It could be an episode of "Fear Factor": A control systems engineer is forced to perform control of a critical process using a desktop
Rational or not, many end users and system integrators fear using PCs for real-time process control. "We have not implemented
PC-based control, and I am not aware of any client applications. Everyone I speak to expresses many reservations regarding PC-based
control," says David Kennedy, PE, a control systems engineer with Fresno, Calif., system integrator Ginosko (
Many engineers and technicians believe PCs are unreliable, and many others don't see a need for PCs in process control. "PCs have
too much overhead, which results in bugs in the Windows software that cause crashes, hang-ups, and re-boots," says Paul Parker, a
plant engineer with KSL Services at Los Alamos National Labs in Los Alamos, N.M. "Why stress out over this, when PLCs cost less
than PCs? What is the advantage?" he asks.
When we visited this topic two years ago, we showed concrete reasons for using PCs instead of PLCs in process control. These
reasons are shown in Table I, and perusal of the table shows how PCs can be a cost-effective alternative when there is a need for
more than just simple discrete and analog control.
Even if one or more of these reasons apply to your application, it still would not make sense to use a PC unless it proved to be a
reliable, inexpensive, and accepted alternative to a PLC or DCS.
Industry acceptance is coming along slowly but surely. PC-based control is a mainstay of discrete parts manufacturing, most notably in
the auto industry. According to the Venture Development (
, a technology market research group, PC control also
has a significant and fast-growing presence in process control.
Table I: Top Ten Reasons for Using a PC Instead of a PLC
1 Networking to higher-level platforms
2 Advanced control algorithms
3 Extensive database manipulation
4 HMI functionality in one platform
5 Integrated custom control routines
6 Complex process simulation
7 Very fast CPU processing
8 Memory requirements exceed PLC specs
9 Interfaces thorough multiple protocols
10 Wireless access
"Distributed and remote I/O for use with PC-based control systems in industrial process industry applications is forecast to increase
from $143 million in 2001 to $254 million in 2005. This gives PC-based control about 10.6% of the total control market in 2001 and a
projected 14.5% of the total in 2005," according to Jim Taylor, a group manager with Venture Development.
PC-based control gains acceptance with new and more practical form factors