Control Loop: PV, SP and MV Explained

What a control loop is

A control loop is the set of elements that keeps a physical variable, such as temperature, level, pressure or flow, at a desired value without a person adjusting it by hand. In a closed loop the result is measured and compared with what was wanted; in an open loop a command is sent and its effect is not checked.

The names used in plants and in the technical literature are these:

In some texts the acronym CV stands for “controlled variable” and in others for the controller output. To avoid confusion with the valve coefficient, which is also written Cv, this guide uses PV, SP and MV.

Defining the error as the difference between the setpoint and the process variable is the basis of all feedback control and is the same definition used in the reference literature on PID controllers.

The four physical elements of the loop

Every closed loop has at least four parts that can be pointed to in the field:

1. Sensor and transmitter. They convert the physical quantity into a standardized signal, normally 4 to 20 mA, that the controller understands.
2. Controller. It can be a loop inside a PLC, a panel controller or a function of a distributed control system. It receives the PV, compares it with the SP and calculates the output.
3. Final control element. The actuator that changes the process: a control valve, a variable frequency drive that changes the speed of a pump, a heater with a power regulator.
4. Process. The tank, furnace, pipe or motor whose variable is to be governed.

The signal travels in a circle: the process produces the PV, the sensor measures it, the controller decides the MV, the actuator applies it and the process responds. If any link fails, the whole loop fails, even if the controller is perfectly tuned.

Open loop and closed loop

In open loop the controller sends a fixed output or one calculated from a table. An example is a fan that switches on at six in the morning for ten minutes. It is simple and cheap, but it does not find out whether the room temperature actually reached where it should.

In closed loop the system uses feedback: it measures the result and corrects the difference. Its advantage is that it compensates for disturbances that nobody foresaw, such as a change in inlet water temperature, and also for slow wear of the equipment. Its cost is that it can become unstable if the correction is too aggressive or arrives late.

Direct action and reverse action

There are two questions everyone commissioning a loop must answer before switching to automatic:

• If the MV increases, does the PV go up or down?
• If the PV increases, should the controller raise or lower the MV?

The controller must correct in the direction that reduces the error. The rule reads like this:

On top of this comes the valve action on loss of air or signal: “fail open” or “fail closed”. The combination of the valve action, the controller action and the direction of the process must, together, give negative feedback. If it gives positive feedback, the loop runs away toward an extreme.

Disturbances and delay

The loop works under two kinds of demand:

• Setpoint tracking. The SP is changed and the PV is expected to follow it quickly without overshooting.
• Disturbance rejection. The SP stays fixed and something in the process changes, for example the load, the composition or the ambient temperature. The PV must return to its value.

A loop well tuned for one of the two cases is not always the best for the other. In continuous processes disturbance rejection usually weighs more.

The main difficulty in process control is delay. There are two types worth distinguishing:

• Dead time. The time between the MV changing and the sensor starting to notice any effect, for example the distance between a valve and the downstream sensor in a long pipe.
• Time constant. How long the process takes to reach its new value once it has started to react, for example the thermal mass of a furnace.

A process with a large dead time relative to its time constant is hard to control: the controller acts on old information and tends to oscillate. This ratio between delay and time constant is the figure that weighs most when choosing the type of control and its tuning, and it comes up again when discussing tuning. To set the PID gains, the Ziegler-Nichols method and its alternatives are explained in Ziegler-Nichols tuning and alternatives.

Controller operating modes

An industrial controller is not just “in automatic”. The usual modes are:

• Manual. The person sets the MV directly. The loop is open.
• Automatic. The controller calculates the MV from the error. The operator sets the SP.
• Remote or cascade. The SP is supplied by another controller or a calculation. This is the case of cascade control.
• Tracking. The output follows another signal while the loop is not active, so that the switch to automatic is bumpless.

The transfer between manual and automatic must be bumpless: the controller takes the current MV as its starting point and not the one it had stored. A sudden output change when switching to automatic can trigger an alarm or an interlock.

Numerical example (illustrative)

Consider a hot water tank whose temperature SP is 60 °C. The values are made up to show the logic and do not correspond to any real equipment.

• Measured PV: 54 °C. Error: e = 60 − 54 = +6 °C.
• The process is heating, so the controller has reverse action: if the PV is below the SP, it must raise the MV.
• A proportional-only controller with a gain of 5 %/°C, starting from a base output of 30 %, would calculate MV = 30 + 5 × 6 = 60 %: the steam valve opens further.
• Over time the PV rises to 58 °C. New error: +2 °C. MV = 30 + 5 × 2 = 40 %.
• The PV may settle at 58 °C because at 40 % opening the heat delivered equals the heat lost. The remaining error is called offset and it is why proportional-only control is rarely enough; the integral term eliminates it.

The example shows the logic of the loop: error, decision and effect, and the trade-off between speed and stability.

Common mistakes

• Controller action backwards. The most common commissioning mistake and the easiest to avoid with a prior review of the loop logic.
• Badly placed sensor. A temperature sensor far from the mixing point or in a zone without flow adds dead time and false readings.
• Different scales. The transmitter is calibrated for one range and the controller scales another. The PV the operator sees is not the real one.
• Oversized valve. With a valve that is too large, only 10 to 20 % of its useful travel is used and the loop loses resolution.
• Loop permanently in manual. If operators leave it in manual, it is a sign the loop does not work and nobody repaired it.
• Noise on the PV. A transmitter without damping or a cable without shield delivers a signal that derivative action amplifies.
• Ignoring dead bands. A valve with friction or backlash does not respond to small changes, and the loop ends up oscillating around the SP.

Loop checklist

1. Clear identification of the loop (tag per the piping and instrumentation diagram) and its variable.
2. Transmitter range equal to the one configured in the controller.
3. Direction of action defined and tested: MV increase, effect on PV, controller action.
4. Valve fail-safe position consistent with the process.
5. Minimum and maximum limits on the controller output.
6. Operating modes tested, including bumpless transfer.
7. High and low PV alarms, and a deviation alarm between SP and PV.
8. Historical record of PV, SP and MV to analyze behavior.
9. Response to a small SP change verified before handing over the loop.
10. Documentation of the adjustments and the tuning values delivered.

When to call a specialist

A loop can be fixed with adjustments or it can have a design flaw: a poorly chosen instrument, the wrong valve, or a control strategy that does not suit the process. Telling the two apart takes field data, tests with the process running and engineering judgment.

At Tiza Group we support that stage on two fronts. To define the strategy, the instrumentation scope and the documentation, see our engineering design and studies service, delivered remotely and worldwide. To check that every loop works before the plant is handed over, see testing and commissioning, in Colombia or elsewhere on request. You can write to us through contact with the process diagram and a description of the problem.

Related guides

• Induction motor and its protections
• Motor control center (MCC)
• Siemens SINAMICS drive
• WEG CFW500, CFW11 and MVW3000 drives
• 4-20 mA signal: how it works and why it is used
• PID controller: what each action does

References

• Åström, K. J. and Murray, R. M., Feedback Systems: An Introduction for Scientists and Engineers, Princeton University Press.
• Kuphaldt, T. R., Lessons in Industrial Instrumentation, chapter on closed-loop control (CC BY 4.0).
• Woolf, P., Chemical Process Dynamics and Controls, University of Michigan (CC BY 3.0).
• Wikipedia, PID controller (error as the difference between setpoint and process variable).

Frequently asked questions