Multi-axis motion control systems are used extensively for positioning in
manufacturing. These systems often rely on feedback from position sensors to maintain
closed-loop control for improved accuracy. In test and measurement applications, it is
often desirable to use additional inputs as part of the feedback loop. For example, in
scanning tunneling microscopes, a sample is moved in the X- and Y-axis while a feedback
loop drives an actuator to maintain a constant tunneling current between a probe and
sample in the Z-axis. The output of this loop can be used to generate a topographic, or
height, profile of the sample. ADwin systems are ideal for these applications. The ADwin
– which incorporates digital and analog inputs and outputs with a real-time processor in a
single unit – can drive stepper motors and/or other actuators, using feedback from various
position sensors or other inputs such as voltage, current, strain, and force.
One such system, designed to test the trigger switches of variable speed power
tools, features six stations which allows up to six separate switches to be tested, in
parallel. Each test station has a stepper motor that drives an actuator to simulate a person
pressing the switch. A home sensor and encoder provide information on the position of
the switch. The ADwin system, in addition to driving the stepper motor and reading the
position information, measures the RMS voltage, RMS current, and duty cycle (in the case
of PWM control) of the output from the switches.
Figure 1 – System Block Diagram
The multitasking capabilities of the ADwin provide independent control of each
station. One main, high-priority task implements the main control loop for all six test
stations. This control loop includes functions that: find the home position; drive the
actuators and move the switch to a specified position; and monitor the voltage, current and
other input parameters. This main loop executes at 10 kHz which allows accurate
measurement of the switch parameters. The move function implements a trapezoidal
velocity profile – an algorithm determines the distance from the current position to the
target position and calculates a velocity profile with a given acceleration, target velocity
and deceleration.
Each test station has an individual process associated with it, which sequences
through the test steps that verify the operation of the switch. Separate processes allow for
independent control of each station: if the switch at test station one failed, it would have no
impact on the test operation of a switch on station two. These processes run at 100 Hz
which is more than fast enough since the RMS data is updated at a line cycle frequency of
60 Hz. A user interface designed with LabView™ controls and monitors the operation of
each test station through the use of global data that is automatically synchronized between
the ADwin system and a PC.
This ADwin-based system meets or exceeds all of the original design goals, and
performs substantially better than previous versions that utilized PC-based data acquisition
boards to perform the measurement and control functions. It also highlights some of the
unique features of ADwin systems including:
- Support for multiple concurrent processes
- Tightly coupled analog and digital I/O and
- Fast real-time operation
These capabilities can be useful in a number of other positioning applications including:
- Dynamic test stands for vibration, force or stress
- Marking or engraving systems
- Automated assembly equipment
- High speed positioning equipment
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