SimMechanics    
Velocity Driver

Specify a linear combination of the linear and angular velocities of two bodies as a function of time

Library

Constraints & Drivers

Description

The Velocity Driver block drives a linear combination of the projected translational and angular velocities of two Bodies. The velocities are projected by inner products on to constant vectors you specify.

Let vB, vF be the two body velocity vectors and B, F be the two body angular velocity vectors. Let cB, cF, dB, dF be constant vectors. The subscripts `B' and `F' refer to base and follower bodies. The Velocity Driver block specifies this linear combination:

as a function of time f(t). You specify the vectors cB, cF, dB, dF. You also connect the Velocity Driver to a Driver Actuator block.

The Simulink input signal into the Driver Actuator specifies the time-dependent driving function f(t) and its first two derivatives, as well as their units. If you do not actuate Velocity Driver, this block acts as a time-independent constraint that freezes the constraint linear combination at its initial value during the simulation.

Drivers restrict relative degrees of freedom (DoFs) between a pair of bodies as specified functions of time. Locally in a machine, they replace a Joint as the expression of the DoFs. Globally, Driver blocks must occur topologically in closed loops. Like Bodies connected to a Joint, the two Bodies connected to a Drivers are ordered as base and follower, fixing the direction of relative motion.

You can also connect a Driver block to a Constraint & Driver Sensor, which measures the reaction forces/torques between the driven bodies.

Dialog Box and Parameters

The dialog box has two active areas, Connection parameters and Parameters.

Connection Parameters

Current base
When you connect the base (B) Connector Port on the Velocity Driver block to a Body CS Port on a Body, this parameter is automatically reset to the name of this Body CS. See the following Velocity Driver base and follower Body Connector Ports figure.
Current follower
When you connect the follower (F) Connector Port on the Velocity Driver block to a Body CS Port on a Body, this parameter is automatically reset to the name of this Body CS. See the following Velocity Driver base and follower Body Connector Ports figure.
Number of sensor/actuator ports
Using this spinner menu, you can set the number of extra Connector Ports needed for connecting Driver Actuator and Constraint & Driver Sensor blocks to this Driver. The default is 0.
To activate the Driver, connect a Driver Actuator.

The base (B)-follower (F) Body sequence determines the sense of positive motion. Positive translation is the follower moving in the direction of the translation axis. Positive rotation is the follower rotating in the right-handed sense about the rotation axis.

Velocity Driver base and follower Body Connector Ports

Parameters

Angular velocity units
From the pull-down menu, choose the common units for all angular velocities. The default is deg/s (degrees/second).
The vectors dB and dF implicitly carry the units conversion of length/angle. The driving function f(t) has the linear velocity units that you set in the Driver Actuator block that you connect to Velocity Driver. If the f(t) units differ from the units set in Linear velocity units in this dialog box, the vectors dB and dF implicitly carry the additional units conversion.
Linear velocity units
From the pull-down menu, choose the common units for all linear velocities. The default is m/s (meters/second).
The driving function f(t) has the linear velocity units that you set in the Driver Actuator block that you connect to Velocity Driver. If the f(t) units differ from the units set here, the vectors cB and cF implicitly carry the units conversion.
Velocity coefficients for base
Under [x y z], enter the Angular velocity and Linear velocity coefficient vectors for the base Body. These are the components of dB and cB, respectively. The defaults are [1 0 0].
In the pull-down menus, choose the coordinate systems (WORLD or BASE) whose coordinate axes the vectors dB and cB are oriented with respect to. The defaults are WORLD.
The vectors dB and cB carry the implicit units conversion to convert all velocities to the common linear velocity units of f(t) that you set in the connected Driver Actuator block.
Velocity coefficients for follower
Under [x y z], enter the Angular velocity and Linear velocity coefficient vectors for the follower Body. These are the components of dF and cF, respectively. The defaults are [1 0 0].
In the pull-down menus, choose the coordinate systems (WORLD or FOLLOWER) whose coordinate axes the vectors dF and cF are oriented with respect to. The defaults are WORLD.
The vectors dF and cF carry the implicit units conversion to convert all velocities to the common linear velocity units of f(t) that you set in the connected Driver Actuator block.

See Also

Angle Driver, Constraint & Driver Sensor, Driver Actuator, Parallel Constraint

See Modeling Constraints and Drivers for more on restricting DoFs with Drivers.

See Checking Schematic Topology and How SimMechanics Works for more on using drivers in closed loops.

See Creating Constraints and Drivers.


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