I would like to animate petals or weeds like this images. If arrow comes closer, petals open, or weeds move away from the arrow. What is the best way? I need to use those bones, so shape keys can be used. I tried to make it using damped track constraint, but it's really hard to control, so I am trying other way.
You can use drivers as recommended by Chris, or you can use the Transformation constraint:
First create the armature with the controller and the chain of bones, give a Copy Rotation to bone 2 and 3 with bone 1 as Target:
Select the controller bone, open the N panel, put your mouse over the Transform > Y location, right click and choose Copy as New Driver:
Then select the first bone and in the N panel right click on the Z rotation value and choose Paste Driver, the value will turn purple:
You can change the driver parameters: Open the Drivers editor, open the N panel > Modifiers, create a Generator, and play with the X^1 value in order to increase or invert the influence of the controller:
You cand also use the Transformation constraint:
Give a Transformation constraint to bone 1, in Local Space/Local Space, Map From > Location and set the Y location to 0 and 1 (or whatever value in local space), Map To > Rotation and set the Z Source Axis to Y (as it is the Y location of the controller that will determine the Z rotation) and choose the Max Angle of rotation:
Here is what it gives when you move the controller:
Now just duplicate the chain of bones in Edit mode and rotate them with CtrlR.
Other answers here are fine, but my personal way to do this would be via locked track and copy rotation constraints.
First, we'll need to set the axes of our bones so that they're pointing at the center point. We can do that by snapping cursor to our control bone and then recalculating roll to cursor. Now all of our bones' +Z axes are pointing at the control:
We'll give each petal root a locked track constraint, lock X, track +Z, targeting our control. We can do this on one bone, and then use copy constraints to selected to apply this to all roots:
Now we can give our child petal bones copy rotation constraints, local->local, targeting their roots:
Now, when we move our control bone, our roots will rotate along their X axes to track it, and their children will bend in and out to follow: