Feedback loops control discrete, continuous processes

Back to Basics
Vance J. VanDoren

Arguably, the most basic tool of the control engineering profession is the feedback loop, shown below. It consists of five fundamental elements:

• The process to be controlled.
• A sensor (or instrument) that measures the condition of the process.
• A transmitter that converts the measurement into an electronic signal.
• A controller that decides whether or not the process condition is acceptable.
• An actuator (or final control element) that applies a corrective action to the process according to the controller's instructions.

This measure-decide-actuate sequence repeats as often as necessary until the desired process condition is achieved.

For a continuous process, a feedback loop attempts to maintain a process variable at a desired setpoint.

For a continuous process, a feedback loop attempts to maintain a process variable (or manipulated variable) at a desired value known as the setpoint. The controller subtracts the latest process variable measurement from the setpoint to generate an error signal. The magnitude and duration of the error signal then determine the value of the controller's output or controlled variable, which in turn dictates the corrective efforts applied by the actuator.

Cruising

For example, a car equipped with a cruise controller uses a speedometer to measure and maintain the car's speed. If the car is traveling too slowly, the controller instructs the accelerator to feed more fuel to the engine. If the car is traveling too quickly, the controller lets up on the accelerator. The car is the process, the speedometer is the sensor, and the accelerator is the actuator.

The car's speed is the process variable. Other common process variables include temperatures, pressures, flow rates, and tank levels. These are all quantities that can vary constantly and can be measured at any time. Common actuators for manipulating such conditions include heating elements, valves, pumps, and dampers.

For a discrete process, the variable of interest is measured only when a triggering event occurs, and the measure-decide-actuate sequence is typically executed just once for each event. There's really no "loop" involved. For example, the eyes of the human controller driving the car measure ambient light levels at the beginning of each trip. If it's too dark to see well, the driver decides to turn on the car's lights. No further adjustment is required until the next triggering event, such as the end of the trip.

Feedback loops for discrete processes are generally much simpler than continuous control loops, since discrete processes do not involve as much inertia. The driver controlling the car gets instantaneous results after turning on the lights, whereas the cruise controller sees much more gradual results as the car slowly speeds up or slows down.

Inertia tends to complicate the design of a continuous control loop, since a continuous controller typically needs to make a series of decisions before the results of its earlier efforts are completely evident. It has to anticipate the cumulative effects of its recent corrective efforts and plan future action accordingly. Waiting to see how each one turns out before trying another simply takes too long.

The ubiquitous proportional-integral-derivative (PID) control algorithm can foresee the future if it is configured or tuned to complement the behavior of the process. A fast-acting PID controller that makes aggressive control decisions works well on a slow process and vice-versa.

Adjustable / Variable Frequency Drives

Drives 101
Adjustable / Variable Frequency Drives

This lesson covers the basic functions of an Adjustable Frequency Drives (AFD).
Functions of an Adjustable Frequency Drive (AFD)



Outline:
3-phase AC Motor

1. Bi-Directional Operation
2. Change Speed
3. Constant Speed
4. Limits
5. Ramping
6. Braking
7. Save Energy

This lesson covers the basic functions of an Adjustable Frequency Drive (AFD) on a 3-phase AC (alternating current) motor. Pictured below is an AC motor.

The one pictured above is for industrial or commercial use, but in your home, AC Motors are used as well. A vacuum cleaner uses an AC motor to clean the carpet; a blender uses an AC motor to process food; and the clothes dryer uses an AC motor to dry clothes. In each of these examples, how is the AC motor controlled?

When controlling motors in the home we control them by applying AC power, and removing it usually through a switch. Obviously when power, 120 or 220 VAC, is applied to the motor it runs. With no power, the motor stops.

With the use of an Adjustable Frequency Drive not only can the AC motor be started and stopped as in the home but more sophisticated controls are accomplished. An AFD can send a modulating signal to the motor, which allows a variety of speeds to be delivered not just an ON/OFF signal. This variety can be used to match a motor speed to a particular process. There are a number of functions that the AFD accomplishes with commercial 3-phase AC motors, which are covered in the pages that follow.

To understand the functions of an AFD better, an example of a conveyor at a Package Delivery Company is used.

