David Pierce - Consultant - 2004-2005
I was hired by Steger & Bizzell, an engineering and surveying
firm, to design the instrumentation and control system for a station
pump water from a one-million gallon tank to one or both of two
pressure planes depending on the levels of the elevated tanks in the
two pressure planes. Highlights:
- Designed the control panel: selected the components and specified
- Specified to the electrical contractor how the sensors,
transmitters, variable-frequency drives, and valve actuators would be
- Designed a program to intelligently control the pumps and valves
based on tank levels, pressures, and flow rates.
- Tested the operation of the station under normal and alarm
Months after we signed off on the pumping station, the customer
stated that it was their only project that required no fixes to the
control logic after delivery from the contractor.
Innovation and Flexibility
- I had been told that a simple Modbus protocol was used to
communicate between the central office and all pumping stations.
When this turned out not to be the case, I had to reconfigure the PLC
so that I could use the programming port as a serial port for
communication with the radio modem. This required finding another way
to communicate between laptop and PLC for programming and debugging.
- I used an MP3 recorder to
capture the output from the radio so that I could develop and test the
code for communication between PLC and radio modem.
- I added pressure-based control as a backup to flow-based
control when it became clear the flow meters were unreliable.
Pressure-based control can be used manually or automatically, at the
For this project, I had to quickly learn the following:
- Principles of pumping station design.
- Practical knowledge of pumps, motors, variable-frequency drives,
pressure sensors, flow meters, and 4-20mA transmitters.
- PanelBuilder, the development environment used to program
Allen-Bradley operator interfaces.
- Modbus, a serial protocol that we used for communication between
PLC and the pump motors' variable-frequency drives.
By this I mean the development of procedures and templates on which
subsequent developers build. Examples:
- Developing a way for Allen-Bradley programmable logic controllers
(PLCs) to communicate via
radio modem. This method will be used in the customer's future
pumping stations and treatment plants.
- Applying good programming practices (documentation, structured
programming, use of
variable names, avoiding copy-and-tweak code) to ladder logic, a very
low-level programming language. The code will be used as the
template for the customer's future control programs.