Configuring PLC HMI Bend Angle Control on Hydraulic Tube Benders

Configuring PLC and HMI Bend Angle Control on Hydraulic Tube Benders

Overview

Hydraulic rotary-draw tube benders require precise angular position control to produce repeatable bends in metal tubing, typically within ±1° to ±2° accuracy. This reference documents the complete architecture, hardware selection, PLC programming, HMI configuration, and tuning procedures required to implement closed-loop bend angle control using a rotary encoder, hydraulic directional or proportional control valves, a SIMATIC S7-1200 PLC, and a SIMATIC HMI panel. The dominant engineering problem in this application class is position overshoot caused by accumulated system delays between the PLC detecting the target position and the hydraulic actuator physically responding. This document describes both the entry-level on/off solenoid approach and the upgraded proportional-valve / motion-controller approach.

Mechanical and Hydraulic System Definition

The reference machine is a rotary-draw tube bender. The principal mechanical chain is:

• Bend head rotated by a hydraulic cylinder pulling on a triplex roller chain
• Chain connected via a sprocket keyed to the bend head shaft
• Tube clamped in the bend die, pressured against the wiper shoe, and wrapped around the bend die as the head rotates

Two solenoid-operated directional valves control the cylinder. One solenoid extends the cylinder (bend direction); the other retracts. Spring centering or a pilot-operated check holds the cylinder in position when both solenoids are de-energized. For ±1° to ±2° accuracy, on/off solenoid control is sufficient only if the cycle is slowed and the stopping position is anticipated. A proportional valve or a two-stage on/off arrangement (fast approach plus slow finish) is required for higher speed at the same accuracy.

System Delay Budget

The total latency from "PLC issues stop command" to "shaft stops rotating" determines the minimum controllable overshoot. The principal contributors are tabulated below.

Delay Source	Typical Value	Notes
PLC scan time	1–10 ms	S7-1200 typical scan 1–3 ms with simple logic
Digital output module update	0.5–2 ms	SM 1223 DQ sourcing response
Relay coil pull-in / drop-out	8–15 ms	Phoenix Contact PLC-RSC or equivalent
Solenoid valve shift	15–40 ms	Direct-acting; pilot-operated 25–60 ms
Hydraulic spool deadband	Variable	±2–5% of command
Cylinder compressibility	Material-dependent	Steel tube springback dominates
Encoder filter / HSC update	1–4 ms	HSC input filter 0.6 µs to 20 ms selectable
Chain backlash	0.5–3°	Compensated by software offset
Tube springback	1–8° per bend	Function of material, radius, wall, mandrel

At a 30°/s bend speed, a 100 ms total latency corresponds to 3° of overshoot at the hydraulic spool before springback correction. The control algorithm must command the stop well before the encoder reading reaches the target.

Rotary Encoder Selection

The encoder is the only position feedback in this system; its resolution and noise rejection set the absolute accuracy floor.

Encoder Type and Output

For a 0–180° bend range and ±1° target, an incremental encoder with 1024 pulses per revolution (PPR) provides 0.088° per pulse before quadrature, 0.044° per edge with x4 decoding. An encoder with HTL push-pull outputs (10–30 V) is recommended for industrial environments; TTL 5 V signals should be avoided beyond 5 m cable runs. A 360 PPR encoder yields 0.25° per edge in x4, marginal for ±1° work.

Parameter	Minimum Recommended	Preferred
PPR	1024	2500 or 5000
Output	HTL push-pull	HTL push-pull, short-circuit protected
Supply	10–30 VDC	24 VDC ±10%
Max frequency	50 kHz	100 kHz
Shaft / hollow bore	Hollow bore with tether	Both acceptable
IP rating	IP65	IP67
Connector	M12 8-pin	M12 8-pin

Encoder output A, /A, B, /B connects to the S7-1200 high-speed counter inputs. The Z (index) pulse is not required for this application but is recommended for home-position verification at power-up. Refer to the SIMATIC S7-1200 Programmable Controller System Manual for HSC wiring details.

HSC Configuration on S7-1200

The S7-1200 CPU provides up to 6 high-speed counters. The HSC supports two-phase quadrature counting (x1, x2, or x4), frequency measurement, and period measurement. For a bender, configure:

• Operating mode: Two phase (quadrature)
• Counting direction: User program (count up = positive bend direction)
• Initial count value: 0
• High / low limit: Defined by bend angle range and PPR

In TIA Portal, configure the HSC under CPU properties → High-speed counters (HSC). For an S7-1214C DC/DC/DC (6ES7214-1AG40-0XB0), the supported HSC inputs are I0.0–I0.5. The function block CTRL_HSC provides the user interface. See the S7-1200 Motion Control Function Overview for HSC function-block reference.

