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| R2011b Documentation → SimMechanics | |
Learn more about SimMechanics |
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Ground and Body Geometry Errors Block Connection and Topology Errors |
SimMechanics simulations can stop before completion with one or more error messages. This section discusses generic error types, and most errors and error-fixing strategies fall into broad categories. These groupings are reflected in the keywords occurring in the error messages. These sections summarize these groupings.
The previous sections, Configuring Methods of Solution and How SimMechanics Software Works, should be useful for identifying and tracing errors. Many common errors also appear in Representing Machines with Models and Validating Mechanical Models in the Modeling Mechanical Systems chapter.
Improving Performance following discusses strategies that can prevent errors.
Every numerical entry you make in a SimMechanics model must be a real numerical expression or MATLAB® equivalent. Spatial vectors are 3-vectors, such as [3 4 5]. Spatial tensors are 3-by-3 matrices, such as rotation matrices and the inertia tensor.
Every machine must have a least one Ground block. Every Body block must have at least one Body CS, defined at the body's center of gravity (CG). You must directly or indirectly define the Body coordinate systems (CSs) of a machine relative to a Ground or to World. You cannot enter cyclic (circular) Body CS definitions. The Body CS definitions must separately satisfy these criteria in the Position and Orientation tabs of the Body dialog.
For example, defining CS3 relative to CS2, defining CS2 relative to CS1, then defining CS1 relative to CS3, results in a definition that is both cyclic and missing any reference to a Ground or World. You can break the cycle by referencing CS1 to a Ground or to World.
To be displayed in visualization, a Body must be connected to at least one Joint that is connected to the rest of the machine. You cannot visualize with equivalent ellipsoids a body whose principal inertial moments do not satisfy the triangle inequalities. (See About Body Color and Geometry in the Introducing Visualization and Animation chapter.)
The geometric configuration of joints, constraints, and drivers can conflict with assembly requirements and restrictions on certain blocks.
Assembled joints must satisfy assembly tolerances on their connected Body CSs at all times. Disassembled joints assembled at model initialization must also satisfy assembly tolerances during the simulation. (See Controlling Machine Assembly in this chapter.)
The initial distance between two Body CS origins connected by a massless connector must be nonzero. The massless connector holds the distance between two Body CS origins constant during motion.
Certain composite Joint blocks place restrictions on their primitive joint axes. For example, Bearing must have its prismatic axis P1 aligned to its third revolute axis R3.
General rules on how to connect SimMechanics blocks are discussed in the Modeling Mechanical Systems chapter. In particular, consult these sections of that chapter:
Some restrictions are properties of individual blocks, as explained in their reference pages. See the SimMechanics block reference.
Inconsistencies in motion arise from misapplication of constraints, drivers, and actuators, from conflicting stiction requirements, and incorrect simulation dimensionality.
Motion simulation errors often occur because of singularities or dividing by very small numbers. Simulink solvers can integrate certain singularities, at a cost. Others, like loss of a degree of freedom (DoF), can be fatal. See Maintaining Constraints, Configuring a Simulink Solver, and Smoothing Motion Singularities in this chapter and the Machine Environment block reference.
A body moving on a prismatic axis must have nonzero mass if you actuate it with forces. A body rotating about a revolute axis or pivoting about a spherical must have nonzero inertial moments about the axis or pivot if you actuate it with torques. If you want a massless rigid body, consider using a Massless Connector from the Joints/Massless Connectors sublibrary.
Note You can use point bodies (nonzero mass but zero moments) in SimMechanics models, if the connected revolute axes and spherical pivots are dislocated from the body. Although the moments are zero about a point body's CG, the displacement of the body from the axis or pivot shifts the moments from zero to nonzero values. |
Within a single Joint block, two distinct prismatic axes or two distinct revolute axes should never align during the simulation. If either occurs, a translational or rotational DoF is lost, and the simulation cannot determine the subsequent motion. An example of such singularities is "gimbal lock." Two of the three revolute primitive axes in the Gimbal block become parallel, reducing the number of independent DoFs in the Joint from three to two.
Two or more physically identical bodies (having the same masses and inertia tensors) should never coincide in space.
Your machine cannot move if it has no degrees of freedom. Each Constraint, Driver, and motion-actuating Actuator block you add to a machine reduces the number of independent DoFs. (See Counting Model Degrees of Freedom in the Modeling Mechanical Systems chapter.) Cure such errors by removing one or more of these blocks from your machine, until you have at least one independent DoF.
You cannot run a three-dimensional machine with a simulation restricted to two dimensions. See Choosing Your Machine's Dimensionality in this chapter.
Some constraints can restrict what another constraint is already restricting. If redundant constraints are present and in conflict, fix these errors by identifying and removing the redundancies. If the simulation misidentifies one or more redundant constraints, adjust the redundant constraint tolerance. See Maintaining Constraints in this chapter.
Some machine motions or simulations might not be able to maintain assembly tolerances at a particular simulation step while simultaneously satisfying the constraints. One or more joints might become disassembled. Any one of these conditions leads to errors.
You can correct this situation in several ways. First, identify the joint, constraint, or driver causing the error and examine its physical configuration when the error occurs to isolate the conflict. Then try any combination of these steps:
Decrease the Simulink solver tolerances or the step size.
Switch to a more robust Simulink solver.
Decrease the constraint solver tolerances.
Increase the redundant constraint tolerance.
Switch to the machine precision constraint solver.
Increase the assembly tolerances.
See Maintaining Constraints and Configuring a Simulink Solver in this chapter.
You cannot put more than one actuator on a joint primitive.
Exceptions You can simultaneously place an initial condition actuator and a force/torque actuator on a joint primitive. The Joint Stiction Actuator block does accept an input signal for nonfrictional forces/torques, which the block adds to the stiction. |
If your machine has two or more stiction-actuated ("sticky") joints, a conflict among them can put the simulation into an infinite loop and prevent determination of the machine motion. Or one locked joint can prevent the other joints, sticky or not, from moving. The machine stops moving.
For example, one sticky joint becomes unlocked and requires the other to lock, which then requires the first to lock.
Remove these conflicts by removing one or more stiction actuators or by changing the Joint Stiction Actuator locking thresholds.
Certain restrictions apply to the analysis modes presented in Choosing an Analysis Mode in this chapter. Consult individual analysis modes for more:
 | How SimMechanics Software Works | Improving Performance |  |
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