Specifications

218
The analysis runs in time from zero to one second, and the pedal angle develops in this time from 100
degrees (1.74 rad) to 145 degrees (2.53 rad). Let us presume that the pedal is loaded by a linear spring
that is slack at 0 degrees and increases its moment linearly with the rotation of the hinge. We might be
wondering: What would be a comfortable spring stiffness for a pedal like that? Not having much experience
with pedal design it might be difficult to imagine just how stiff the spring should be, and we could find
ourselves developing a series of hardware prototypes with different springs and perhaps conducting
subjective investigations with test subjects. A simple task like this could potentially be very time consuming
and expensive.
Let us do it with AnyBody instead. We shall start out by declaring an AnyForce to play the role of the spring.
Since this is not a part of the body it is logical to place it in the Environment.any file. Here's what to add:
AnyRevoluteJoint HingeJoint = {
Axis = z;
AnyFixedRefFrame &Ground = .GlobalRef;
AnyRefNode &Pedal = .Pedal.Hinge;
};
AnyForce Spring = {
AnyRevoluteJoint &Hinge = .HingeJoint;
F = -0.0*.HingeJoint.Pos;
};
This looks easy, does it not? The AnyForce contains a reference to the HingeJoint. Since the degree of
freedom in HingeJoint is rotational, the force is automatically turned into a moment and applied it to the
hinge. The specification of F is the actual size of the force. We have made it proportional to the
HingeJoint.Pos, which is the hinge angle, and we have initially set the spring stiffness to 0.0, to investigate
the effect of having no spring before we start adding a spring force. Notice, by the way, the minus sign in
front of spring constant. It has no importance now, but when we start adding non-zero stiffnesses it will
signify that the spring force goes against the angle, i.e. pushes back onto the foot.
There are just a couple of things we need to do before we can do the InverseDynamicAnalysis operation and
compute the forces: All the drivers we added in the previous lesson have motors built into them. This means
that whatever force or moment is necessary to perform the movement will be provided by the drivers, and
there will be nothing for the muscles to do. Motors in drivers are technically reaction forces, and they can be