Open Loop vs Closed Loop Tension Control

Open loop vs closed loop tension control is the decision that decides whether your line holds set tension or only holds set torque. Open loop commands a torque and assumes the web obeys. Closed loop measures the web and corrects the torque until the force is right. Cost, sensor choice, commissioning time and how much loop tuning your maintenance team will tolerate all follow from that single difference.

Most buyers assume closed loop is simply better and move on. It usually is, but not always, and the exceptions are expensive. This article covers both architectures, the two ways open loop tracks roll diameter, what a sensor actually measures, when a closed loop performs worse, and how to decide.

The Difference in One Sentence

Open loop controls the cause. Closed loop controls the result.

In an open loop station the controller sends current to a brake or clutch and stops there; nothing measures the web. In a closed loop station a load cell measures the web, the controller compares that reading against the setpoint, and current is adjusted to close the gap. Both still obey the same equation:

T (N·m) = F (N) × R (m)

Torque equals web force times roll radius. Open loop calculates T from an estimate of R and hopes. Closed loop keeps changing T until F is correct, whatever R happens to be.

What Open Loop Actually Controls

An open loop unwind or rewind station is a torque source with a diameter correction. On unwind, a magnetic powder brake holds the parent roll back so the web must be pulled off under load. On rewind, a magnetic powder clutch pulls the web onto the core. In both cases torque tracks coil current almost linearly, which is precisely why powder devices suit open loop duty.

Open loop handles steady running well. What it cannot do is see a disturbance: a splice passing through, a parent roll with a hard and a soft side, a bearing starting to drag, a nip that slips. Nothing in an open loop knows any of that happened, so tension drifts silently.

The Two Diameter Compensation Methods

Open loop only works if the controller knows R. There are two ways to supply it.

Method 1 — measure the radius. An ultrasonic or laser distance sensor aimed at the roll surface reports remaining radius directly. With no extra sensor at all you can also derive it: measure roll speed and line speed, then compute R = V ÷ (2πn), where V is line speed in m/min and n is roll speed in rev/min. Both approaches track the true radius continuously and self-correct as conditions change.

Method 2 — accumulate thickness. Start from core diameter and add two material thicknesses per revolution. It needs only a proximity pick-up and a thickness entry, so it is the cheapest option and the least accurate. A wound roll is never the sum of its thicknesses: entrained air, compressibility and caliper variation all make real diameter larger than calculated. Paper, nonwoven and textile are the worst offenders. The error compounds with every wrap, so torque is furthest wrong where the roll is biggest and a tension error costs the most material.

Rule of thumb: if your material is compressible, or a roll builds for more than about twenty minutes, measure the radius instead of accumulating thickness.

What a Closed Loop Sensor Really Measures

Adding a sensor does not automatically give you a truthful tension reading. A load cell under a measuring roller reads the resultant of two web strands plus everything else acting on that roller:

F_sensor = 2 × T × sin(θ ÷ 2) + roller weight component + bearing drag

θ is the wrap angle. At 180° the cell sees 2T, at 90° it sees 1.41T, and at 30° only 0.52T. A small wrap angle does more than shrink the signal — it amplifies every other error, because the web contribution falls while roller weight and drag stay the same. Use the largest wrap angle the layout allows and always tare the roller weight before calibrating.

The bearing-drag term is why a seized idler can hide inside a loop that looks healthy: the controller faithfully holds a number that is no longer web tension. Sensor architecture matters here. Pillow block tension sensors support the roll at both ends, a cantilever tension load cell suits narrow webs and quick roll changes, and a through-shaft web tension sensor turns a dedicated roller into the measuring element where space is tight.

When Closed Loop Performs Worse Than Open Loop

This is the part most selection guides skip.

• Oversized sensor range. A cell rated many times the working force puts the signal in the bottom few percent of its range, so noise dominates and the loop chases noise — modulating the brake and printing a visible pattern into the roll. A correctly tapered open loop is smoother than that.
• Sensor too far from the disturbance. Every metre of free span between the sensor and the actuator adds compliance and delay. You then lower gain and get a sluggish loop, or raise it and get oscillation. Relocating the sensor beats retuning it.
• Fast acceleration. A load cell cannot separate web tension from the force needed to spin the measuring roller up. During a hard ramp the reading jumps and a stiff loop reacts to a disturbance that is not in the web at all. Ramp-rate limiting, or acceleration feed-forward, is the fix.
• Measuring a dead zone. If the sensor sits downstream of a nip that already fixes tension, the loop has no authority there. It will drive the brake for a long time without changing the number.

In all four cases the honest answer is not a better controller. It is a moved sensor, a resized sensor, or a slower ramp.

What Closed Loop Costs You

Item	Open loop	Closed loop
Hardware	Controller plus actuator	Adds one to two sensors, brackets, cabling
Commissioning	Taper setup only	Zero and span, tare, wrap-angle calculation, loop tuning
Skill needed	Any competent electrician	Someone who can tune a loop
Failure modes	Torque drifts with diameter	Sensor drift, noise, oscillation, cable faults
Best fit	Paper, board, tolerant webs, slow lines	Film, foil, lithium electrode, high speed

Payback comes out of scrap and rewind rejects: estimate your current reject rate minus the expected rate, times annual throughput and material cost per unit. If that does not cover the installed cost within a year, stay open loop. A full automatic web tension controller handles diameter following, taper and multiple zones, while a compact tension controller covers a single narrow-web station for less.

How to Decide: A Five-Point Checklist

1. Is the material thin, stretchy or expensive? If yes, go closed loop.
2. Is there an existing roller with a large wrap angle close to the disturbance? If not, fix the web path first.
3. Can you obtain a reliable radius signal? If not, open loop will drift.
4. Does the line start and stop frequently? Then use closed loop with ramp limiting.
5. Who will tune the loop after commissioning? If the answer is nobody, choose open loop with good taper.

FAQ

Q: Is a dancer open loop or closed loop? Neither, strictly. A dancer measures position, not force, so it is a third architecture: it holds a mechanical storage arm in mid-travel and infers tension from the air pressure or weight setting. Dancers absorb short disturbances better than either and are common on unwinds with splices.

Q: Do I need sensors on both unwind and rewind? Only if both zones need tight control. Many lines run a closed loop rewind — where roll quality is judged — with an open loop unwind, and that is a perfectly sound compromise.

Q: Can I upgrade from open loop to closed loop later? Yes, and it is a common retrofit path. You need a roller you can convert to a measuring roll, room for the sensor, and a controller with a sensor input. Confirm both before you buy the actuator.

Q: Why does my closed loop hunt at low tension? Usually one of three causes: the sensor range is far above your working force, the wrap angle is too small, or the gain is set for a heavier product. Check range and geometry before touching gain.

Conclusion

Open loop vs closed loop tension control is a trade between torque you can predict and force you can verify. Open loop is cheaper, simpler and honestly adequate for tolerant materials and steady duty — provided the diameter is measured rather than estimated. Closed loop is the right answer for films, foils, battery electrodes and high-speed lines, but only when the sensor is correctly sized, well placed and given a ramp it can follow.

XW Machinery supplies the whole chain, from powder brakes and clutches through sensors to controllers, so both architectures can be specified as one supported system. Contact XW Machinery with your material, width, speed and roll build time for a recommendation and a factory-direct quote.