Zhang et al. Robot. Biomim.  (2018) 5:3  
https://doi.org/10.1186/s40638-018-0085-7
RESEARCH
A 6-DOF robot-time optimal trajectory 
planning based on an improved genetic 
algorithm
Jiayan Zhang, Qingxi Meng* 
, Xugang Feng and Hao Shen
Open Access
Abstract 
By using quintic polynomial function to interpolate several given points of each joint of the robot, the mathematical 
expressions of each joint variable of the robot with time are established. In addition, to improve the search algo-
rithm performance crossover operator and mutation operator of the genetic algorithm are improved in cosine form. 
Furthermore, the improved adaptive genetic algorithm is applied to optimize the time interval of interpolation points 
of each joint, so as to realize time optimal trajectory planning. Moreover, MATLAB simulation is carried out, and the 
results show that the method proposed in this paper reduces the running time of the robot tasks. Meanwhile, the 
curves of angle position, velocity and acceleration of each joint are smooth enough, which ensure accomplish its 
tasks in a stable and efficient way.
Keywords:  Industrial robot, Trajectory planning, Adaptive genetic algorithm (AGA), Time optimal
Background
Robot  trajectory  planning  usually  refers  to  track  points 
given  several  expectations  and  target  pose,  and  timely 
adjust the rotation angle of each joint of the robot to the 
end effector at a prescribed trajectory followed by each 
point to eventually reach the target point. The trajectory 
planning  in  joint  space  is  simpler  and  convenient  than 
that  of  Cartesian  space.  Therefore,  several  fixed  points 
which located at the end of several robotic arms are usu-
ally given. Then, these track points for the robot are com-
puted by using the inverse kinematics so as to convert it 
from Cartesian space to joint coordinate space. Next, the 
track  points  are  used  to  carry  out  interpolation  opera-
tion using various spline functions, polynomial functions 
or other forms of curves, and the expressions about the 
time of each joint variable for the robot are obtained. In 
addition, in light of the mechanical characteristics of the 
robot, the speed and acceleration of each joint should be 
limited to the allowable range. Therefore, it is necessary 
to optimize the speed and acceleration of each joint arm, 
*Correspondence:  mqx3021@qq.com 
College of Electrical Engineering and Information, Anhui University 
of Technology, Ma’anshan 243002, China
not only to ensure the smooth operation of the joint arm 
but  also  to  reduce  the  wear  and  impact  to  prolong  the 
working life of the robot.
The  method  of  optimal  trajectory  planning  generally 
includes time optimal trajectory planning [1–3], energy 
minimum  trajectory  planning  [3,  4]  and  impact  mini-
mum  trajectory  planning  [5],  or  multi-objective  trajec-
tory optimization combining these estimation schemes. 
Among  them,  the  optimal  trajectory  planning  with  the 
robot running time as main consideration is favored by 
many  scholars.  In  recent  years,  many  researchers  have 
made some achievements in the field of robot trajectory 
planning. Tohfeh and Fakharian [6] constructed a func-
tion  expression  for  the  omnidirectional  robot’s  energy 
dissipation  by  combining  obstacle  avoidance  perfor-
mance,  and  the  optimization  problem  was  transformed 
into  a  parameter  minimization  problem  by  the  poly-
nomial  interpolation  method,  which  provided  a  more 
effective  way  for  the  study  of  robot  obstacle  avoidance. 
However, due to the complexity of this method, there is a 
certain degree of difficulty in practice. Bende [7] studied 
