Dynamic Simulation - Output Grapher question

[Wapster]

Hi there,

I’m having trouble understanding the Output Grapher in Dynamic Simulation.

I have a door, which I’m going to open and close using an actuator.

I set it up in Simulation and use an imposed motion on the prismatic joint in the actuator.

Then I use the output grapher to look at the force (Z) of the joint, needed for the actuator.

When I look at the force, there is a negative force showing.

How is that possible? I’m stuck on this one.

If there is anyone who can tell me what I’m doing wrong I would be very thankful.

Cheers,

Wap.

[admaiora]

Hi Wap,

it's correct.

Each Joint has its reference (created at the joint creation)

1 arrow  x

2 arrows y

3 arrows z

[Wapster]

Hi admaiora,

Thank yo for looking in to this.

I don't know if I understand your answer tough.

I know the joint is OK, but why is the force negative in the Grapher..?

The model is a simple model now, but I'm gonna add glass, (wind) pressure, and friction later.

The goal here is to find out if the actuator is strong enough but I first wanna know if I'm "reading" the Grapher the right way.

A negative force suggests that the door would open by itself (the first few seconds) only using gravity

I just don't believe that (and tested it removing the imposed motion), so I must be doing something wrong.

Thanks again,

Wap.

[admaiora]

Hi Wap,

you have set an imposed motion (velocity, 30 mm/s).

To ensure that, it's necessary a force (mainly) oriented in the opposite direction of the three joint blue arrows.

Opposite direction means negative.

If i reverse the prismatic joint orientation and velocity

[Wapster]

Hi,

It seems that when it comes to DS you’re on a much higher level then me.

So.... could you perhaps tell me how much force needs to be applied on (the back off) the actuator to open the door in the current setup (no other loads applied).

Many thanks in advance.

Wap.

[admaiora]

Hi Wap,

you have done  a globally good dyn sim work.

Just some consideration:

- there is not actual opposing forces (no forces, no frictions)

- conseguence there are only inertial forces, (not always so easy to be interpreted)

- as consequence,  you have a velocity of 30 mm/s, if you reduce it you will need a reduced cylinder axial force (it will tend to 0)

I suggest to suppress contact joint in this fase, because they can push result in an system like that.

[jan_priban]

Hi Wap,

basically when moving part (you door) is decreasing speed - decelerates, then force can be negative value. I think it could be your case, doors at initial position move faster than in the end position while piston speed/imposed motion is constant. See please the attached image what I am trying to explain. Speed in position 1 is faster than speed in position 2, due to angle change.

Regards

Jan Priban, Autodesk Inventor team

[jan_priban]

Hi Wap,

see please my better answer/update

I created similar model, 

And define imposed motion like you. Graph shows angular velocity of rotating door, see it is decreasing, means deceleration from start to time 2.8 sec. Than angular speed is growing, door accelerates. That's why Imposed motion force or FZ in prismatic joint has negative value to t= 2.8 s, after that positive.

and

 

Regards

Jan Priban