Improving on servo systems for tension control

For tension control applications there is a highly efficient, cost effective and simple mode of control provided by unique hysteresis brake and clutch products from ZF Industrial

When asked to provide tension control of winding/unwinding operations the natural reaction of today's control engineers would be to reach for a servo system.

This is not surprising because servo systems are routinely used for many precision tasks in industrial engineering, including, positioning, winding, interpolation and registration.

They are used for so many applications, in fact, that in relative terms they could actually be called "non-specialist".

The nature of the equipment itself underlines this point.

The servomotor and its amplifier are standard pieces of equipment, which have to integrated and adapted to the applications for which they are specified, usually with high-powered software, proprietary or otherwise.

The costs of servo systems are coming down all the time, however this does tend to obscure the fact that with servo systems one is looking at total installed cost, not just the cost of the equipment itself.

The cabling costs for servos are generally steep, being accrued for encoder feedback as well as the main cabling for the motor/drive and its associated power supply.

When the installation is inside a control enclosure the cabling costs are even higher still.

What all these points serve to underline is that whilst a servo system is almost always the first choice for precision applications, it may not always be the best choice in terms of cost and complexity - and especially not if some alternative proprietary method of achieving the desired mode of control is available.

For tension control applications there is a highly efficient, cost effective and simpler alternative mode of control.

It is provided by unique hysteresis brake and clutch products from ZF Industrial.

The exact levels of control, and the unique combination of contact free and, therefore, wear free torque transmission, provided by these units, is ideally suited to all manufacturing processes where control of tension is required, e.g the production of endless products such as wire, cable, paper and foil.

With servo systems, producing constant torque means modulation of motor speed.

Therefore, additional to the application parameters of the system, the speed and torque curves of the servo-motor have to be factored by the servo drive.

This is a complex process, especially so at low speeds where the small incremental control adjustments may result in "cogging" - a condition cause resulting from torque pulsations at low speed.

In contrast, the operation of the hysteresis brakes and clutches is virtually speed independent, as such these units are much more straightforward in operation and simpler and quicker to set-up because much less specialised electrical knowledge is required and harmonisation of the various system components is not necessary.

In addition, less equipment is required in the set-up operation - just the hysteresis unit, its electronic control unit and diameter sensors (0-10V) - so costs are also reduced considerably.

Uniquely the hysteresis brakes transmit torque via a magnetic flux.

Consequently, there is no physical contact and none of the usual disadvantages resulting from friction.

There is also no fluctuation in friction levels due to external influences, no stick-slip effect and no wear during slip operation.

This reduction in friction and wear right across the board with the hysteresis products means that in many instances users can expect an operating life much in excess of alternative systems.

The hysteresis brakes are used mostly in slip mode, but they also have a holding torque and provide full torque at zero speed.

In contrast, the clutches transmit torque in both synchronous and slip modes.

A major benefit is that the torque transmitted is dependent only on the current though the armature coil.

The torque, moreover, is continuously variable and is controlled precisely from zero to the maximum.

Compared to the complex procedures required to set-up servos, the corresponding procedure for the hysteresis products is both fast and uncomplicated.

The slip and synchronous torques of the brakes and clutches are simply set manually on the ERM electronic control unit, in the range from zero up to the maximum value.

As well as being largely speed-independent the torque set is effective during standstill.

Because of these factors there are essential differences between hysteresis components and eddy-current units.

In synchronous operation and at standstill the latter units cannot transmit torques.

They are also difficult to control in the lower speed range.

The simplicity in set-up and straightforward operation which characterises the hysteresis clutches and bakes is due in no small part to the universal electronic control, which is an integral part of the system package.

The functionality of the control unit compares favourably with that of servo controllers.

For example; in braking systems requiring constant torque/tensile force the servo controller has a comparatively high expenditure in control functions, because the intrinsic parameter that is being controlled (torque) cannot be adjusted directly.

This is not the case with the ZF hysteresis control unit because torque can be adjusted directly by setting the system current.

With its proprietary design, ZF's ERM control unit offers users the flexibility of a number of operating options.

The simplest of these is basic open loop control, without any form of feedback.

For applications requiring more precise control a number of closed loop control options are available.

These include simple sensing with encoders, and systems with both sensing and the capability to perform calculations and provide feedback variables.

Other major options are closed loop force control, in conjunction with a dancer arm, closed loop position control and full PID control.

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