a method of modeling underwater robot based on bond 
graph theory and optimized the model parameters with 
the genetic algorithm to obtain the optimized trajectory 
© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License 
(http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, 
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and indicate if changes were made.
Zhang et al. Robot. Biomim.  (2018) 5:3 
Page 2 of 7
of the robot. Experiments were carried to show the sig-
nificance  of  this  method.  Zhu  and  Liu  [8]  applied  the 
seventh-order B-spline curve for the interpolation opera-
tion  of  robot’s  articulation  arm  trajectory  and  applied 
the sequential quadratic programming for the trajectory 
planning, which achieved the optimal planning goal and 
the angular displacement, velocity and acceleration curve 
of each joint of the robot are smoother.
In  this  paper,  from  the  point  of  view  of  robot  run-
ning time, the trajectory of robot joint is planned by the 
quintic  polynomial  interpolation  under  the  premise  of 
smooth operation of the manipulator, and the time inter-
val of trajectory interpolation point is optimized by the 
improved  adaptive  genetic  algorithm,  so  that  the  robot 
accomplishes task time as short as possible.
Model
In  this  paper,  we  consider  PUMA560  robot  as  the 
research  object,  which  belongs  to  a  conventional  six-
arm-type  robot.  Its  first  three  joints  are  used  to  deter-
mine the position of the robot end effector, and the latter 
three joints are used to determine its attitude. In order 
to  make  it  easier  to  analyze  the  motion  of  articulation 
arm of the robot, it is necessary to establish the kinematic 
equation of the robot. The link pole coordinate system is 
established by D-H method, and through the translation, 
rotation and other coordinate transformation to establish 
the relationship between the coordinate system. The link 
pole coordinate system is shown in Fig. 1.
0T1 =
2T3 =
4T5 =
1T2 =
3T4 =
5T6 =
cθ4 −sθ4 0 a3
1 d4
0
−sθ4 −cθ4 0 0
0 1
cθ2 −sθ2 0 0
1 d2
0
−sθ2 −cθ1 0 0
0 1
cθ6 −sθ6 0 0
0
1 0
−sθ6 −cθ6 0 0
0 1
cθ5 −sθ5 0 0
0 −1 0
0
sθ5
cθ5
0 0
0
0
0 1
cθ3 −sθ3 0 a2
cθ3 0 0
sθ3
1 0
0
0
0
0
0 1
cθ1 −sθ1 0 0
cθ1 0 0
sθ1
1 0
0
0
0
0
0 1
0
0
0
0
0
0
0
0
0
(1)
z0 (z1)
1
x0
d2
z2
a2
x3
2
x2
a3
y0 (x1)
6
6
0
.
4
3
4
d4
z3
x4 (x5)(x6)
z5
5
z4 (z6)
o(y)
56.25
6
n(x)
a(z)
Fig. 1  The link pole coordinate system of PUMA560 robot according 
to the link pole coordinate system of Fig. 1, the transformation matrix 
of each link can be obtained as follows
nx ox ax
ny oy ay
nz oz az
the posture of the end effector relative to the base system 
where the third-order sub-matrix 
 represents 
 represents its position relative to the base 
{0}, and 
px
py
pz
system {0}.
Trajectory planning analysis
The  number  of  desired  track  points  of  the  robot  end 
effector  in  the  Cartesian  space  is  transformed  into  the 
corresponding  joint  variables  by  the  kinematic  inverse 
operation,  and  then,  track  points  of  the  joint  space  are 
interpolated to obtain the joint variables of the robot a 
function  of  time-varying  expression  [9].  In  this  paper, 
the  quintic  polynomial  is  used  to  interpolate  trajectory 
points in the robot joint space.
Denote  by  θ (t)  the  joint  angle.  Assume  θ (t0) = θ0 , 
θ (tf ) = θf . Obviously, there are several quintic polynomial 
curves satisfying the above conditions, as shown in Fig. 2.
Multiplying  the  above  transformation  matrice 
in 
turn,  the  comprehensive  transformation  matrix  of  the 
PUMA560 robot is obtained:
0T6 = 0T1(θ 1)1T2(θ 2)2T3(θ 3)3T4(θ 4)4T5(θ 5)5T6(θ 6)
nx ox ax px
ny oy ay py
nz oz az pz
0 0 0 1
=
(2)
