Constant Jerk Trajectory Generator

Сентябрь 14, 2022

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Constant Jerk Trajectory Generator

Содержание

2. In particular, you will Determine why S-curves are necessary Review the ideal S-curve. Consider constant acceleration
3. Why S-curves? Reviewing the trapezoidal trajectory profile in speed v, we examine points 1, 2, 3,
4. Ideal S-curve t t = T vo vs v 1 as ar 1 Concave Convex t
5. Ideal S-curve equations The form assumed for the S-curve velocity profile is v(t) = co +
6. Concave period The concave conditions are v(0) = vo a(0) = 0 a(T/2) = as j(0)
7. Concave period Applying the initial and final conditions, we get the equations for s (position), v,
8. Ideal S-curve observations If we let Δv = vs - vo and define ar = Δv/T
9. Convex period This period applies for T/2 ≤ t ≤ T. Letting time be zero measured
10. Convex period Applying the initial and final conditions, we get the equations for s (position), v,
11. Distance traversed Adding in the distance at the halfway point gives the total distance traversed in
12. Max jerk transitions An ideal S-curve cannot transition smoothly between any speed change using a specified
13. Max jerk transitions Given a jerk jm, a starting speed vo, and the ending speed vs,
14. Speed transitions If v1 > v2 (overlap), we can determine an intermediate transition point using speed
15. Speed transitions
16. Speed transitions The pertinent equations are: vo + ao Tt + jm Tt 2/2 = vs-
17. S-curve with linear period If v1
18. S-curve with linear period Motion conditions: Phase 1 - Concave Phase 2 – Linear Phase 3
19. S-curve with linear period Phase 1 – Concave motion conditions: s(t) = vo t + jm
20. S-curve with linear period Phase 2 – Linear motion conditions: s(t) = v1 t + as
21. S-curve with linear period Phase 3 – Convex motion conditions: s(t) = v2 t + as
22. S-curve context How is the S-curve applied in the real world? Robots and machine tools are
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In particular, you will
Determine why S-curves are necessary
Review the ideal

S-curve.
Consider constant acceleration jerk transitions.
Consider the speed transition when the velocity change is too small to reach the desired accel (or decel) value.
Consider the trajectory generator in the context of joint moves or curvilinear moves.

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Why S-curves?
Reviewing the trapezoidal trajectory profile in speed v, we

examine points 1, 2, 3, and 4. Each of these points has a discontinuity in acceleration. This discontinuity causes a very large jerk, which impacts the machine dynamics, also stressing the machine’s mechanical components.
An S-curve is a way to impose a limited jerk on the speed transitions, thus smoothing out the robot’s (or machine tool) motion.

The trapezoidal profile to the right was the trajectory generator of choice for many years, but is now being replaced by S-curve profiles. Why?

Area under curve is move distance

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Ideal S-curve
t
t = T
vo
vs
v
1
as
ar
1
Concave
Convex
t
j
T/2
T/2
jm
T

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Ideal S-curve equations
The form assumed for the S-curve velocity profile

is
v(t) = co + c1t + c2 t2 (5.1)
giving the acceleration and constant jerk equations:
a(t) = c1 + 2 c2 t (5.2)
j(t) = 2 c2 (5.3)
The rise motion can be divided into 2 periods - a concave period followed by a convex period.

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Concave period
The concave conditions are
v(0) = vo
a(0) = 0
a(T/2) = as
j(0)

= jm
where jm is the jerk set for the profile (near the maximum allowed for the robot), and as is the maximum acceleration encountered at the S-curve inflection point.

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Concave period
Applying the initial and final conditions, we get the equations

for s (position), v, and a along the concave portion of the S-curve:
s(t) = vo t + jm t3/6 (5.7)
v(t) = vo + jm t2/2 (5.8)
a(t) = jm t (5.9)
Note: It is assumed that s is 0 at the beginning of the S-move. Thus, s represents a position delta.

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Ideal S-curve observations
If we let Δv = vs - vo

and define ar = Δv/T to be the acceleration of a constant acceleration ramp from vo to vs, then we note that as is twice ar. It is also true that T = 2Δv/as.
The trapezoidal profile can be used to predict the time and distance required to transition the accel and decel periods of the ideal S-curve. This exercise is commonly called motion or path planning.