PLC Hardware Selection

For a bender with 2–4 solenoid outputs, 1–2 limit switch inputs, 1 encoder, and HMI, the minimum S7-1200 selection is listed below.

Module	Catalog Number	Function
CPU 1214C DC/DC/DC	6ES7214-1AG40-0XB0	14 DI / 10 DO / 2 AI onboard
SM 1223 DQ 8 x 24V	6ES7223-1BH32-0XB0	8 DO if not on CPU
KTP700 Basic PN	6AV2123-2GB03-0AX0	7" HMI for setpoint, actual, manual mode
PM 1207	6EP1332-1SH71	24 V / 2.5 A power supply
CSM 1277	6GK7277-1AA10-0AA0	4-port unmanaged switch

If the application is upgraded to a proportional valve (4–20 mA or ±10 V), add an SM 1232 AQ 4 x 14-bit module (6ES7232-4HD32-0XB0). For higher performance, the S7-1500 platform (CPU 1511-1 PN, 6ES7511-1AK02-0AB0) with the TO_PositioningAxis technology object provides a direct path to 0.1° accuracy closed-loop control. Refer to the SIMATIC S7-1500 Motion Control Function Manual.

HMI Panel and Tag Configuration

A SIMATIC Basic HMI (KTP700 Basic PN, 6AV2123-2GB03-0AX0) is sufficient for the entry-level implementation. A Comfort Panel (TP700 Comfort) is recommended for diagnostics, trends, and recipe management. The HMI design guidance is in the WinCC Engineering Manual.

The minimum HMI tag list:

Tag	PLC Address	Data Type	Function
hmi_SetAngleDeg	DB1.DBD0	Real	Operator entry 0.0–180.0
hmi_ActualAngleDeg	DB1.DBD4	Real	Encoder-derived position
hmi_CommandStart	DB1.DBX8.0	Bool	Bend cycle start
hmi_CommandReset	DB1.DBX8.1	Bool	Fault reset
hmi_ManualMode	DB1.DBX8.2	Bool	Manual jog enable
hmi_SpringbackOffset	DB1.DBD12	Real	Per-recipe springback compensation
hmi_SlowZoneTrigger	DB1.DBD16	Real	Angle at which bend slows down
hmi_StatusWord	DB1.DBW20	Word	PLC status flags
hmi_FaultWord	DB1.DBW22	Word	Active fault bits
hmi_RecipeNumber	DB1.DBW24	Int	Recipe selection 0–15

HMI screen layout:

• Main screen: Numeric input for setpoint (0.0–180.0, 1 decimal), I/O field for actual position, two pushbuttons START and RESET, status bar showing Ready / Running / Complete / Fault, fault text list
• Calibration screen: Encoder home, zero offset, PPR multiplier
• Recipe screen: 16 recipes storing setpoint, springback, slow-down trigger angle, approach speed
• Diagnostics screen: Status word, fault word, HSC count, last cycle time, overshoot trend

Enable the HMI audit trail if the bent part must be traceable to a recipe. Use the HMI recipe view to write structured data from the panel to a recipe DB on the PLC.

PLC Program Structure

The PLC program is organized into the following organization blocks and function blocks:

• OB1 Main: Cyclic scan, calls FB_HmiControl, FB_BendSequence, FB_Encoder
• OB35 (or OB30–OB38 cyclic interrupt): Position loop execution at 10 ms, calls FB_PositionController
• OB82 / OB121: Fault and time-error handling
• FB_Encoder: HSC configuration and conversion of raw counts to degrees
• FB_PositionController: Closed-loop control with two-stage velocity profile
• FB_BendSequence: State machine driving outputs based on operator commands
• FB_HmiControl: HMI tag exchange and recipe handling
• DB_HMI: HMI interface tags
• DB_Recipes: 16-recipe array, each record 16 bytes

Encoder to Degrees Conversion

For a 1024 PPR encoder with x4 decoding mounted directly to the bend shaft:

Counts_per_degree = (1024 * 4) / 360 = 11.378 counts/°

Allow for sprocket ratio if the encoder is not on the bend shaft. A common configuration is a 2:1 reduction; the formula becomes:

Counts_per_degree = (PPR * 4 * GearRatio) / 360

The conversion in Structured Text:

IF b_EncoderValid THEN
    i_EncoderCounts := HSC1_Count;
    r_AngleDeg := INT_TO_REAL(i_EncoderCounts) / r_CountsPerDeg;
END_IF;

At every power-up, the program must force the home position. Because the bender has no absolute reference, the home procedure retracts the cylinder to a mechanical end-stop, then sets HSC1_Count := 0 after a debounce delay (typically 500 ms). Alternatively, an absolute single-turn encoder (SSI or CANopen) removes the home routine.