Fig. 2  Quintic polynomial function of the same starting point and 
end point
Zhang et al. Robot. Biomim.  (2018) 5:3 
Page 3 of 7
It is now necessary to find a smooth curve with starting 
point θ0 and ending point θf . In addition, the expression 
of the quintic polynomial is:
θ (t) = a0 + a1t + a2t2 + a3t3 + a4t4 + a5t5
(3)
At the start and the end points, the displacement con-
straint, speed constraint and acceleration constraint are 
expressed in (4)–(6), respectively:
θ (tf ) = θf
 θ (0) = θ0
 θ ′(0) = θ ′
 θ ′′(0) = θ ′′
0
θ ′(tf ) = θ ′
f
0
θ ′′(tf ) = θ ′′
f
(4)
(5)
(6)
Deriving Eq. (3), the velocity expression of the robot’s 
trajectory is obtained as:
θ ′(t) = a1 + 2a2t + 3a3t2 + 4a4t3 + 5a5t4
(7)
Similarly,  by  means  of  the  second  derivative  of  t  in 
formula Eq. (6), we can get the acceleration function as 
follows:
θ ′′(t) = 2a2 + 6a3t + 12a4t2 + 20a5t3
(8)
Combining Eqs. (4), (5) and (6), we obtain the coeffi-
cients of the quintic polynomial as follows:
a0 = θ0
a1 = θ ′
0
θ ′′
0
a2 =
2
20θf −20θ0−(12θ ′
a3 =
a4 =
a5 =
)tf −(3θ ′′
0 −θ ′′
f
)t2
f
0+8θ ′
f
2t3
f
0+14θ ′
f
30θf −30θ0+(16θ ′
)tf +(3θ ′′
0 −2θ ′′
f
)t2
f
12θf −12θ0−(6θ ′
)tf −(θ ′′
0 −θ ′′
f
)t2
f
2t4
f
0+6θ ′
f
2t5
f
(9)
Introducing  the  above  factors  into  Eq.  (3),  the  robot 
trajectory  equation  of  the  quintic  polynomial  can  be 
derived.
Improvement in genetic algorithm
The basic knowledge of the kinematics of the robot and 
the  basic  principle  of  the  interpolation  of  the  trajec-
tory points of the joint space by the quintic polynomial 
have  been  introduced  in  the  preceding  narrative.  On 
this  basis,  it  is  necessary  to  optimize  the  time  interval 
of the interpolation point of the track point by using the 
genetic algorithm to realize the optimal planning of the 
robot time and further make the angular displacement, 
velocity and acceleration curve of each joint movement 
smoother.
The genetic algorithm, which is an artificial intelligence 
optimization  algorithm  for  simulating  the  genetic  and 
evolutionary processes in nature [10–12], owns the char-
acteristics of simplicity and robustness, and starts from 
the parallel solution of the problem (rather than a single 
solution). Therefore, it not only has excellent global opti-
mization but also is used in the optimization process of 
practical problems [13].
However, the crossover operator and mutation opera-
tor of a simple genetic algorithm are invariant in the pro-
cess of algorithm implementation and do not satisfy the 
dynamic requirement of biological evolution. To this end, 
Ren and San [14] proposed an adaptive genetic algorithm 
according  to  the  individual  fitness  of  different  dynamic 
adjustment of crossover probability and mutation prob-
ability.  The  adjustment  of  the  crossover  and  mutation 
probabilities is shown in Eqs. (10) and (11), respectively:
pc = pc1 −
pc1,
(pc1−pc2)(f ′−favg)
fmax −favg
, f ′ ≥ favg
f ′ < favg
pm = pm1 −
pm1,
(pm1−pm2)(f ′−favg)
fmax −favg
, f ≥ favg
f ′ < favg
(10)
(11)
where  pc1 = 0.9 ,  pc2 = 0.6 ,  pm1 = 0.1 ,  pm2 = 0.01  f ′  is 
the parent of the larger fitness value;  favg is the average 
fitness value in the population;  fmax is the fitness value of 
the largest individual in the current population.
Equation (10) shows that a larger fixed crossover prob-
ability is given when the fitness value of the cross parent 
is  small.  And  the  greater  the  fitness  of  the  two  chro-
mosomes,  the  smaller  the  crossover  probability  is.  The 
adjustment of the mutation probability in Eq. (11) is con-
sistent with the crossover probability.
It is noted that the above method has two drawbacks. 
Firstly, the crossover probability and the mutation prob-
