What is automation?

What is automation?

       Today we are discuss about HAND-OFF-AUTO circuit

HAND-OFF-AUTO Circuits (Full Lecture)

 Good day  ,Today's topic of discussion is Hand-Off-Auto Circuits. Our objective's to take a brief look at a common industrial application of two wire control circuits known as a Hand-Off-Auto circuit. We'll discuss the devices that make up a Hand-Off-Auto circuit, and how they govern the function of the different modes. This lecture operates under the assumption  the two and three-wire control circuits.

          Control circuits are divided into two general categories;

          two and three-wire control circuits. Two-wire circuits are ordinarily characterized by automatic switches like temperature, float or pressure switches and are suited for applications in which the automatic movement of fluid power actuator or an electric motor would not present a safety hazard.     

         Three-wire circuits are ordinarily characterized by momentary switches like push buttons and a holding circuit. A three-wire control circuit is suitable for applications where the unexpected movement of fluid power actuator or an electric motor would present a safety hazard.

         It's sometimes desirable to override the automatic nature of a two-wire control circuit. For this reason, some industrial processes incorporate Hand-Off-Auto circuits where the three different modes dictate the system's behavior.

 As implied by their titles,

       Hand means a manual override, which runs the system whether the automatic input devices like pressure, temperature or float switches call for it or not. Off means the system is turned off, even if the automatic input devices want it turn on.

       Auto means the automatic input devices govern the operation of the system. Consider this simple sump pumping circuit incorporating a Hand-Off-Auto circuit. The mode selection is provided by a maintained contact three-position selector switch. The target table, sometimes called the contacts chart for the selector switch indicates A is closed in the Auto position. Neither contact is closed in the Off position and contact B is closed in the Hand position. The top rung of the ladder logic diagram contains the A contact of the selector switch, a normally open float switch, the P contactor coil and the normally closed overload contacts. When the selector switch is in the Auto position, the pump motor will run when the float switch closes. The overload serve to protect the pump motor from sustained overload conditions.

          Note this circuit does not use momentary manual switches or a holding contact as would a three-wire control circuit. Once a quantity of liquid has been emptied from the sump, the float switch would reach its reset level and reopen, disabling the pump. In the automatic mode, this circuit does this day in and day out, well into the night and doesn't take sick days, holidays nor show up drunk to work. This is largely the purpose of a two-wire control circuit, it functions without human intervention.

           This being said, if the pumping system is to remain reliable it necessitates routine maintenance. This is one of the purposes of the Off position on the selector switch. If a technician needed to service the motor, pump, float switch or tank, the selector switch could be placed in the Off position and the system locked out and tagged out. Additionally, the off position could serve some function critical to the industrial process.

        For example, the accumulation of a quantity of liquid before it's pumped out. An operator would place the system in Off then once the desired quantity of liquid accumulated in the tank, the operator would flip it to Auto to pump it out. Similarly, the Hand setting overrides the automatic nature of the system. In a Hand mode, the selector switch provides an alternate power that directly energizes the P contactor coil circumventing the automatic logic. This could be used to function-test the system or bypass a malfunctioning float switch that despite being submersed in liquid just refuses to close.    

         Switches, like human employees, have off days too. This brings this short lecture to a close. But before we do, I'll leave you with this subtly modified Hand-Off-Auto circuit. This time utilizing a maintain contact, single pole triple throw with center-off switch used instead of the three-position selector switch. The float switch has been replaced with a normally closed temperature switch, set to a reasonable 70 degrees Fahrenheit, 21 degrees Celsius. Instead of a pump motor, this circuit controls a three-phase heater.

        The switch in position one allows the temperature switch to automatically govern the heating process. If the temperature switch senses temperature is below 70 degrees Fahrenheit or 21 degrees Celsius, it is in its deactivated state and the heater contactor coil would be energized and the heater would come on. As heating in the conditioned space rises to the setpoint, the temperatures which would open and de-energize the contactor coil and turn off the heater. If heat were to fall below the reset value of the temperature switch, it would return to its deactivated closed state and the heater would come back on.

           Note the difference between the set and reset value of the switch would establish the range of the system. Over time, the thermal inertia would stabilize and this simple circuit would keep the conditioned space at a reasonable 70 degrees Fahrenheit, 21 degrees Celsius, habitable by most of humanity. Much, much later lectures we'll discuss fine tuning of the response of similar circuits using proportional integral derivative control, sometimes called PID control. Characteristics like rise time, overshoot and attenuation can be tuned to fit the process. The switch in the center-off two position disables the heater whether the temperature switch calls for it or not. This position can be used to save energy when you go on holiday or prop open the doors to the conditioned space on a nice day. The switch in position three is a manual bypass which energizes the H contactor coil and turns on the heater whether temperature switch calls for it or not. That paint could be peeling off your walls and your goldfish could be belly up in a bowl full of boiling water and the heater would still be on at full blast in position three.

          This setting could be used by your girlfriend or old people in the habit of bitching about being cold all the time. The legend play shows the intended purpose of each mode. I'll leave it to you to figure out how to pay the electric bill anytime these people come over and diddle with your heating system. All right, this wraps up this super brief lecture on Hand-Off-Auto circuits. In conclusion, this lecture examined a common modification to automatic circuits that allows the automatic nature of the system to be disabled or manually overridden with the use of a three-position selector switch or a single pole triple throw switch with the center off. Remember to review these concepts as often as you need to really drive it home. Imagine how well lab will go if you know what you're doing. Thank you very much for your attention and interest and we'll see you again during the next lecture of our series. Remember to tell your lazy lab partner about this resource.