A Package Delivery Service uses a conveyor to deliver packages from the shipping area to the sorting area. Once sorted, the conveyor is used to return the boxes backto shipping.

Looking at the example above, see if you can identify some of the functions that must be performed by the AFD and AC motor? In other words, what must the conveyor do? Take a couple of minutes to jot down the functions of the conveyor.

The conveyor must …
_______________________________ __________________________
_______________________________ __________________________

A few of the basic functions of an AFD in controlling the AC motor and conveyor are covered here.

1. Bi-Directional Operation

FORWARD
One function of the AFD is to operate the motor in a forward direction, to move the packages from the shipping area to the sorting area.

REVERSE
Power going to the motor must be changed to move the packages backward (Reverse) from the sorting area back to the shipping area. If there were no AFD, 2 of the 3 leads of the 3-phase motor would be switched in order for the motor to change its direction and go backwards.

2. Change Speed
The speed of the conveyor must be adjustable to allow for a slower speed when fewer packages and employees are present and a faster speed for large volumes. This allows the operator to match the speed of the conveyor to a particular process. The setting of this speed is known as the Reference. Commonly reference refers to speed in Hertz (Hz), maximum reference of 60Hz, and minimum reference of 0Hz. It could also be used in regards to a pressure setting, maximum reference of 100psi, minimum reference of 40psi, if a transmitter were attached to the AFD.

In the picture below, the display of an AFD, a Danfoss VLT 5000, is shown. Speed in Hz is the reference. The plus (+) key is used to increase the reference making the conveyor go faster and the minus (-) is used to decrease the reference point.



3. Constant Speed
Another function of the AFD is to maintain the speed of the conveyor regardless of the number of packages. The AFD automatically compensates the current and torque to accommodate changes in the load, from hundreds of boxes to a few.

4. Limits
It is important that limits be placed on an AFD. Speed limits can be placed in the program of the AFD so an operator can not go beyond a maximum speed or less than a minimum speed. If a package gets stuck, there are torque limits that the AFD monitors stopping the motor if they are exceeded.

Current limits are also important for protection of the drive and motor. In the picture below the maximum reference is set to 50 Hz. Notice that in the diagram there is a minimum speed or reference of 10Hz.

5. Ramping
The AFD also ramps the conveyor up and ramps it down. When the conveyor starts, acceleration, it is important that there is no sudden lurch to the reference speed, or packages can go rolling backward. In the example below, a ramp-up slowly increases the speed from stopped or 0 Hz up to the reference, 34 Hz, over a certain amount of seconds perhaps 10 seconds. If this ramp is too short, the drive can trip on an over current alarm or torque limit.


A ramp is also present on the stop side. This is referred to as a ramp down or deceleration. It is important that packages are not jerked to a halt. A ramp-down of 10 seconds might be entered into the program for this application. If the ramp is too short, the drive can trip on over voltage.


6. Braking
There are special challenges when the conveyor is stopping. With many packages on the conveyor there is a great deal of momentum, inertia, so that the motor continues to spin when it is trying to stop. The continued spinning causes the motor to generate energy instead of using it. This extra energy must be handled in the drive or it will trip. If the drive trips, the drive releases control of the motor and the conveyor would then coast to a stop, which might take some time. A special drive is used which has external brake resistors added to dissipate this extra energy as heat as pictured below.



Other ways of braking can also be used. DC Braking is an example that places a DC signal onto the AC motor, which can produce a certain amount of braking. DC braking works best at very slow speeds. Other arrangements have been engineered to handle this extra power, such as DC
Load sharing, but will be discussed later. Only certain applications require special arrangements for braking.

7. Saving Energy
The major function of the AFD is to save energy and equipment. In the example below a drive is used on a screw compressor. Before drives, shown on the left, the motor was cycled On at full power 60Hz until the pressure setting (80psi) was reached. When 80 psi was reached the motor
was turned OFF coming back ON when the pressure dropped to perhaps 60psi. This arrangement used a great deal of energy and the frequent cycling caused a great deal of wear on equipment.

A drive is placed on the screw compressor, shown below, which slows the screw compressor down to perhaps 45Hz to constantly maintain the required pressure. The motor speeds up or slows down following load changes. This saves approximately 35% in energy costs and greatly reduces the wear on equipment.