ability are fixed for the parent chromosomes whose fit-
ness is lower than the average fitness of the population. 
Secondly,  the  adjustment  of  crossover  and  mutation 
probability is linear, and it can not meet the objective of 
population  evolution.  To  overcome  the  two  drawbacks, 
this  paper  presents  an  adaptive  genetic  algorithm  that 
uses  a  cosine  function  to  adjust  crossover  probability 
and mutation probability. Adjusting Eqs. (10) and (11) as 
follows:
pc0+pc min
2
pc0+pc max
2
pc =
+ pc0−pc min
+ pc0−pc max
2
2
fmax −favg
π�, f ≥ favg
cos� f −favg
cos� favg−f
π�,
f < favg
favg
(12)
Zhang et al. Robot. Biomim.  (2018) 5:3 
Page 4 of 7
Fig. 4  Mutation probability adjustment curve of IAGA 
T = T1 + T2 + · · · +T i−1 =
Pcmax
Pc0
Pcmin
Fmin
F0
Fmax
Fig. 3  Crossover probability adjustment curve of IAGA 
Pmmax
Pm0
Pmmin
Fmin
F0
Fmax
pm =
pm0+pm min
2
pm0+pm max
2
+ pm0−pm min
2
+ pm0−pm max
2
fmax −favg
cos� f ′−favg
π�, f ′ ≥ favg
cos� favg−f ′
π�,
f ′ < favg
favg
(13)
where  pc min and  Pc max are the minimum and the maxi-
mum  probability,  respectively.  pc min ≤ pc0 ≤ pc max  is 
a  crossover  probability;  pm min  and  pm max  denote  the 
smallest and largest mutation probabilities, respectively, 
and  pc min ≤ pm0 ≤ pc max  is  a  mutation  probabilities; 
fmax is the fitness of the best individual in the population 
and  favg is the average fitness;  f  is the greater fitness of 
crossed parent;  f ′ is the fitness of the individual perform-
ing the mutation.
For  the  crossover  probability  adjustment  method  in 
Eq.  (12),  pc0  represents  a  crossover  probability  for  the 
average fitness of the population, and the size of  pc0 can 
be  determined  based  on  the  required  problem  and  the 
algorithm optimization process. If  pc0 is larger, it means 
raising the crossover probability of the individual in the 
population  and  promoting  the  change  in  the  individual 
gene pattern from a wide range. If x is small, the oppo-
site is true. Therefore, in order to improve the optimiza-
tion performance of the algorithm, we need to adjust the 
value of  pc0 to balance the global optimization ability and 
local optimization ability of the algorithm.
According  to  the  formula,  the  crossover  probability 
and mutation probability of the improved algorithm can 
be  approximately  calculated,  which  are  automatically 
adjusted according to the individual fitness in the popula-
tion, as shown in Figs. 3 and 4.
The adaptive genetic algorithm is used to adjust crosso-
ver operator and mutation operator by the cosine func-
tion. The crossover probability and mutation probability 
are  adjusted  nonlinearly  according  to  the  fitness  of  the 
population. The algorithm flow is shown in Fig. 5.
Combining the improved algorithm with trajectory 
planning
Problem description
Assume that the robot performs an action with its end 
effector passing n points (including the start and the end 
points). N points can be converted into n corresponding 
joint variables of the joint space by the inverse kinemat-
ics of the robot, that is produced n − 1 time segment with 
length  of  Ti(i = 1, 2, 3, . . . , n − 1) ,  Ti = ti+1 − ti ,  where 
x  represents  the  moment  when  the  robot  end  effector 
reaches the i-th path point. The total time is:
Ti
n−1
i=1
(14)
is  the  objective 
where  T  is  the  total  time  of  the  robot  movement 
which 
function  of  the  problem; 
Ti (i = 1, 2, 3 . . . , n − 1)  is  the  time  interval  of  the  joint 
variable; the constraint is the maximum angular velocity, 
acceleration and jerk of the joints of the robot. Therefore, 
the problem of time optimal trajectory planning for robot 
is described as follows:
(1) Objective function:
 
