The short answer
Control Strategies
The control strategy is chosen by what the process needs and what the final control element can do. On-off control with a deadband fits a level held between two points by a fixed-speed pump. Staging fits several fixed units serving one demand. PID fits a variable held at a setpoint by a modulating element such as a drive or a control valve. Ratio and feedforward fit a dose that follows a measured flow. Cascade fits a slow variable that is best controlled by setting the setpoint of a faster inner loop. Override fits a loop that must respect a constraint on a second variable. The simplest strategy that meets the requirement is the right one, and combinations such as PID speed control with staging are normal.
Key points
- Start from the process and the final control element, not from the instruction set.
- On-off with deadband is the right choice for a level between two points with a fixed-speed pump; do not add PID to it.
- PID needs a modulating element and a measurable variable that responds to it; otherwise it is decoration.
- Ratio and feedforward act on the disturbance before the error appears; feedback trims what they miss.
- Cascade only helps when the inner loop is several times faster than the outer, and the inner loop is tuned first.
- Every loop gets a documented fallback: what happens when the transmitter fails, and what the operator does in manual.
The strategies
- On-off with deadband
- The output is fully on above one threshold and fully off below another. The deadband between them sets the cycle time. Wet well pumping between start and stop levels, tank filling, a heater in a panel.
- Staging
- Several fixed-capacity units are started and stopped in turn to match demand: lead, lag, lag 2 pumps by level or pressure; blowers by air demand; compressors by receiver pressure. Alternation shares wear.
- PID feedback
- A modulating element is adjusted continuously to hold a measured variable at a setpoint: pump speed for discharge pressure, valve position for flow, blower speed for dissolved oxygen, chemical pump speed for residual.
- Ratio
- One flow is held in proportion to another: chemical feed paced to plant flow at a dose setpoint. The controller computes the feed rate from the flow and the dose; there may be no feedback at all.
- Feedforward with feedback trim
- A measured disturbance sets most of the output and a feedback loop corrects the remainder: flow-paced hypochlorite with the dose trimmed by a residual analyzer.
- Cascade
- The output of a slow outer loop is the setpoint of a fast inner loop: dissolved oxygen sets an air flow setpoint, and air flow controls the blower. Level sets a flow setpoint that controls a pump speed.
- Split range
- One controller drives two elements over different parts of its output: a small valve for the first half of the range and a large valve for the second, or heating below 50 percent and cooling above.
- Override and selector
- Two controllers compete for one element and a selector picks the safer output: pressure control overridden by a maximum flow, or a low-select so that a suction pressure limit takes over from discharge pressure control.
Choosing
- 1
What does the process need?
A variable held between two limits, or held at a value? Between two limits is on-off or staging. At a value is modulating control of some kind.
- 2
What can the final control element do?
A fixed-speed pump can only run or stop; the strategy is on-off or staging no matter what the process would prefer. A drive, a control valve, or a metering pump can modulate; PID and its relatives become possible.
- 3
How does the variable respond?
Fast and direct, such as pressure to pump speed: PID alone. Slow with dead time, such as chlorine residual at a downstream sample point: feedforward or ratio does the bulk of the work and PID trims slowly.
- 4
Is there a measurable disturbance?
Plant flow, influent load, a tank being filled: measure it and feed it forward or use it as the ratio basis, rather than waiting for the error.
- 5
Is there a faster secondary variable?
If the element affects a fast variable that in turn affects the slow one, cascade: the fast inner loop rejects disturbances before the slow outer loop sees them.
- 6
Are there constraints?
A pressure that must not exceed a limit, a flow that must not fall below one: override control with a selector, and an alarm when the override is active.
- 7
What is the fallback?
Transmitter failure, communication loss, and manual operation: what does the loop do, and what does the operator do. Write it before writing the loop.
Applications
| Application | Strategy | Why | Common mistake |
|---|---|---|---|
| Lift station with fixed-speed pumps | On-off with lead-lag staging and alternation | Level between two points; the pumps cannot modulate | Adding a PID that has nothing to modulate |
| Lift station with drives | PID on level to lead pump speed, staging when the lead is at maximum | Smoother flow to the plant and fewer starts | Running the drive at minimum speed for hours below the pump curve |
| High service pumping | PID on discharge pressure to drive speed; staging by speed and pressure | Pressure held at a value; several pumps serve one demand | Staging on pressure alone, which fights the PID |
| Hypochlorite feed | Ratio to plant flow with residual trim | Dose follows flow immediately; residual corrects slowly for demand | Pure residual feedback with a long sample lag, which oscillates |
| Aeration dissolved oxygen | Cascade: DO sets air flow, air flow controls blower | DO responds slowly; air flow responds fast | DO directly to blower speed, which hunts |
| Filter effluent | PID on filter level to effluent valve | Constant level keeps the head loss curve honest | Tuning too aggressively and cycling the valve |
| Chemical day tank | On-off fill with deadband, high-high shutoff | Level between limits; a solenoid valve | Filling on a PID and hunting a solenoid |
| Pressure with a flow limit | Override: low-select between pressure PID and flow limit PID | Pressure held unless flow would exceed the limit | Alarming on nothing, so nobody knows the override is active |
Ratio and feedforward
For chemical feed the ratio calculation does most of the work. The feed rate is the plant flow times the dose setpoint, converted to the units of the feed pump and corrected for the strength of the chemical.