How to produce 440V ac from 110V ac Source

By using 2 pieces 110V ac to 220V ac step up transformers, one can produce 440V ac to power up certain industrial equipment for testing by following the setup above (parallel input, series output). Low power transformer can be used to power up variable frequency drives and similar equipment at no load to test programming and simulation. High power transformers should be used if used to power up with loads such as motor. At the place where I work, I usually use two variable transformers of high power rating that weighs about 10kgs for each to be be able to produce output voltages from 220 to 600Vac. It is such a big help for me instead of laying out power cables from to the 440V ac source towards the area where I usually do my work. But be be careful when doing this for one mistake could be hazardous. I mean high voltages here and that is fatal. Watch out for shorts and grounds.

Cleaning and Maintaining the Magnetic Contactor

Cleaning and maintaining magnetic contactors (MC) are fairly easy and fast. Tools needed are usually screwdrivers, brush, rubber eraser (use the plastic rubber type, not the powdery rubber one). You can also spread a light colored material usually bond paper to contrast small items that comes from the magnetic contactor itself and small tools.




Start off by cleaning the outside of the MC with a brush. You can also use a high percentage ethanol solution to clear out contacts. (Ethanol is highly volatile and flammable so keep it away from flames and sparks!)




Unscrew the screw that holds the upper and lower parts together of the MC. Sometimes a locking notch at the opposite side from which the screw was found helps keep the upper and lower parts be together. This can be easily be disengaged using a flat screw driver.




Once the upper and lower parts are separated, the coil can be removed, the spring and the upper and lower cores. Check for corrosion and rust. You can use a rubber eraser to clean out minor surface problems on the cores or use an abrasive if rust has already settled. Check the resistance of the coil. It must be from a few tens of an ohms to more or less a hundred but should never be below or beyond by means of another hundred or more. Ohmic values of the core varies with the corresponding operating voltage of the coil.

Also check the resistance between the line side and load side of the terminal contacts of the MC. L1 and T1, L2 and T2, L3 and T3 should each have a resistance of less than an ohm, otherwise clean with a contact cleaner available in most of the industrial and electrical supply in your neighborhood.


After cleaning and making the necessary checks, put back the magnetic contactor ensuring that screws and locking notches are securely in place. Power up the said contactor with the correct operating voltage and check again the resistance. When power upped, always be careful of live voltages and ensure your safety first.

Our Deadwieght Tester

Our Budenberg brand hydraulic deadweight tester is used to calibrate various pressure gauges ranging up to thousands of Psi of pressure. Standard weights are used with graduation on how much they are lifted up by the hydraulic medium to indicate great amount of pressure. Two gauges are on the said tester. One is the standard and the is the one to be tested and calibrated. Other hydraulic equipment or instrument can be attached aside from the said gauges to indicate and test their accuracy and capability. Many digital deadweight testers are now available in the market which can indicate pressures with very high accuracy unlike our obsolete pure analog tester. But it does show how the principle of deadweight testing and hydraulic systems work.

Multimeter Safety by Fluke

As an industrial instrumentation and electronics technician, i am sharing to you these safety notes and practices shared by Fluke (maker of my issued digital multimeter). Here it goes...

Three levels of electrical reaction:

You should be aware of the electrical shock levels at which you are likely to have a mild reaction. Higher levels may be accident-provoking or even fatal. (Current rather than voltage is used below because the reaction to voltage varies more from person to person (body impedance changes with contact condition - wet or dry, extent of bodily contact, etc).)

Perception Level. This refers to the ability of the operator to sense an electrical shock. Levels as low as 1 mA current with a 1-second contact can be detected as a slight warm in the palm of the hand (dc) and a mild tingling sensation (ac).

Surprise Level. The reaction caused by the surprise level current is sudden, involuntary and totally without warning. It will trigger an unexpected reaction such as suddenly pulling one's hand away from the source of shock with enough force to cause an accident. The reaction becomes more violent with higher levels of current and voltage. Specific surprise levels vary from person to person.

