Deadband Controller

On/off control with hysteresis to prevent hunting

Overview

The Deadband Controller implements on/off control with hysteresis (dead zone) to prevent rapid cycling of equipment. This protects motors and actuators while providing simple, reliable control.

When to Use

• Equipment protection: Prevent motor wear from rapid cycling
• Binary actuators: On/off valves, pumps, blowers
• Cost reduction: When continuous control is unnecessary
• Simple systems: When PID is overkill

Common Applications

Application	Deadband	Min On/Off Time
Blower on/off	0.2-0.5 mg/L DO	5-15 min
Pump cycling	0.1-0.3 m level	2-5 min
Heating/cooling	1-2°C	5-10 min
Sludge blanket	0.1-0.2 m	10-30 min

Control Theory

Concept

graph TD
    subgraph Hysteresis
        H1[High Limit = SP + DB]
        H2[Low Limit = SP - DB]
        SP[Setpoint]
    end

    PV[Process Variable] --> Compare{PV vs Limits}
    Compare -->|PV > High| ON[Turn ON]
    Compare -->|PV < Low| OFF[Turn OFF]
    Compare -->|In Deadband| HOLD[Hold State]

Mathematical Model

Direct Action (turn ON when PV rises):

• Turn ON when: $PV > SP + deadband$
• Turn OFF when: $PV < SP - deadband$

Reverse Action (turn ON when PV falls):

• Turn ON when: $PV < SP - deadband$
• Turn OFF when: $PV > SP + deadband$

State Machine

stateDiagram-v2
    [*] --> OFF
    OFF --> ON : PV > High AND time > minOffTime
    ON --> OFF : PV < Low AND time > minOnTime

Parameters

Parameter	Unit	Range	Default	Description
Setpoint	varies	Process dependent	-	Center of deadband
Deadband	same as PV	0.1-5% of span	0.5	Half-width of dead zone
High Limit	same as PV	SP + deadband	-	Turn-on threshold
Low Limit	same as PV	SP - deadband	-	Turn-off threshold
On Value	varies	0-100%	1 (100%)	Output when ON
Off Value	varies	0-100%	0 (0%)	Output when OFF
Min On Time	days	0.001-0.1	0.01	Minimum time in ON state
Min Off Time	days	0.001-0.1	0.01	Minimum time in OFF state
Action	-	direct/reverse	direct	Control direction

Tuning Guidelines

Deadband Sizing

1. Too narrow: Frequent cycling, equipment wear
2. Too wide: Poor control, large deviations
3. Optimal: 2-5% of control range

Minimum Cycle Time

Consider equipment requirements:

Equipment	Minimum Cycle Time
Large motors	10-15 min
VFD-controlled	2-5 min
Solenoid valves	30 sec
Pumps	5-10 min

Recommended Settings

Application	Setpoint	Deadband	Min On/Off
DO blower	2.0 mg/L	0.3 mg/L	10 min
Level pump	2.0 m	0.2 m	5 min
Temperature	35°C	1.0°C	5 min

Configuration

Example: Blower On/Off Control

{
  "id": "blower-deadband-1",
  "type": "Deadband",
  "enabled": true,
  "targetNodeId": "aerobic-reactor-1",
  "pvNodeId": "aerobic-reactor-1",
  "pvVariable": "DO",
  "manipulatedVariable": "blowerState",
  "setpoint": 2.0,
  "deadband": 0.3,
  "highLimit": 2.3,
  "lowLimit": 1.7,
  "onValue": 1,
  "offValue": 0,
  "action": "reverse",
  "minOnTime": 0.007,
  "minOffTime": 0.007,
  "mode": "auto"
}

Note: action: "reverse" because we turn ON the blower when DO falls LOW.

Integration Example

Blower Cycling for DO Control

Scenario: Single-speed blower with no VFD

1. DO drops below 1.7 mg/L → Blower turns ON
2. DO rises above 2.3 mg/L → Blower turns OFF
3. DO between 1.7-2.3 → Maintain current state
4. Minimum times prevent cycling faster than 7 min

Timing Diagram

DO     2.5 ─────────────┐              ┌─────
(mg/L)              │              │
       2.3 - - - - -│- - - - - - -│- - HIGH
       2.0 ─────────────────────────────── SP
       1.7 - - - - - - - - - - - - - - - LOW
       1.5 ─────────────┘              │
                                       └─────

Blower OFF─────────────┐              ┌─────
       ON              └──────────────┘

Best Practices

1. Size deadband appropriately

   • Consider measurement noise
   • Account for process lag
   • Protect equipment

2. Set appropriate minimum times

   • Check motor manufacturer specs
   • Consider thermal limits
   • Account for process response

3. Choose correct action

   • Direct: ON when PV high (cooling, draining)
   • Reverse: ON when PV low (heating, filling, aeration)

4. Monitor cycling frequency

   • Track on/off transitions
   • Alert if cycling too fast

Troubleshooting

Issue	Cause	Solution
Rapid cycling	Deadband too narrow	Increase deadband
Poor control	Deadband too wide	Reduce deadband
Stuck ON/OFF	Min time too long	Reduce min time
Wrong direction	Action reversed	Toggle action setting
Noise triggering	Measurement noise	Increase deadband or filter PV

Comparison with PID

Aspect	Deadband	PID
Control quality	Good within band	Precise at setpoint
Equipment wear	Low (limited cycles)	Moderate (continuous)
Complexity	Simple	Moderate
Energy	Step changes	Gradual changes
Best for	Binary actuators	Modulating actuators

Related Controllers

• Switch-Controller - Smooth transitions instead of steps
• TimeOnOff-Controller - Time-based cycling
• PID-Controller - Continuous modulation

References

1. Shinskey, F.G. "Process Control Systems" (4th Edition)
2. Smith, C.L. "Practical Process Control" (2009)