Quanser 6 DOF Hexapod Laboratory Manual page 21

2.8.3 Position Control
The Hexapod implements the PID-based controller described in Section 3.2 in the PID subsystem block, shown
in Figure 2.12. The control force, i.e., effort required to attain the desired position, is converted to current for the
amplifier in the Hexapod IO subsystem.
The joint velocities used in the PID controller can be generated through hardware or software. Use the manual switch
in the Hexapod IO API subsystem shown in Figure 2.14 to select between the two sources for the joint velocity:
1. Hardware-based velocity obtained directly from the DAQ board using HIL Read Timebase block (see the
Hexapod User Manual for channel details). This is the default.
2. Software-based velocity obtained by passing the joint values through a second-order high-pass filter.
2.8.4 Kinematics
The forward and inverse kinematic block are available in the Hexapod Kinematics block shown in Figure 2.17. For-
ward kinematics are used to find the world-based end-effector position (X, Y, Z, Roll, Pitch, Yaw) from the six joint
positions, q, and the leg lengths. It also the rotation matrix, R.
Inverse kinematics calculates the six joint positions from the world coordinate of the end-effector position. The Inverse
Kinematics block is used in the Hexapod_World_Control.mdl model to find the joint-level commands required to attain
the desired end-effector position. But it is also used in Hexapod_Joint_Control.mdl to compare it again the actual,
measure joint positions.
Figure 2.17: Forward and inverse kinematic blocks for the Hexapod
2.8.5 Performance Tracking
The Performance Tracking subsystem, depicted in Figure 2.18, includes various scopes to monitor the position and
speed of the Hexapod.
Joint Position Tracking: The yellow trace displayed in this plot is the measured position of the joint and the
purple plot line is the commanded or desired position. The joint to be displayed on the scope can be selected
using the Joint Index block (values between 0 and 5).
Joint Speed Tracking: Similarly as Joint Position Tracking scope, but displays the desired and measured
joint-level velocities instead.
HEXAPOD Laboratory Guide
v 1.3