Drake
ContactResultantForceCalculator< T > Class Template Reference

This is a utility class for replacing a set of force/torques by an equivalent force/torque (defined by the ContactForce class). More...

#include <multibody/rigid_body_plant/contact_resultant_force_calculator.h>

Public Member Functions

 ContactResultantForceCalculator ()
 Default constructor – no accumulation. More...
 
 ContactResultantForceCalculator (std::vector< std::unique_ptr< ContactDetail< T >>> *detail_accumulator)
 Accumulator constructor. More...
 
void AddForce (const ContactForce< T > &force)
 Adds a new contact force to the calculator. More...
 
void AddForce (std::unique_ptr< ContactDetail< T >> contact_detail)
 Adds a new force to the calculator from a contact detail. More...
 
void AddForce (const Vector3< T > &application_point, const Vector3< T > &normal, const Vector3< T > &force)
 Adds a new force to the calculator. More...
 
void AddForce (const Vector3< T > &application_point, const Vector3< T > &normal, const Vector3< T > &force, const Vector3< T > &pure_torque)
 Adds a new force with an arbitrary pure torque to the calculator. More...
 
ContactForce< T > ComputeResultant () const
 Compute the resultant contact force – it's translational force, pure torque, and application point. More...
 
ContactForce< T > ComputeResultant (const Vector3< T > &reference_point) const
 Computes the resultant contact spatial force with respect to a given reference point. More...
 
Does not allow copy, move, or assignment
 ContactResultantForceCalculator (const ContactResultantForceCalculator &)=delete
 
ContactResultantForceCalculatoroperator= (const ContactResultantForceCalculator &)=delete
 
 ContactResultantForceCalculator (ContactResultantForceCalculator &&)=delete
 
ContactResultantForceCalculatoroperator= (ContactResultantForceCalculator &&)=delete
 

Detailed Description

template<typename T>
class drake::systems::ContactResultantForceCalculator< T >

This is a utility class for replacing a set of force/torques by an equivalent force/torque (defined by the ContactForce class).

See also
ContactForce

The equivalent ContactForce consists of the set's resultant force applied at a point P, together with a torque equal to the moment of the set about P summed with any torques defined in the set members. Point P is chosen to minimize the magnitude of the moment of the normal forces (i.e., P lies along the "central axis" of the normal forces). Note: only the normal components of the input contact forces affect the location of point P. Tangential components of the input contact forces affect the set's resultant, but not the calculation of point P.

Center of Pressure (more precisely center of normal forces for planar contact)

For point P to be a center of pressure (as returned by this class):

  • The normal components of all forces must lie in the same direction, n.
  • The points, where each force is applied, must lie on one plane, F.
  • Plane F must be perpendicular to n. If these conditions are met, P will be the center of pressure and lie on plane F, and the minimum moment due to the normal forces will be zero. Note: this class does not rely on a center of pressure existing.

Usage

The class is designed to be exercised by a contact response model. As each pair of collision elements is evaluated, the contact model should instantiate a ContactResultantForceCalculator. As each contact point between the elements is processed and a contact force is computed, the details of the contact force are provided to the calculator (via calls to AddForce). Currently, the contact force is defined by four values (see ContactForce):

  • position of the force's point of application from a common origin O,
  • normal component of the contact force (i.e., in the normal direction),
  • tangential component of the contact force (e.g., friction force), and
  • Optional torque term (e.g., torsional friction). All input vectors must be expressed in a common frame and the position vector must be measured from that frame's origin.

After all of the forces have been added to the calculator, an equivalent ContactForce (force/torque) can be requested using the appropriate method.

The order in which the forces are added has no impact on the final result.

By default, the contact forces that are added to the calculator are destroyed when the calculator is destroyed. There is an alternative constructor which allows the caller to provide an STL vector which will accumulate the contact information and allow the caller to persist the data beyond the life span of the calculator.

A Single Force

If the set consists of a single contact force, the minimum moment point and resultant force will be the details of that force: i.e., its application point, response force, and pure torque.

A Non-zero Minimum Moment

For an arbitrary set of forces, there may not be a well-defined center of pressure as with the planar case outlined above. Generally, there is an infinite set of minimum moment points for a set of contact forces; it is a line called the "central axis". Any point on this line will lead to the same minimum moment. The ContactResultantForceCalculator selects one of those points.

We assume that the "ideal" point would be where the line intersects the (deformed) contact surface. Generally, this can't be solved because it depends on a geometric query that is outside the scope of this calculator class. Furthermore, in many cases, it is unnecessary. A point on the surface is good for visualization, but in contexts where only a mathematically meaningful point is all that is needed, then one point is as good as another. That said, the calculator employs a method to cheaply approximate the intersection of the line with the contact surface by doing the following.

The central axis can be thought of as a line defined by a point and direction. The point can be any point on the line. The direction is defined by the direction of the resultant normal force (i.e., the vector sum of the normal components of all forces.) The direction vector defines "positive" and "negative" directions on the line. The force originated from the negative direction and accelerates the body in the positive direction. If we had access to the geometry, the point we would be interested in, would be the intersection of the line and (deformed) geometry that is farthest in the "negative" direction (i.e., closest to the originating source of the contact).