Two-Stage Velocity Profile and Stop Anticipation

The dominant technique for ±1° accuracy on a hydraulic system is a two-stage approach:

1. Fast bend from 0° to (setpoint − 5°), full valve command, ~30°/s
2. Slow approach from (setpoint − 5°) to (setpoint − springback), partial valve command or single-solenoid PWM, ~5°/s
3. Coasting zone from (setpoint − springback) to setpoint, all valves de-energized, allow cylinder to coast and lock

The 5° slow-zone trigger is a tunable parameter in the recipe. The transition from fast to slow is triggered when the actual angle enters the slow-zone window. Springback compensation is material-specific: 304 stainless bends 2–4° back after load release; mild steel A513 typically 1–2°; aluminum 6061-T6 typically 1–3°.

The bend state machine implemented in OB35 (simplified SCL):

IF "DB_HMI".hmi_CommandStart AND NOT "DB_HMI".b_Running THEN
    "DB_HMI".b_Running := TRUE;
    "DB_HMI".r_TargetDeg := "DB_HMI".hmi_SetAngleDeg;
    "DB_HMI".r_SlowZoneDeg := "DB_HMI".hmi_SetAngleDeg - "DB_HMI".hmi_SlowZoneTrigger;
    "DB_HMI".r_FinalDeg := "DB_HMI".hmi_SetAngleDeg - "DB_HMI".hmi_SpringbackOffset;
END_IF;

IF "DB_HMI".b_Running THEN
    IF "DB_HMI".r_AngleDeg < "DB_HMI".r_SlowZoneDeg THEN
        "Q_BendForward" := TRUE;      // fast extend
        "Q_BendReverse" := FALSE;
    ELSIF "DB_HMI".r_AngleDeg < "DB_HMI".r_FinalDeg THEN
        "Q_BendForward" := TRUE;      // slow extend via PWM
        "Q_BendReverse" := FALSE;
    ELSE
        "Q_BendForward" := FALSE;     // coast and stop
        "Q_BendReverse" := FALSE;
        "DB_HMI".b_Running := FALSE;
        "DB_HMI".b_Complete := TRUE;
    END_IF;
END_IF;

PWM on the S7-1200 is generated by the PTO/PWM generator of the onboard outputs Q0.0–Q0.3. PWM frequency 50–200 Hz, duty cycle set to 30% for slow approach. This is a coarse but effective speed control on a direct-acting solenoid valve. For higher precision, replace the on/off solenoid pair with a single proportional valve (4-way, 4–20 mA command) and use the PID_Compact block to control actual angle to setpoint with an aggressive derivative term to anticipate overshoot.

PID Position Controller (Proportional Valve Alternative)

If the system is upgraded with a proportional valve, use the technology object PID_Compact (firmware V4.0 and later on S7-1200) configured as position control:

• Input (process value): r_AngleDeg
• Setpoint: r_TargetDeg
• Output: 4–20 mA proportional valve command
• Sampling time: 100 ms (matches hydraulic response)
• Derivative action: enabled, smoothing time 1 s
• Proportional gain: tune manually with Ziegler-Nichols step test
• Output limits: 0%–100% of valve range

The PID_Compact self-tuning can be used at first commissioning; the resulting gain is conservative. Manually trim by increasing GainValue until the system shows a single overshoot of 0.5°–1°, then back off 20%. Refer to the PID Control with PID_Compact Function Manual.

Tuning Formula

The effective steady-state gain of a hydraulic position system can be estimated by:

K_plant = Δθ / (Q_valve × t)

Where Δθ is the angle change in degrees over time t at a constant valve command Q_valve (% of 4–20 mA span). For a representative bender: K_plant ≈ 0.6 °/s per % valve. The closed-loop bandwidth is:

ω_c ≈ Kp × K_plant

For 1 s response with no overshoot, ω_c ≈ 3 rad/s, giving Kp ≈ 5.0.

Wiring and Electrical Design

Encoder wiring must use twisted-pair shielded cable. Ground the shield at the panel end only; the encoder end is isolated by the bearing. Encoder 24 V supply must be sourced from the same 24 VDC bus as the PLC inputs to avoid ground-loop noise.

Solenoid outputs on the S7-1200 DC outputs are rated 0.5 A per channel. A typical hydraulic solenoid draws 1.0–1.5 A inrush, 0.3–0.5 A holding. Use an interposing relay (Phoenix PLC-RSC-24UC/21, 2966171) or a solid-state relay (Phoenix PLC-OSC-24DC/24DC/2, 2966634) sized to the inrush. If the solenoid is 110 VAC or 230 VAC, a 24 VDC coil relay is required; never switch AC directly from a transistor output.

Safety Considerations

A tube bender presents several hazards: high hydraulic pressure (typically 1500–3000 PSI), rotating mass, and pinch points at the bend die. Functional safety must be addressed even if the basic implementation does not use a safety PLC.