min T =
n−1
i=1
(2) constraint condition:
Ti
(15)
(A)  angular velocity
 
 ∀t ∈ [ti, ti+1]
|θ ′
where θ ′
by the arm joint.
(B)  acceleration
 
i (t)| ≤ θ ′
(16)
max is the maximum angular velocity allowed 
max
 ∀t ∈ [ti, ti+1]
|θ ′′
i (t)| ≤ θ ′′
(17)
max  is  the  maximum  angular  acceleration 
max
where  θ ′′
value allowed by the robot joint.
Zhang et al. Robot. Biomim.  (2018) 5:3 
Page 5 of 7
Fig. 5  Cosine genetic algorithm evolution process
(C) 
jerk 
 ∀t ∈ [ti, ti+1]
|θ ′′
where θ ′′′
arm joint.
i (t)| ≤ θ ′′
max is the maximum jerk value allowed for the 
max
(18)
Time‑optimal simulation
According to the parameters of the PUMA560 robot, we 
can get the constraints of the robot joints (see Table 1).
The  improved  algorithm  is  coded  based  on  the  real 
coding, and the parameters are selected as follows: pop-
ulation  size  M = 80;  maximum  crossover  probability 
Pc1 = 0.9 ,  minimum  value  Pc2 = 0.4 ,  Pc0 = 0.7 ;  maxi-
mum  mutation  probability  Pm1 = 0.1 ,  minimum  value 
Pm2 = 0.01 ,  Pm0 = 0.7 ;  Evolutional  generation  G = 100. 
A MATLAB program for the optimal trajectory planning 
of the first three joints of PUMA560 is written by com-
bining  the  quintic  polynomial  interpolation  trajectory 
[15, 16]. In the optimization process, the trajectory of the 
robot joint is composed of the seven-segment polynomial 
curve,  and  its  optimization  precision  is  0.001  s.  The 
results are shown in Table 2.
It  can  be  seen  from  Table  2  that  after  the  optimiza-
tion of the running time of the robot, the time taken by 
the robot to reach the target point is obviously reduced 
under the constraint of the joint angular velocity, accel-
eration  and  jerk  of  the  robot.  For  the  first  three  joints, 
the  total  time  of  the  original  run  is  28  s.  After  optimi-
zation of this method, it is shortened to 13.729, 14.381 
Table 1  The constraints of the robot joints
Joint i
Constraints
θ ′(t)/(◦/s)
θ ′′(t)/(◦/s2)
θ ′′′(t)/(◦/s3)
1
2
3
4
5
6
100
95
100
150
130
110
45
40
75
70
90
80
60
60
55
70
75
70
Zhang et al. Robot. Biomim.  (2018) 5:3 
Page 6 of 7
Table 2  Optimization Results
Time interval
Joint i
Initial value
1
h1
h2
h3
h4
h5
h6
h7
Total time (s)
4
4
4
4
4
4
4
28
2.785
2.032
2.316
1.578
1.824
1.487
1.707
13.729
100
)
°
(
n
o
i
t
i
s
o
P
l
 
r
a
u
g
n
A
80
2
2.113
2.128
1.764
1.918
1.706
1.874
2.878
14.381
3
3.349
2.064
1.626
1.659
2.113
1.629
2.208
14.648
Joint 1 
5
0
-5
Position
Velocity
Acceleration
← A
← V
← P
)
2
s
/
°
(
n
o
i
t
l
a
r
e
e
c
c
A
l
 