We will approximate this by finding the contact force application point that similarly lies farthest in the negative direction (simply by projecting the application points on the line.) This most-negative projection point will serve as the reported minimum moment point.

This reported minimum moment point can be moved along the central axis by the caller if additional information is available. Movement along the axis preserves its "minimal-moment" property. For example, if the caller had access to the (deformed) geometry, the ray defined by the reported minimum moment point and the resultant ContactForce normal direction can be intersected with the geometry to create an alternate, but equally valid, minimum moment point.

A Zero-Resultant Force

It is possible for all of the contact forces to sum up to a zero resultant. But there may still be a resultant moment, i.e., the forces are "coupled". In this case, the minimum moment point can be literally any point in space. In this case, the ContactResultantForceCalculator defines the minimum moment point to be the centroid of all application points (the "average" application point.) Similarly, the normal direction of this resultant force is likewise meaningless but will be set to the +x direction (i.e., <1, 0, 0>).

Computation considerations

Even though the resultant is reported as a ContactForce instance, it should not be construed to mean that the results of the calculator can be meaningfully composed. For example, given a set of contact forces: S = {f_0, ..., f_n-1}, the result of computing the resultant for the set S (i.e., F = ComputeResultant(S) will not necessarily provide the same answer as would be produced by creating two disjoint subsets, S_a and S_b and then performing:

F_a = ComputeResultant(S_a);
F_b = ComputeResultant(S_b);
F_ab = ComputeResultant({F_a, F_b});

Do not expect F to be equal to F_ab.

Template Parameters
TThe scalar type. Must be a valid Eigen scalar.

Instantiated templates for the following ScalarTypes are provided:

  • double

Constructor & Destructor Documentation

Default constructor – no accumulation.

As contact forces are added to the calculator, the force will be added to the set of forces for calculation, but they will be destroyed when the calculator is destroyed.

ContactResultantForceCalculator ( std::vector< std::unique_ptr< ContactDetail< T >>> *  detail_accumulator)
explicit

Accumulator constructor.

This allows the caller to provide a vector into which the contact forces/details can be persisted beyond the life span of the ContactResultantForceCalculator instance. See the various AddForce methods for details on what is placed in the detail_accumulator.

Parameters
detail_accumulatorIf non-null, ContactDetail instances will be appended to the vector as they are assigned to the calculator.

Member Function Documentation

void AddForce ( const ContactForce< T > &  force)

Adds a new contact force to the calculator.

If the calculator was initialized with a detail accumulator, an instance of PointContactDetail, with this contact force, will be appended to that accumulator.

Parameters
forceThe contact force.

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void AddForce ( std::unique_ptr< ContactDetail< T >>  contact_detail)

Adds a new force to the calculator from a contact detail.

The result of ContactDetail::ComputeContactForce will be used in the calculation.

If the calculator was initialized with a detail accumulator, the detail will be appended to that accumulator. Otherwise, the detail will be destroyed at the conclusion of this method invocation.

Parameters
contact_detailThe contact detail which will provide a ContactForce for computation.
void AddForce ( const Vector3< T > &  application_point,
const Vector3< T > &  normal,
const Vector3< T > &  force 
)

Adds a new force to the calculator.

If the calculator was initialized with a detail accumulator, an instance of PointContactDetail, with this contact information, will be appended to that accumulator.

Parameters
application_pointThe application point of the force.
normalThe translational force's unit-length normal direction.
forceThe translational force.
void AddForce ( const Vector3< T > &  application_point,
const Vector3< T > &  normal,
const Vector3< T > &  force,
const Vector3< T > &  pure_torque 
)

Adds a new force with an arbitrary pure torque to the calculator.

If the calculator was initialized with a detail accumulator, an instance of PointContactDetail, with this contact information, will be appended to that accumulator.

Parameters
application_pointThe application point of the force.
normalThe translational force's unit-length normal direction.
forceThe translational force.
pure_torqueThe pure torque for the wrench.
ContactForce< T > ComputeResultant ( ) const

Compute the resultant contact force – it's translational force, pure torque, and application point.

The rotational component of this ContactForce is pure torque only. It does not include an r X f moment term. It will be non-zero due to contributions from sources such as:

 - the minimum moment (which may be non-zero in the general case),
 - the moments induced by the tangential components of the forces
   shifted to the minimum moment point, and
 - the sum of the pure torques of the individual input contact forces.

The responsibility of computing the moment belongs to the code that knows what frame the input contact forces are defined and what the origin around which the moment would be induced.

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ContactForce< T > ComputeResultant ( const Vector3< T > &  reference_point) const

Computes the resultant contact spatial force with respect to a given reference point.

The force part is the summation of all f_i, where f_i is the individual force. The torque part is the summation of all tau_i + (p_i - r) X f_i, where tau_i is the ith pure torque, f_i is applied at p_i, and r is the given reference point.

Parameters
reference_pointis in the same frame as all the individual forces.

The documentation for this class was generated from the following files: