Arduino controlled two servos based robot - achieving a straight line

Introduction

In this article I will discuss how I came close to syncing the rotation speed of two continuous rotation servo motors to allow movement in straight line and control the speed of the robot forwards and backwards.

The Problem

If you are making a robot and want it to move in a straight line, and assuming you are using two motors for the drive, you need them to rotate in the same speed.

As most people know choosing the correct motor type for the job is important, but sometimes you don't have options and need to work with what you have.

Motor Types:

Motor Types
Source: https://backyardbrains.com/experiments/MuscleSpikerShield_GripperHand

Hobbyists & makers usually choose one of these motor types to drive their robot. To achieve a straight line, stepper motors seems to be the right choice since they can be controlled in terms of speed and direction accurately without encoders or feedback circuit. In my case I was stuck with servo motors, and thought why not tackle this challenge, but couldn't find much help online.

No two motors are exactly the same, that means you will find slight differences in the rotation speed for each motor, which causes the robot to drift when trying to move in a straight line.

On top of this, mounting the servo motors on opposite sides of the robot means that one rotates in clockwise direction and the other in counter clockwise direction. Which adds to the difference in rotation speed, I don't believe servo motors rotation speed is symmetrical!

Solution suggestions

First thought was to add an encoder to the continuous rotation servos, but if you worked with servos before you'll find that there isn't much space for one! So my approach was to take an attempt at this statistically!

Main idea:

Log the rotation speed of each motor individually at each pulse width speed signal and then map both motors results together.

Which means:

1. Set motor speed speed signal to 'X' microseconds
2. Measure the time it requires to complete a rotation (I actually measured the time needed to rotate three times)
3. Change X and measure again
4. Do the same for the other motor

Steps:

1. Measure time required to rotate at each speed setting

I originally setup the servo with the 6 spike arm but I then used the 2 spike arm instead
       --->      
Using the Arduino millis() and an optocoupler as the encoder I was able to measure the time to complete three rotations (with a 2 spike arm, that means 6 counts)
2. Measurement
   
   I started plotting data in the range 500 us to 2500 uS, but there was no significant changes in speed from 500 uS to 1200 and from 1800 uS to 2500 uS.
   Consequently I used the data range 1200 - 1800 uS in 25 uS speed increments.
   
   Measuring 9 readings and plotting the average time taken to complete 3 rotations:
   
   

   encoder count	Speed (uS)	Avg. Time (Sec)	(mS) 1	(mS) 2	(mS) 3	(mS) 4	(mS) 5	(mS) 6	(mS) 7	(mS) 8	(mS) 9
   6	1200	1.941	1950	1926	1945	1951	1944	1941	1940	1934	1941
   6	1225	2.019	2014	2009	2018	2019	2025	2016	2027	2019	2026
   6	1250	2.320	2322	2297	2294	2303	2305	2308	2363	2351	2339
   6	1275	2.680	2689	2669	2673	2673	2688	2686	2686	2670	2688
   6	1300	3.180	3170	3182	3144	3166	3177	3181	3191	3201	3211
   6	1325	3.960	3894	3906	3938	3974	3985	3964	3980	3989	4012
   6	1350	5.142	4984	5046	5074	5139	5163	5210	5206	5230	5230
   6	1375	7.270	6982	7003	7135	7209	7319	7343	7429	7460	7548
   6	1400	11.977	11181	11779	11799	12081	12068	12148	12138	12271	12325
   6	1575	13.697	12368	13080	13486	13765	13767	13974	14085	14393	14359
   6	1600	7.979	7727	7745	7889	8012	8059	8054	8081	8102	8140
   6	1625	5.511	5383	5429	5472	5532	5527	5554	5543	5589	5568
   6	1650	4.157	4110	4111	4166	4181	4189	4165	4141	4173	4177
   6	1675	3.307	3271	3281	3291	3308	3308	3317	3316	3335	3340
   6	1700	2.756	2767	2732	2741	2748	2748	2745	2759	2776	2788
   6	1725	2.350	2366	2354	2334	2355	2335	2339	2351	2356	2359
   6	1750	2.069	2077	2071	2075	2083	2077	2071	2051	2056	2056
   6	1775	1.951	1944	1956	1969	1978	1961	1971	1946	1918	1919
   6	1800	1.953	1946	1953	1962	1981	1969	1985	1942	1916	1924

   
   Motor 1 - Time required (m sec) to complete three rotations at each motor speed signal (uS)
   
   

   

   Motor 1 - Time required to complete three rotations VS. motor speed signal

   
   I noted the minimum and maximum time deviations in each speed signal to log the error range:
   
   
   

   Speed (uS)	MIN	MAX	Error Range
   1200	1926	1951	25
   1225	2009	2027	18
   1250	2294	2363	69
   1275	2669	2689	20
   1300	3144	3211	67
   1325	3894	4012	118
   1350	4984	5230	246
   1375	6982	7548	566
   1400	11181	12325	1144
   1575	12368	14393	2025
   1600	7727	8140	413
   1625	5383	5589	206
   1650	4110	4189	79
   1675	3271	3340	69
   1700	2732	2788	56
   1725	2334	2366	32
   1750	2051	2083	32
   1775	1918	1978	60
   1800	1916	1985	69

   
   

   

   Motor 1 - Error range in ms for each speed signal.
   Note: As the speed decreases, approaching motor stop @1500uS signal, error range increases

   
   Did the same for Motor 2...
   