r
a
u
g
n
A
 
/
 
)
s
/
°
(
y
t
i
l
c
o
e
V
l
 
r
a
u
g
n
A
60
0
0
2
2
4
4
6
6
8
8
Time (s)
10
10
12
12
-10
14
14
Fig. 6  Kinematic curve of joint 1
and 14.648 s, which was at least 47.7% shorter than 28 s. 
Compared with several literatures that use PUMA560 as 
the object for MATLAB simulation, the same constraints 
apply. The reference literature [17] describes the method 
of interpolating the trajectory of the robot with seventh-
order  B-spline  curves  and  optimizing  the  robot  trajec-
tory using the genetic algorithm. After optimization, the 
time is shortened from the original 20–15.620 s, which is 
shortened by 21.9%. In reference literature [18], a cubic 
B-spline  curve  is  used  to  interpolate  the  robot  motion 
trajectory,  and  then  an  improved  genetic  algorithm 
based on the crossover operator and mutation operator 
adjusted  with  evolutionary  algebraic  average  fitness  is 
used to perform the time optimal trajectory planning for 
the robot motion trajectory. The time after optimization 
was  shortened  from  the  original  20–13.072  s,  a  34.6% 
reduction. It can be seen that the improved genetic algo-
rithm,  which  the  crossover  operator  and  the  mutation 
operator  in  the  general  adaptive  genetic  algorithm  are 
adjusted cosine, based on quintic polynomial interpola-
tion described in this paper satisfies the goal of the short-
est time trajectory planning.
Simulation and analysis of smoothness of joint operation
The optimization of the robot’s running time is shown in 
the previous section, and then, we simulate the angular 
displacement, velocity and acceleration curves of the first 
three joints of the robot.
From  Figs.  6,  7  and  8,  it  can  be  seen  that  the  angu-
lar  displacement,  velocity  and  acceleration  curves  are 
smooth  and  can  reduce  shock  and  impact  of  the  robot 
arm and ensure smooth operation of the robot.
Conclusions
Based on the PUMA560 robot model, this paper briefly 
describes the method of locating the trajectory with the 
quintic polynomial interpolation in the joint space. Then, 
the crossover operator and the mutation operator in the 
general  adaptive  genetic  algorithm  are  adjusted  cosine 
Joint 2 
← A
← V
-40
-50
-60
-70
)
°
(
n
o
i
t
i
s
o
P
l
 
r
a
u
g
n
A
Joint 3 
Position
Velocity
Acceleration
 ← A 
 ← P
 ← V
50
)
°
(
n
o
i
t
i
s
o
P
l
 
r
a
u
g
n
A
0
Position
Velocity
Acceleration
2
0
)
2
s
/
°
(
n
o
i
t
a
r
e
e
c
c
A
l
-2
-4
← P
l
 
r
a
u
g
n
A
 
/
 
)
s
/
°
(
y
t
i
l
c
o
e
V
l
 
r
a
u
g
n
A
5
 
0
-5
)
2
s
/
°
(
n
o
i
t
a
r
e
e
c
c
A
l
l
 
r
a
u
g
n
A
 
/
 
)
s
/
°
(
y
t
i
l
c
o
e
V
l
 
r
a
u
g
n
A
-80
0
0
2
2
4
4
6
6
8
8
Time (s) 
10
10
12
12
-6
14
14
Fig. 7  Kinematic curve of joint 2
 
0
0
2
2
4
4
6
6
8
8
Time (s) 
10
10
12
12
14
14
-10
Fig. 8  Kinematic curve of joint 3
to improve the performance of the algorithm. Using the 
improved  algorithm  to  optimize  the  interpolation  time 
of  the  robot  trajectory,  the  simulations  show  that  the 
Zhang et al. Robot. Biomim.  (2018) 5:3 
Page 7 of 7
running time of each joint of the robot has been greatly 
reduced. The angular displacement, velocity and acceler-
ation curve of the joint operation shows that the smooth-
ness of the robot is better and the oscillations and shocks 
of  the  manipulator  can  be  reduced.  In  conclusion,  the 
method of this paper realizes the optimal trajectory plan-
ning target for robot time.
Authors’ contributions
JYZ completed the system modeling and the main algorithm improvement. 
QXM worked on algorithm optimization and simulation. XGF and HS studied 
the improved method of adaptive genetic algorithm, analyzed the simula-
tions and revised the manuscript. All authors read and approved the final 
manuscript.
Acknowledgements
This research study was supported financially by the National Natural Science 
Foundation of China under Grant No. 61473171 and the Natural Science 
Foundation of Anhui Province (KJ2015A058).
Competing interests
The authors declare that they have no competing interests.
Ethics approval and consent to participate
Not applicable.
Funding
This research has been funded by the National Natural Science Foundation of 
China under Grant No. 61473171 and the Natural Science Foundation of Anhui 
Province (KJ2015A058).
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in pub-
lished maps and institutional affiliations.
Received: 8 November 2017   Accepted: 14 May 2018
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