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Convex period
This period applies for T/2 ≤ t ≤ T. Letting

time be zero measured from the beginning of the convex period (0 ≤ t ≤ Τ/2), the pertinent motion conditions are:
v(0) = vh = (vs + vo)/2
a(0) = as
a(T/2) = 0
j(0) = -jm

where -jm is the jerk set for the profile, and as is the maximum acceleration encountered at the S-curve inflection point.

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Convex period
Applying the initial and final conditions, we get the equations

for s (position), v, and a along the convex portion of the S-curve:
s(t) = vh t + as t2/2 - jm t3/6 (5.11)
v(t) = vh + as t - jm t2/2 (5.12)
a(t) = as - jm t (5.13)
Note: It is assumed that s is 0 at the beginning of the S-move. Thus, s represents a position delta.

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Distance traversed
Adding in the distance at the halfway point gives

the total distance traversed in the S-curve, including both concave and convex sections:
S = (vs2 - vo2)/as

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Max jerk transitions
An ideal S-curve cannot transition smoothly between any

speed change using a specified max jerk value!
Why?

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Max jerk transitions
Given a jerk jm, a starting speed vo,

and the ending speed vs, we can determine v1 and v2, where these are the velocities that end the concave transition and begin the convex transition at max accel as for the ideal S-curve transition:
v1 = vo + as2/(2jm)
v2 = vs- as2/(2 jm)
By setting v1 = v2, we can also determine the max jerk for a given as and Δv = vs - vo:
jm = as2 /Δv

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Speed transitions
If v1 > v2 (overlap), we can determine an

intermediate transition point using speed and acceleration continuity.
Note that the velocity and acceleration for the previous concave curve and the new convex curve must be equal at Tt where the velocity is vt. We cannot reach the maximum acceleration as by applying maximum jerk transitions. Nevertheless, there exists a point where the concave profile will be tangent to the convex profile. This point will lie between vo and vs. At this point the acceleration and speed of both profiles are the same, although there will be a sign change in jerk.

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Speed transitions

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Speed transitions
The pertinent equations are:
vo + ao Tt + jm

Tt 2/2 = vs- jm (T - Tt)2/2 (5.20)
ao + jm Tt = jm (T - Tt) (5.21)
Solving these we get:
T = [-ao + sqrt( 2 ao2 + 4 Δv jm) ]/ jm (5.22)
Tt = (jm T - ao)/(2 jm) (5.23)
where Δv = (vs - vo).

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S-curve with linear period
If v1 < v2, then we must insert

a linear (constant acceleration) period. The desired maximum S accel (as) is known, as is the maximum jerk (jm).

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S-curve with linear period
Motion conditions:
Phase 1 - Concave Phase 2 –

Linear Phase 3 - Convex
0 ≤ t ≤ t1 0 ≤ t ≤ T -2t1 0 ≤ t ≤ t1 _________________________________________________________________________________________________________________

v(0) = vo
a(0) = 0
a(t1) = as
j(0) = jm
v(t1) = v1

v(0) = v1
a(0) = as
a(T-2t1) = as
v(T-2t1) = v2

v(0) = v2
a(0) = as
v(t1) = vs
a(t1) = 0
j(0) = - jm

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S-curve with linear period
Phase 1 – Concave motion conditions:
s(t) = vo

t + jm t3/6 (5.25)
v(t) = vo + jm t2/2 (5.26)
a(t) = jm t (5.27)

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S-curve with linear period
Phase 2 – Linear motion conditions:
s(t) = v1

t + as t2/2 (5.30)
v(t) = v1 + as t (5.31)

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S-curve with linear period
Phase 3 – Convex motion conditions:
s(t) = v2

t + as t2/2 - jm t3/6 (5.35)
v(t) = v2 + as t - jm t2/2 (5.36)
a(t) = as - jm t (5.37)

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S-curve context
How is the S-curve applied in the real world?
Robots

and machine tools are commanded to move in either joint space or Cartesian space.
In joint space the slowest joint becomes the controlling move. Its set speed and joint distance is used for the trajectory motion planning. Desired acceleration and jerk values are applied for this joint to specify the S-curve profiles.
In Cartesian space either the path length or tool orientation change dominates the motion. The associated speeds , accelerations, and jerk values specify the S-curve profiles. The trajectory generator processes length or orientation change, whichever is dominant. The other change is proportioned.