Freeze Level. As voltage and amperage levels increase, the "freeze level" is reached. The hand muscles contract to the point where the victim has no control. Inability to let go of the conductor could "freeze" the victim to the electrical source long enough to cause lung and heart stoppage.

Electrical circuits can cause burns, heart fibrillation and eye injuries.
The DMM user working with or near high voltage, high-current electricity is exposed to potential danger from both actual contact and from exposure to flash hazards. Electrical contact may cause burns either two ways: (1) by touching a high voltage supply, when the resulting arc causes sudden tremendous heat and a burn; or (2) by touching a high frequency current, causing a burn under the skin.
The heart can go into fibrillation when sufficient current passes through it; it seldom recovers spontaneously.
Flash hazards occur when a short circuit creates an arc. Shorts in high-energy circuits can cause the equivalent of welding arcs, which can flash-burn the eyes or cause injury from flying molten metal.

Electrical shock danger varies by wet or dry contact.
Dry skin has a resistance perhaps 10 times greater than wet skin. (Typical impedance of a wet gripping contact is 1500 ohms in parallel with 0.22 uF. Internal body resistance between hands is about 400 ohms.) Perspiration greatly decreases the meter's users skin resistance. The same voltage that would be safe with dry skin may put the user at the "surprise level" with wet skin.

Industrial high energy circuits (>220V ac).
The dividing line between high and low energy circuits is often given as 3,600 volt-amps (such as 240V ac, 15A circuit). Above this, a poorly designed DMM (digital multimeter), or one designed for light duty, could break down, resulting in damage and possible operator injuries. Only meters having special built-in protection such 600V fusing, ohms overload capability and transient protection should be used in these conditions.

Fault Conditions. These can occur when a circuit is accidentally shorted, or the DMM itself is overloaded. Faults cause arcing, sparks, fire explosions or other immediate hazards. (A simple overload is not a fault.) Common situations that lead to faults are:

• Accidental contact with a 480V ac power line while in the current mode.
• Accidental contact with power lines while in the resistance mode.
• High voltage transients from motor switching, load change, or indirect lightning surges.

Unsheltered locations. Any locations allowing wet conditions, including condensation, could lower the insulation value of the DMM and its accessories by creating a current path. This could present a danger to the user, or cause errors in the measurement which could lead to indirectly to hazardous decisions by the user. Take precautions to shield your meter from moisture.

Hazardous locations (potentially explosive atmospheres). A DMM may be safe by itself under normal operating conditions. But if it were to fault, it potentially could cause a spark or overheat a component to cause an explosion. Only specially designed and approved (and clearly labeled as such) meters should be used in hazardous locations. Even simply disconnecting test leads from live circuits on explosive environments, especially while measuring current with inductive loads, may create a dangerous spark.

Proper procedures for using DMMs

Always:
1) Inspect the test leads for insulation damage or exposed metal. Damaged leads should be replaced.
2) Reduce the risk of accidental contact by using leads with shrouded connectors and finger guards, and meters with recessed jacks.
3) Check continuity of the test leads.
4) Be certain the DMM itself is in good condition. During the continuity test, a meter reading that goes from overload to 0 generally means the circuitry is working properly.
5) Select the proper function and range for your measurement.
6) Electrically disconnect "hot" end first.

Safe Practices:
1) Insulate yourself from earth ground, using an insulating floor mat. If possible, employ one hand operation, keeping the other hand in your pocket to avoid accidentally allowing current to travel through your body to earth ground.
2) Follow all equipment safety procedures, disconnecting the power and discharging high-voltage capacitors prior to testing with the DMM. (This applies to dc power supplies, tube circuit transmitters, motor control circuits and x-ray machines.)
3) Working with a CRT? wear safety glasses and protective clothing to avoid injuries should an implosion cause flying glass.
4) Avoid working alone.
5) Using a current shunt? Turn the power off before connecting into the circuit. Overloading a current shunt will cause excessive heating.
6) Current greater than 2 Amps? Use clamp-on probes for maximum protection.
7) Measuring current transformer output or motor winding current? Check the fuse first. An open fuse will allow high voltage buildup, potentially damaging to equipment and operator.
8) Measuring automotive circuits? Be aware of potential danger from high voltage (up to 30,000V dc) and fire hazard from gasoline fumes/leakage.