Measurement Frames

You can sense the spatial relationship between two frames. When you do so, Simscape™ Multibody™ resolves the measurement in a measurement frame. For most joint blocks, the measurement frame is the base frame. However, if you use either Transform Sensor or a joint block with a spherical primitive, you can select a different measurement frame. Measurement frames that you can select include Base, Follower, and World. The Transform Sensor block adds the choice between rotating and non-rotating versions of the base and follower frames.

Measurement Frame Purpose

The measurement frame defines the axes that Simscape Multibody uses to resolve a measurement. The measurement still describes the relationship between base and follower frames. However, the X, Y, and Z components of that measurement refer to the axes of the measurement frame. Simscape Multibody takes the measurement and projects it onto the axes of the measurement frame that you select. The figure illustrates the measurement frame concept.

The arrow connecting base and follower frame origins is the translation vector. If you select the base frame as the measurement frame, Simscape Multibody resolves that translation vector along the axes of the base frame. If you select the World frame as the measurement frame, Simscape Multibody instead resolves the translation vector along the axes of the World frame. The translation vector remains the same, but the frame Simscape Multibody expresses that measurement in changes.

Note that you can select the measurement frame only with certain blocks. Among joint blocks, only those with a spherical primitive offer a selection of measurement frames. All other joint blocks resolve their measurements in the base frame. The Transform Sensor block offers the most extensive selection of measurement frames.

Measurement Frame Types

Simscape Multibody offers five different measurement frames. These include World as well as rotating and non-rotating versions of the base and follower frames. The table describes these measurement frames.

Measurement FrameDescription
World

Inertial frame at absolute rest. World is the ultimate reference frame in a model. The World Frame block identifies this frame in a model.

Base

Frame that connects to the B port of the sensing block. Unless you rigidly connect it to World, Base is generally non-inertial.

Follower

Frame that connects to the F port of the sensing block. Unless you rigidly connect it to World, Follower is generally non-inertial.

Non-Rotating Base/Follower

Non-rotating versions of the Base and follower frames.

A non-rotating frame is a virtual frame which, at every point in time, Simscape Multibody holds coincident with the rotating frame, but which has zero angular velocity with respect to the World frame.

Measurements that can differ between rotating and non-rotating frames are the linear velocity and linear acceleration.

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