how to implement an inverse cosine lookup table function? Perhaps using kevin watson’s code posted above?

Thanks,

Michael Michniewski

p.s see my new post below

What are the two angles your looking trying to find for? I know that i had to find a few angles for our Mast on our bot. One for it at 45, one at 90, and then i had to find the two angles for where the Pnuematic ram would be placed and what distance on our mast and frame. This only took me about 15 Min in autocad… Autocad is the best mathmatical cheater. But i dont know what your looking for

You can use a similar method to what Kevin uses, but replace the data with the appropriate trig function. If you know your trig identities you might be able to use his functions to get the data you need.

However, depeding on how many calculations you are doing, you should be fine with a call to arccos() from <math.h>. Consider posting the code.

BTW, I don’t think squaring is nearly as processor intensive as the other functions.

I can’t help with the inverse stuff but I did just cobble together a SIN/COS/TAN function lookup table in Java to show one of the students how it could be done. Remember, it’s java, so you’ll have to adjust the parameters and the data types as appropriate (signed, unsigned, etc).

``````/*
* Main.java
*
* Created on February 16, 2007, 12:54 PM
*
* To change this template, choose Tools | Template Manager
* and open the template in the editor.
*/

package triglookup;

/**
* Table Values provided from: http://www.industrialpress.com/en/tabid/63/default.aspx
* @author purduePhotog
*/
public class Main {
//TRIGTAB is x1000 values for added precision.
private static int] TRIGTAB = {0,17,35,52,70,87,105,122,139,156,174,191,208,225,242,259,276,292,309,326,342,358,375,391,407,423,438,454,469,485,500,515,530,545,559,574,588,602,616,629,643,656,669,682,695,707,719,731,743,755,766,777,788,799,809,819,829,839,848,857,866,875,883,891,899,906,914,921,927,934,940,946,951,956,961,966,970,974,978,982,985,988,990,993,995,996,997,998,999,1000,1000};

/** Creates a new instance of Main */
public Main() {
}

/**
* @param args the command line arguments
*/
public static void main(String] args) {
// TODO code application logic here
//Test the values of each function:
//int n = -360;
int n = 0;
while (n <= 365) {
System.err.println("Angle Test: " + n + "	TAN: " + (double) TAN(n*100) / 1000);
n = n + 5;
}
}

//Expects ANG to be x100
//public signed int SIN( signed long ANG, int SCALE) {
public static int SIN(long ANG) {

//Get us in the proper range for Sin to work.
while (ANG > 36000) {
ANG=ANG - 36000;
}
while (ANG < 0) {
ANG= ANG + 36000;
}

//if we're from 0-90 (0-9000) then we simply return the lookup value
if (ANG <= 9000 ) {
return (TRIGTAB(int)ANG/100]);
} else if ((ANG > 9000) && (ANG <= 18000) ) {
return (TRIGTAB[180 - (int)ANG/100]);
} else if (( ANG > 18000) && (ANG <= 27000) ) {
return (-1 * TRIGTAB(int)ANG/100 - 180]);
} else {
return (-1 * TRIGTAB[360 - (int)ANG/100]);
}
}

//Expects ANG to be x100
public static int COS(long ANG) {
//Simply offset the SIN by 90 degrees phase
return (SIN(9000 + ANG));
}

//Expects ANG to be x100
public static int TAN(long ANG) {
//Simply offset the SIN by 90 degrees phase for COS
int cosResult = COS(ANG);
int sinResult = SIN(ANG);
if (cosResult == 0) {
cosResult = 1;
}
return ( sinResult * 1000 / cosResult );
}
}

``````

Thanks for the replies.

I’m not just looking for a given number of angles (ie set positions), but rather within a range (almost 360 degrees for at least one of two angles).

I’ll try the suggestion of replacing the cos function in Kevin’s code with the cosine inverse.

Here’s the current code. I put things that are used or calculated more than once in a variable, which is called later.

Basically I’m just looking for the quickest way to execute this code, my main idea being using a trig lookup table.

``````
//
// For Calculations
//
l_upper_squared = arm.length_upper^2;
l_lower_squared = arm.length_lower^2;

// dist. of arm infront of bot + offset of arm from front of bot
temp_o_d = (arm.offset_front + arm.distance);

temp_R_squared = (arm.height^2) + (temp_o_d^2);
temp_R = sqrt(temp_R_squared);

//
// Calculate Angles
//

// calculate joint1 angle 1
j1_angle1 = acos( arm.distance / temp_R  );

// calculate joint1 angle 2
temp_var = ( (l_upper_squared) - (l_lower_squared) - (temp_R_squared) ) / ( (-2) * (arm.length_lower) * temp_R );
j1_angle2 = acos( temp_var );

// calculate joint2 angle
temp_var =  ( temp_R_squared - (l_lower_squared) - (l_upper_squared) ) / ( (-2) * arm.length_lower * arm.length_upper );
j2_angle = acos( temp_var );

//
// Set Joint Angles
//
arm.j1_set_angle = j1_angle1 + j1_angle2;	// joint1
arm.j2_set_angle = j2_angle;				// joint2

// done

``````

I’ve tried to keep all the trig functions the same (so they’re all arccos, or inverse cosine).

Thanks,

Michael

Edit: the arccos in the code should be (and now is) acos, right?

The ^ operator is bitwise XOR, there is no power operator in C. You need to use the pow() function or one of its brothers or just square it manually (which would be better if they are fixed point):

``````#include <math.h>
double pow(double x, double y);
long double powl(long double x, long double y);
float powf(float x, float y);
``````

Example of new code:

``````

temp_R_squared = pow(arm.height,2) + pow(temp_o_d,2);
temp_R_squared = arm.height*arm.height + temp_o_d*temp_o_d;

``````

Be sure you type-cast correctly.

I’ve tried to keep all the trig functions the same (so they’re all arccos, or inverse cosine).
This is a good way to keep it fast if you go with the lookup table solution.

The arccos in the code should be (and now is) acos, right?
Yes, if you are using <math.h>. But I thought you were going to use a lookup table for inverse cosine, no? In which case you would have to name it to not conflict with the definition in <math.h> which you will need for sqrt(), eg. acos_lookup().

Also, if arm.length_upper, arm.length_lower, arm.offset_front, arm.distance are all constant consider doing the math on them ahead of time. And if all of these are floating point variables (which they need to be if you put them into functions like sqrt() and acos() then you are probably not going to get done quick enough. Consider making a lookup table for the whole operation (if possible) or just casting to floats for the <math.h> calls.

Good luck,
Robinson

I’ve been meaning to post this for about a week or so, but for some reason never seemed to have any time. This is the type of method we have used for the past couple of years for calculating sine & cosine (and arcsine & arccosine). It uses both a lookup table and linear interpolation to determine the approximate value. It utilizes signed shorts rather than floating point to keep the execution speed decent. I’ve only extracted the sine/cosine routines but the documentation shows some other examples of how you could use some of underlying routines to generate your own non-linear functions.

Mike

sincos_1126.zip (73.8 KB)

sincos_1126.zip (73.8 KB)