Mandatory safety functions:

• Emergency stop wired through a safety relay (Pilz PNOZ s4, 750104) or a fail-safe S7-1200F CPU (CPU 1214FC, 6ES7214-1AF40-0XB0)
• Two-hand control for cycle start, or light curtain on operator side
• Hydraulic pressure relief valve set 10% above operating pressure
• Mechanical crash stops in case of hydraulic runaway
• Light curtain or area scanner on operator side per ANSI B11.19 / ISO 13849 PL d

Wire the E-stop as a hardwired circuit, not a software flag. The PLC's safety input is supplemental. Refer to the SIMATIC Safety Integrated for S7-1200/1500 Manual for fail-safe wiring patterns.

Commissioning Procedure

1. Verify wiring against schematic. Check 24 VDC polarity on every terminal. Megger insulation on the encoder cable if installed in a tray with VFD cables.
2. Power up PLC. Connect TIA Portal. Download hardware configuration. Confirm no diagnostic LEDs on CPU or SM modules.
3. Open the HSC configuration screen. Manually rotate the encoder. Confirm HSC1_Count increments in the correct direction. Reverse the A and B leads if direction is wrong.
4. Wire the home limit switch. Force the cylinder to retract to the home stop. Verify HSC1_Count = 0 at home.
5. Configure HMI tags. Set up the screens in WinCC within TIA Portal.
6. Test outputs in manual mode from the HMI. Confirm extend/retract solenoid actuation.
7. Run a 10° bend with springback compensation disabled. Record actual final angle. Compute springback as target minus actual.
8. Enter springback into recipe. Run 30°, 60°, 90°, 120°, 150° at slow speed. Plot actual vs. target.
9. Tune the slow-zone trigger angle and the slow approach speed to achieve target accuracy.
10. Lock the cabinet. Perform the operator acceptance test with full production tubes.

Verification and Acceptance

Acceptance criteria for ±1° accuracy on production tubes, post springback correction:

• All bends within ±1.0° of programmed setpoint, verified with a digital protractor
• 95% of bends within ±0.5° after recipe tuning
• Cycle time not more than 110% of pre-PLC retrofit cycle
• No nuisance faults over 1000 cycles

Troubleshooting Matrix

Symptom	Likely Cause	Verification	Action
Position reading jumps at high speed	Encoder frequency exceeds HSC input	Check HSC count stability at full speed; set HSC input filter to 0.6 µs	Reduce bend speed; replace encoder with higher PPR
Reading is correct direction reversed	A and B swapped	Jog bend and observe count sign	Swap A and B at the terminal block
Overshoot of 3°–10°	Springback not compensated; slow zone too late	Record actual vs. setpoint curve	Increase springback offset, reduce slow zone trigger angle
Cycle stops short of setpoint	Slow zone trigger angle too aggressive; encoder lost counts	Compare HSC count to expected	Recalibrate encoder; widen slow zone
E-stop does not stop motion	E-stop wired to standard DI not F-DI	Inspect wiring	Re-wire to safety input or safety relay
Hydraulic chatters in slow zone	PWM frequency too high for valve; valve hysteresis	Listen for chatter; reduce PWM freq	Lower PWM to 30–50 Hz; use proportional valve
HMI shows "Encoder Fault"	Wire break or short; HSC diagnostic	Check SF LED on CPU; HSC input status	Inspect cable; replace encoder
Position drift after power cycle	No absolute reference; home procedure skipped	Verify home routine execution	Add home routine on every power-up
Reading noisy in noisy plant	Encoder cable routed with VFD cable; ground loop	Inspect cable routing; measure noise with scope	Reroute cable; ground shield at panel only

Advanced Motion Control

If ±1° is not achievable at the required cycle time with a hydraulic on/off valve, upgrade the control loop:

• Use a SIMOTION D425-2 (6AU1425-2AD00-0AA0) or a SIMATIC S7-1500T (CPU 1511T-1 PN, 6ES7511-1TK02-0AB0) motion controller
• Use a servo-driven rotary actuator (Siemens 1FK7 with HIPERFACE DSL) or a closed-loop hydraulic servo valve
• Use the TO_PositioningAxis technology object
• Position loop update 1 ms
• Override the on/off valve with a closed-loop proportional or servo valve

A Siemens SINAMICS V90 servo drive (6SL3210-5FB10-1UF0) with a 1FK7 motor replacing the hydraulic cylinder delivers ±0.1° accuracy, eliminates springback as a control variable, and removes the hydraulic power pack. The trade-off is reduced peak torque compared to a 2000 PSI hydraulic cylinder. See the SIMATIC S7-1500T Motion Controller Manual for commissioning details.

FAQ