Specifications

VA6x User’s Reference405-00022-00
31Rev 00; 16 October 2007
using a standard I/O line, and are turned on before the camera’s exposure is triggered and turned
off after the exposure is done. This minimizes the intensity variation on different exposures.
Using Contact Closures
Mechanical contacts, such as switches or relays, typically exhibit “bounce.” The moving contact
makes the electrical circuit by touching a fixed contact, but then bounces off this fixed contact.
The result is a series of rapid closing and openings of the contact until the moving contact stops
bouncing. Bouncing typically continues for less than 10 milliseconds, but the duration depends
on mechanical factors of the switch. This oscilloscope trace shows about 5 ms (milliseconds) of
bounce when a switch is closed:
Figure 18. Example of switch “bounce” during a contact closure
From Switch Bounce and Other Dirty Secrets, Maxim Integrated Products, Inc., Sept. 2000
The problem is, the Vision Appliance “sees” the bouncing as multiple, rapid input signals. For
example, if your “part in place” sensor is a mechanical switch (say, a photocell running a relay),
the contact bounce will make it appear as if many parts were being presented to the Vision Ap-
pliance in a few milliseconds.
Here are three ways to deal with bounce. (1) Use a signal that does not bounce; for example, from
a photoelectric sensor. (2) Use the built-in de-bouncing circuits. The de-bounce circuit delays the
VA4x from responding to the input for some number of microseconds (us) to allow time for the
contact to settle. The de-bounce time can be programmed through the camera configuration file.
(3) Externally de-bounce the switch closure using commercial de-bounce chips (for example, the
Maxim MAX6816), or a low-pass filter and Schmitt trigger.
Both the Vision Appliance and external de-bounce circuits delay the input signal by the de-
bounce period. This delay is rarely long enough to be a problem, but might have to be considered
in very high-speed applications where any delay might mean the parts being inspected move par-
tially out of the field of view.