   

   encoder steps	Speed (uS)	Avg. Time (Sec)	(mS) 1	(mS) 2	(mS) 3	(mS) 4	(mS) 5	(mS) 6	(mS) 7	(mS) 8	(mS) 9
   6	1200	1.953	1949	1953	1961	1954	1948	1972	1944	1955	1944
   6	1225	2.017	2027	2002	2026	2006	2023	2031	2016	2018	2008
   6	1250	2.240	2236	2240	2240	2241	2237	2248	2245	2260	2213
   6	1275	2.559	2569	2556	2565	2550	2569	2554	2554	2550	2561
   6	1300	3.004	3016	2997	2975	2998	2992	3011	3005	3027	3013
   6	1325	3.687	3695	3662	3697	3681	3684	3663	3689	3701	3713
   6	1350	4.739	4679	4684	4682	4734	4710	4778	4773	4806	4804
   6	1375	6.464	6341	6317	6400	6385	6441	6461	6544	6610	6680
   6	1400	9.967	9606	9750	9705	9853	9854	10035	10152	10336	10414
   6	1575	11.460	11311	11235	11050	11186	11239	11621	11652	11836	12013
   6	1600	7.021	6970	6912	6950	6952	6996	7027	7071	7140	7171
   6	1625	5.012	5002	4984	4977	4991	4983	5034	5027	5065	5044
   6	1650	3.868	3903	3857	3882	3872	3846	3850	3845	3878	3878
   6	1675	3.111	3121	3099	3096	3104	3099	3113	3112	3129	3124
   6	1700	2.621	2625	2605	2607	2612	2620	2621	2626	2642	2632
   6	1725	2.278	2282	2271	2266	2267	2268	2280	2276	2300	2296
   6	1750	2.016	2031	2017	2016	2025	2025	2022	2000	2009	1996
   6	1775	1.932	1909	1924	1929	1935	1932	1942	1951	1937	1929
   6	1800	1.932	1911	1926	1933	1928	1934	1936	1952	1942	1929

   
   
   Motor 2 - Time required (m sec) to complete three rotations at each motor speed signal (uS)
   

   

   Motor 2 - Time required to complete three rotations VS. motor speed signal

   
   

   Again, noted the minimum and maximum time deviations in each speed signal to log the error range:

   
   

   Speed (uS)	MIN	MAX	Error Range
   1200	1944	1972	28
   1225	2002	2031	29
   1250	2213	2260	47
   1275	2550	2569	19
   1300	2975	3027	52
   1325	3662	3713	51
   1350	4679	4806	127
   1375	6317	6680	363
   1400	9606	10414	808
   1575	11050	12013	963
   1600	6912	7171	259
   1625	4977	5065	88
   1650	3845	3903	58
   1675	3096	3129	33
   1700	2605	2642	37
   1725	2266	2300	34
   1750	1996	2031	35
   1775	1909	1951	42
   1800	1911	1952	41

   
   

   

   Motor 2 - Error range in ms for each speed signal.

   Note: As the speed decreases, approaching motor stop @1500uS signal, error range increases



3. Compare the two motors performance
   
   

   Speed mR (uS)	motorR	motorL	Speed mL (uS)
   1200	1.95	1.95	1800
   1225	2.02	1.95	1775
   1250	2.24	2.07	1750
   1275	2.56	2.35	1725
   1300	3.00	2.76	1700
   1325	3.69	3.31	1675
   1350	4.74	4.16	1650
   1375	6.46	5.51	1625
   1400	9.97	7.98	1600
   1575	11.46	13.70	1575
   1600	7.02	11.98	1400
   1625	5.01	7.27	1375
   1650	3.87	5.14	1350
   1675	3.11	3.96	1325
   1700	2.62	3.18	1300
   1725	2.28	2.68	1275
   1750	2.02	2.32	1250
   1775	1.93	2.02	1225
   1800	1.93	1.94	1200

   Note: Motor 1 sorted ascendingly and Motor 2 descendingly, as they will be mounted oppositely on the robot, each motor on one wheel.
   

   

   Motor 1 & 2 comparison. Time (sec) to rotate three rotations at each speed signal (uS)

   
   It can be noticed that some of the values are close to each other, but we need to shift the use of motors speed signal.
   
   From the above table, I eliminated the use of 1800uS speed signal (in dark gray) from both motors, as it added no significant speed change, and would allow both sides of the table to shift up to a closer values on both sides.
   
   I also ignored the signals between 1400 - 1500 uS as the error range was already too high in the slow range near the stopping speed signal 1500uS.
   
   The result table would then be:
   
   

   Speed mR (uS)	motorR	motorL (white)	Speed mL (uS)
   1200	1.95	1.95	1775
   1225	2.02	2.07	1750
   1250	2.24	2.35	1725
   1275	2.56	2.76	1700
   1300	3.00	3.31	1675
   1325	3.69	4.16	1650
   1350	4.74	5.51	1625
   1375	6.46	7.98	1600
   1600	7.02	7.27	1375
   1625	5.01	5.14	1350
   1650	3.87	3.96	1325
   1675	3.11	3.18	1300
   1700	2.62	2.68	1275
   1725	2.28	2.32	1250
   1750	2.02	2.02	1225
   1775	1.93	1.94	1200

   
   Note A: Motor 1 sorted ascendingly and Motor 2 descendingly, as they will be mounted oppositely on the robot, each motor on one wheel.
   Note B: Both Motors time is now closer in value. which brings us closer to moving in straight line

   

   Motor 1 & 2 comparison at each speed signal after mapping the results

   
   At this point, we could see that we have improved the matching in speed between the two motors. But it's still not quite accurate. We need as much as we can to use values for each motor that would bring them closer to rotating at the same speed.
   
   
4. To be continued...