Posts Tagged ‘PLC’

The Depositor’s Industrial Control System

Friday, August 24th, 2018

    Last time we saw how a solenoid valve operates a pneumatic actuator in a jelly depositor in a food manufacturing plant.   The operation was manual.   In other words, an electrical switch had to be thrown by hand each time to get the solenoid to work.   This can be rather tedious, when you consider the thousands of pastries that must be filled on each production run.   Now, let’s see how the solenoid can be automatically turned on and off by an industrial control system.

    In food manufacturing plants, industrial control systems are typically made up of programmable logic controllers, otherwise known as “PLCs.”   The PLC is an industrial computer that is used to control equipment like conveyor belts, motors, pumps, robots, and solenoid valves.   The PLC is connected to Input/Output Modules, or “I/O Modules.”

    The I/O modules act as an interface between the computer and the equipment in the plant.   As such, they contain a means to connect electrically to the computer and the plant equipment.   In the case of our solenoid valve, the PLC computer program would turn the valve’s solenoid on and off.   Whether it is turned on or off depends on the computer program’s timing and/or external sensors and how it feeds in conveyor belt/pastry position data to the PLC.    The result is the automatic depositing of jelly filling as each pastry passes by the depositor nozzle.

The Depositor’s Industrial Control System

The Depositor’s Industrial Control System


    That wraps things up for our blog series on depositors.   Next time we’ll move on to a new topic.

Copyright 2018 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog



Machine Safety, Operator Safety, And Keeping Those Fingers

Sunday, September 27th, 2009

     Crushed fingers, amputations, burns, blindness, these are all too common undesirable occurrences involving moving machinery.  Eliminating the risk of such accidents is an integral part of the engineering design process, where risk assessment goes hand and hand with industry standards in order to provide adequate machine safeguards and protection to operators as well as bystanders.

     Machine safeguards fall into three basic categories: Guards, Devices, and Distance.

     Guards are physical barriers that are added to machines with the goal of keeping body parts, clothing, etc., separated from potentially hazardous areas.  An example would be a metal cage surrounding drive belts and pulleys.  Guards can also serve to keep material fragments and debris from flying out of machines while in operation, such as when an enclosure is built around the grinding wheel of a bench grinder.

     Devices can consist of automatic controllers, often connected to sensors on machine componets.  These controllers use a form of “safety interlock logic” to monitor the operating state of machinery.  They must act quickly and automatically to stop the normal operation of a machine if they sense that an undesirable object, say a person’s forearm, is in danger of entering a hazardous area.

     Controllers can be in the form of hard-wired electromechanical relays, embedded microprocessors, or programmable logic controllers (PLCs).  Their sensors can include electrical switches embedded in floor mats or mounted on movable guards, incorporated into control handle grips, or linked to an access door latch.  Still other sensors are more elaborate, using more sophisticated methods to maintain safety, such as photoelectric devices known as laser curtains.  These act by spreading beams of light across an opening which may be a gateway to a dangerous area.  If the beam is broken by an object, the controller takes appropriate action and renders the machinery inoperable.

     Distance safeguards operate as you would infer them to, by designing machinery so that hazardous areas are kept a great enough distance from body parts, etc., so as to eliminate any danger of them being drawn into an unsafe area.  An example of this factor at work would be when machinery is developed so that moving gears and other potential hazards are kept far out of the reach of someone by virtue of their overall design. 

      Sometimes even the best machine safeguard designs can be rendered ineffective after a piece of machinery is put into actual operation.  The reasons for this are varied, from poor maintenance of equipment, to lack of training for operating personnel, to inadequate supervision of workers, or perhaps the machine has been modified to operate outside the parameters of its design capacity.  Whatever the reason, people can be put at risk for serious injury and even death if machine safeguards are bypassed, eliminated, and defeated.