F = Q × D / (C × k)
- F = chemical feed rate, in the units the feed pump uses
- Q = plant flow
- D = dose setpoint, in mg/L as the active chemical
- C = concentration of the chemical as supplied, as the same active chemical
- k = the unit conversion between flow times dose and feed rate
Feedback trim then adjusts the dose setpoint slowly from a residual analyzer. The trim loop is tuned gently, because the analyzer is downstream and reports the result minutes after the change. A common arrangement limits how far the trim can move the dose, so a failed analyzer cannot drive the dose to zero or to the maximum.
Cascade
In a cascade the outer loop does not touch the equipment; it writes the setpoint of the inner loop, which does. The inner loop must be faster, several times faster as a rule, or the two loops interact and the result is worse than a single loop. Commission the inner loop first, with the outer loop in manual, and tune it to be fast and stable. Then close the outer loop and tune it slowly. If the inner loop is switched to manual or loses its transmitter, the outer loop must stop integrating, which the PID instruction on most platforms handles if the cascade is configured rather than improvised.
Combining strategies
Real loops are usually combinations. A high service system holds pressure with PID on the lead pump speed, stages the lag pump when the lead has been at maximum speed for a time and pressure is still low, and destages when the lead has been at minimum for a time and pressure is high. The staging logic uses speed and time as its basis so that it cooperates with the PID rather than competing with it on pressure. An aeration system cascades DO to air flow, with a most-open-valve strategy setting the blower pressure setpoint so the blowers work no harder than the neediest zone requires. Each layer is a simple strategy; the design is in how they hand off.
Fallbacks
- Transmitter failure: the loop holds its last output, goes to a safe fixed output, or switches to a backup measurement, and alarms. Never lets the failed value drive the output.
- Manual mode: the operator sets the output directly, with the setpoint tracking so the return to automatic is bumpless.
- Communication loss to a remote setpoint: hold the last setpoint, or revert to a local default, and alarm.
- Override active: an alarm that says which controller has the element, so a pressure that is not being held is explained.
Frequently asked questions
- When is on-off control better than PID?
- Whenever the final control element cannot modulate, and whenever the process only needs to stay between two limits. A wet well pumped by fixed-speed pumps, a tank filled by a solenoid valve, a heater: on-off with a suitable deadband is the correct design, not a compromise.
- How do I combine drive speed control with staging pumps?
- Let PID control the lead pump speed. Stage the lag pump when the lead has been at or near maximum speed for a set time and the controlled variable is still below setpoint; destage when the lead is at minimum for a set time and the variable is above. Base staging on speed and time, not on the variable alone, so the two mechanisms do not fight.
- What is override control for?
- Holding one variable at setpoint while respecting a limit on another. A discharge pressure loop with a low-select against a maximum flow controller keeps pressure until flow would exceed the limit, and then the flow controller takes the pump speed. Alarm when the override is active so operators know why the pressure is off.
- Why tune the inner loop first in a cascade?
- Because the outer loop sees the inner loop as part of its process. An inner loop that is slow or oscillating makes the outer loop impossible to tune, and any outer tuning done first becomes wrong the moment the inner loop is changed. Inner loop fast and stable, then outer loop slow.
Related topics
- PID Control for Pumps and ValvesWhat each term actually does, how to tune a loop on a real pump station, and the handful of mistakes that cause most of the oscillation people blame on tuning.
- Deadband and HysteresisWhy a single setpoint chatters, how a deadband turns it into a start point and a stop point, how wide to make it, and where deadbands belong in alarms, level control, and controller outputs.
- How to Create a PID Loop in a PLCSet up a PID instruction from scratch: scale the PV and CV, pick the action, set the execution rate, configure limits and anti-windup, tune conservatively, and test the manual and auto transitions before it controls anything.
- Lead/Lag Pump ControlWhich pump runs first, when the second joins, how they trade places, and the design decisions that determine whether a duplex station wears evenly.
- Wet Well Level PID ControlHolding a wet well level with a variable speed pump instead of bang-bang control: the loop structure, the minimum speed floor, the stop and restart logic at low inflow, tuning for a slow integrating process, and where level PID is the wrong idea.
- VFD Pump Control in WastewaterThe control modes for a variable speed wastewater pump: level control, flow pacing, fixed speed with soft start, and drawdown modes, plus the wastewater-specific rules: minimum speed, ragging at low speed, force main velocity, and the periodic full-speed run.
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