# What are the equations of a s-curve position path?

I have a decoupled mechatronic system with two motors. I want for each motor to give a s-curve path as reference. You can see in the image below the position path, velocity and acceleration. All information that I have are extracted from these plots.

The way I have designed the system, the position is given as the reference. So I need the equations for it. From what I saw, this is what is referred to as a "s-curve" path. The way I understand it, the equation is broken down in to 4 different parts:

• $$0\le t\le 0.05$$
• $$0.05
• $$0.2
• $$t>0.25$$

This seems like a fairly standard equation but I have not been able to find anything that works. I have tried to used the equations from Constant Jerk Equations For A Trajectory Generator1 but the results where not the same. I also found Mathematics of Motion Control Profiles, and maybe I don't understand it, but the equations found there do not seems to be enough for what I want.

Can anybody provide the correct equations (preferably in a generalized form, so I can apply them else where in the future) or some other help?

• This doesn't look like a traditional s-curve used by motion controllers to me, it looks like a cubic spline acceleration profile, where jolt (jerk) is constant. Akshay Kumar's answer shows what an s-curve motion controller profile would normally look like. As Akshay Kumar suggested, a traditional s-curve profile will look like this only when the maximum acceleration needed is less than the acceleration limit defined for moves. – Mark Booth Jan 29 at 16:42
• When creating links it's good to avoid using here or similar as the link text. Links tend to rot and if this happens, here doesn't help anyone find the page. Often missing pages haven't been removed, they have just been moved to another location. If you give the page title as the link text then a search for that text will often find the new location. – Mark Booth Jan 29 at 17:03
• @MarkBooth I am not that well informed to be able to object or agree with what you said regarding the s-curve. However, the jerk is not constant. It is $80m/s^3$ for the first period, $-80m/s^3$ for the third, and zero elsewhere. Regarding the link format you suggested, I thank you. In the future I will try to do it as you proposed. – Dimitris Pantelis Jan 30 at 10:31
• By constant jolt I meant constant within each acceleration and decceleration phase. Obviously jolt has to change when you stop or start accelerating, otherwise you would continue accelerating or deccelerating. *8') Welcome to Robotics by the way, Dimitris Pantelis, I look forward to your contributions in the future. If your first few questions are anything to go by, you're on an interesting journey. – Mark Booth Jan 30 at 10:38

To come up with a mathematical expression of the position reference $$x\left(t\right)$$ as a function of time $$t$$, we can inspect the profile of the acceleration $$a\left(t\right)$$. It is piece-wise linear and defined as follows:

$$a\left(t\right)=\begin{cases} \Delta \cdot t, & t \in \left[0,0.025\right] \\ 2000 - \Delta \cdot \left(t-0.025\right), & t \in \left[0.025,0.05\right] \\ 0, & t \in \left[0.05,0.2\right] \\ -\Delta \cdot \left(t-0.2\right), & t \in \left[0.2,0.225\right] \\ -2000 + \Delta \cdot \left(t-0.225\right), & t \in \left[0.225,0.25\right] \\ \end{cases},$$

where $$\Delta=\frac{2000}{0.025} \cdot \frac{\text{mm}}{\text{s}^3}$$.

If we now integrate $$a\left(t\right)$$ twice in $$t$$, we will get two expressions representing the velocity $$v\left(t\right)$$ and the position $$x\left(t\right)$$, respectively, that contain $$10$$ unknown constants in total. These unknowns can be easily retrieved if we guarantee that both $$v\left(t\right)$$ and $$x\left(t\right)$$ are continuous at the extrema of the time intervals given above.

Alternatively, you may consider letting the machine do the integration for you, offline or in real-time, depending on your kind of application. Your code might look something like this:

float get_acceleration(const float t) {
// put here the code implementing the above
// definition of the acceleration profile

float delta = 2000.0f / 0.025f;
float a;
if (t <= 0.025f) {
a = delta * t;
}
else if (t <= 0.05f) {
a = 2000.0f - delta * (t - 0.025);
}
// ...
return a;
}

int main() {
float T = 0.001f;    // the integration time
float v = 0.0f;      // the starting velocity
float x = 0.0f;      // the starting position
float t = 0.0f;      // the increasing time

while (t <= 0.25f) {      // the main control loop
float a = get_acceleration(t);
v += a * T;
x += v * T;
// use x to feed the motor controller
// (or store it for later usage)

// make sure that you wait T seconds
// to enforce the control rate (use e.g. a thread)
t += T;
}
return 0;
}

`
• hmm I see. So what I needed to do is go from the accelerations to towards the position. Initially, I though that one should go from position towards acceleration (by taking derivatives). I now see that it was incorrect. Thanks for the equations and the code, super helpful. – Dimitris Pantelis Jan 29 at 12:40

The S-Curve profile can have several divisions along the time axis

• 7 divisions as per this image. This example has a constant positive jerk zone, a constant acceleration zone, a constant negative jerk zone, a zero acceleration zone and then the vice-versa. This is the S-Curve in its most general form • 5 divisions if there exists no constant acceleration zones like t1-t2 and t5-t6

• 4 divisions if there exists no constant velocity (zero acceleration zone) like t3-t4

There are more possible cases that depend upon initial and final desired conditions and constraints on the positions, velocities, accelerations and jerks.

While the equations are pretty straight forward, mentioned in this paper Jeong, Soon Yong, et al. "Jerk limited velocity profile generation for high speed industrial robot trajectories." IFAC Proceedings Volumes 38.1 (2005): 595-600. for almost all possible test cases, there are issues with constant jerks and other constraints that limit the feasibility of an S-curve.

• thanks for the theoretical back ground. Properly understanding the relationship between Jerk-Acceleration-Velocity-Position is very useful. Thanks! – Dimitris Pantelis Jan 29 at 12:42
• 7, only 7 divisions or segments... – 50k4 Jan 30 at 12:44
• Yes, it was a typo. It should have been 7 divisions and 8 timestamps. Thanks for pointing out. However, as mentioned in my answer, there can be lesser number of divisions as well. – Akshay Kumar Jan 30 at 17:52