12Level 4

Two pumps with alternation and backup

Two pumps fill a tank. Each filling cycle uses a different one to share the wear, and if one fails the other takes over and an alarm lights up.

Open in the simulator

The simulator is designed for computers (screens 1024 px or wider).

Virtual plant

Shown at rest. In the simulator the plant moves according to your program's outputs.

Inputs and outputs

Address	Name	Description
I0.0	SYSTEM	System switch
I0.2	LEVEL_LOW	Low level switch: 1 when covered (plant)
I0.3	LEVEL_HIGH	High level switch: 1 when covered (plant)
I0.4	PUMP1_OK	Pump 1 NC fault contact: 0 = fault (plant)
I0.5	PUMP2_OK	Pump 2 NC fault contact: 0 = fault (plant)
Q0.0	PUMP_1	Pump 1 contactor
Q0.1	PUMP_2	Pump 2 contactor
Q0.2	ALARM	Fault alarm light
M0.0	DEMAND	The tank needs water (between the two switches)
M0.1	TURN_P2	Pump 2's turn in this cycle
M0.2	SWITCH_TO_P2	Pulse: turn goes to pump 2
M0.3	SWITCH_TO_P1	Pulse: turn goes to pump 1

Program

Ladder diagram (LD)

Rung 1 — Water demand with hysteresis between the low and high switches.
Rung 2 — New cycle and it was pump 1's turn: it goes to pump 2.
Rung 3 — New cycle and it was pump 2's turn: it goes to pump 1.
Rung 4 — Turn switches to pump 2.
Rung 5 — Turn switches to pump 1.
Rung 6 — Pump 1: in its turn, or as backup if pump 2 fails.
Rung 7 — Pump 2: in its turn, or as backup if pump 1 fails.
Rung 8 — Alarm on any fault.

Structured text (ST)

(* Two pumps with alternation and backup — converted from Ladder *)

VAR
  R_TRIG_1 : R_TRIG;
  R_TRIG_2 : R_TRIG;
END_VAR

(* Rung 1: Water demand with hysteresis between the low and high switches. *)
DEMAND := (SYSTEM AND NOT LEVEL_LOW OR SYSTEM AND DEMAND) AND NOT LEVEL_HIGH;

(* Rung 2: New cycle and it was pump 1's turn: it goes to pump 2. *)
R_TRIG_1(CLK := DEMAND);
SWITCH_TO_P2 := R_TRIG_1.Q AND NOT TURN_P2;

(* Rung 3: New cycle and it was pump 2's turn: it goes to pump 1. *)
R_TRIG_2(CLK := DEMAND);
SWITCH_TO_P1 := R_TRIG_2.Q AND TURN_P2;

(* Rung 4: Turn switches to pump 2. *)
IF SWITCH_TO_P2 THEN TURN_P2 := TRUE; END_IF;

(* Rung 5: Turn switches to pump 1. *)
IF SWITCH_TO_P1 THEN TURN_P2 := FALSE; END_IF;

(* Rung 6: Pump 1: in its turn, or as backup if pump 2 fails. *)
PUMP_1 := DEMAND AND PUMP1_OK AND NOT TURN_P2 OR DEMAND AND PUMP1_OK AND NOT PUMP2_OK;

(* Rung 7: Pump 2: in its turn, or as backup if pump 1 fails. *)
PUMP_2 := DEMAND AND PUMP2_OK AND TURN_P2 OR DEMAND AND PUMP2_OK AND NOT PUMP1_OK;

(* Rung 8: Alarm on any fault. *)
ALARM := NOT PUMP1_OK OR NOT PUMP2_OK;

Instruction list (IL)

(* Two pumps with alternation and backup — converted from Ladder *)

VAR
  R_TRIG_1 : R_TRIG;
  R_TRIG_2 : R_TRIG;
END_VAR

(* Rung 1: Water demand with hysteresis between the low and high switches. *)
  LD    SYSTEM
  ANDN  LEVEL_LOW
  OR(   SYSTEM
    AND   DEMAND
  )
  ANDN  LEVEL_HIGH
  ST    DEMAND

(* Rung 2: New cycle and it was pump 1's turn: it goes to pump 2. *)
  CAL   R_TRIG_1(CLK := DEMAND)
  LD    R_TRIG_1.Q
  ANDN  TURN_P2
  ST    SWITCH_TO_P2

(* Rung 3: New cycle and it was pump 2's turn: it goes to pump 1. *)
  CAL   R_TRIG_2(CLK := DEMAND)
  LD    R_TRIG_2.Q
  AND   TURN_P2
  ST    SWITCH_TO_P1

(* Rung 4: Turn switches to pump 2. *)
  LD    SWITCH_TO_P2
  S     TURN_P2

(* Rung 5: Turn switches to pump 1. *)
  LD    SWITCH_TO_P1
  R     TURN_P2

(* Rung 6: Pump 1: in its turn, or as backup if pump 2 fails. *)
  LD    DEMAND
  AND   PUMP1_OK
  ANDN  TURN_P2
  OR(   DEMAND
    AND   PUMP1_OK
    ANDN  PUMP2_OK
  )
  ST    PUMP_1

(* Rung 7: Pump 2: in its turn, or as backup if pump 1 fails. *)
  LD    DEMAND
  AND   PUMP2_OK
  AND   TURN_P2
  OR(   DEMAND
    AND   PUMP2_OK
    ANDN  PUMP1_OK
  )
  ST    PUMP_2

(* Rung 8: Alarm on any fault. *)
  LDN   PUMP1_OK
  ORN   PUMP2_OK
  ST    ALARM

Step by step

1Press ▶ Run (F5) and turn on SYSTEM. DEMAND turns on at low level and stays on until high level, as in the tank example.
2Rungs 2 to 5: on each rising edge of DEMAND, TURN_P2 changes value. That way each filling is done by a different pump; see each pump's running time in the plant.
3Rungs 6 and 7: each pump runs in its turn, or in the other's turn if the other one has a fault. In the plant, press “Fault” on a pump and watch the other one back it up.
4Rung 8: any fault turns on the ALARM. Faults are read with the equipment's NC contacts: a broken wire also shows up as a fault.

Open in the simulator