Continuous process industries benefit from torque measurement

To most people, 'process control' means monster-sized central computers running barely-fathomable SCADA software in a control room that would not be out of place at Cape Canaveral. But out on the plant floor, there may be literally thousands of simple sensors and switches, collecting data and feeding it back to the computer.

These sensors are tracking every little change in the plant's operating parameters, indicative of either the state the materials being worked or of the state of the machinery itself. For instance, an increase in torque on a mixer drive may suggest that a mixture within has thickened as expected or, alternatively, that a seal or bearing is sticking and may soon fail altogether; either way, this is vital information.

Ultimately, process engineers want to transform material from one state to another, and to monitor variables indicative of the various stages of the process. Some parameters can be measured directly and simply - such as temperature. Others are more difficult to measure, so an often-used technique is to measure a related parameter (typically one related to the plant or machinery rather then the process material) and interpolate from this.

Significantly, many types of process plant - mixers, pumps and conveyors, for example - are motor-driven, and measuring the motor output characteristics will often provide process information. For instance, the torque of the motor could suggest the quantity, speed or viscosity of the process material being worked.

Measuring the processing is the primary concern of the production engineers, but torque measurement has a second, equally important function. Because you are actually measuring plant performance, you get to see how the machinery is holding up. Knowing what to look for will give you early warning of breakdowns, allowing you to schedule pre-emptive maintenance. For a continuous process where downtime can cost thousands of pounds an hour in lost production, this can be critical - ultimately the difference between a health profit and a catastrophic loss.

This all sound very useful, and actually measuring torque can be very simple. Not so long ago torque sensors required a fairly complicated and fragile array of slip rings connected to the rotating drive shaft of the machine under test. But now TorqSense provides a non-contact means of taking the readings. In use, a couple of simple pads are bonded to the the drive shaft and a TorqSense unit mounted close by. The TorqSense then starts monitoring torque and feeding it as a data signal to the SCADA control system

The pads are small piezoelectric combs encased in plastic. The combs are designed to open or close minutely under the torque effect of their rotational speed on the drive shaft. The greater the torque, the more the distortion.

Each TorqSense unit emits a low-powered radio-frequency signal towards the combs, which reflect it back. The reflected signal is changed in proportion to the distortion of the combs, and hence to the torque in the drive shaft.

Applications

Charles Austen Pumps (CAP) has recently upgraded its test facilities with Surface Acoustic Wave (SAW) equipment from Sensor Technology. Charles Austen Pumps manufactures pumps individually designed to meet specific customer requirements that cannot be satisfied by off-the-shelf units. Much of its work involves optimising drive dynamics to produce the desired characteristics, be it a smooth flow in a critical medical situation, ultra-low-noise for pumps in home and office installations, or the guaranteed extra-long life of pumps in inaccessible locations.

The cyclic nature in many pumps' operation tends to induce torsional oscillations in the drive shaft, which can have a significant adverse effect on performance if unchecked.

CAP has recently built a new test station based on Sensor Technology's SAW sensors, and it is proving its worth time and time again. Advantages of SAW techniques include a broader signal bandwidth than other analogue-based technologies and the elimination of electronic interference. As CAP found, it often also proves lower cost, simpler to use, is more reliable and has a wider operating than contact alternatives.

Currying flavour

Real-time process control for food manufacture involves characterising the flow and mixability of highly non-Newtonian fluids. TorqSense transducers monitoring the constantly changing flow characteristics of materials as diverse as tomato ketchup, chocolate, pasta sauce and chicken tikka masala as they are mixed.

Many manufactured foods are presented in a sauce (or 'neo-liquid') and can be produced in a process-type environment. But to date, real-time control has been virtually impossible due to the non-uniform nature of the food, which may contain particulates, fibres, vegetables, meat, nuts, raisins, biscuits, etc.

To achieve real-time control the TorqSense has to be able to detect the changes with sufficient sensitivity, yet be robust enough for regular wash-downs and general industrial abuse. Of course, it must not compromise hygiene standards and regimes either. TorqSense has been found to meet all these requirements and is being used in by a number of food processors.

Often the key requirement is to mix sufficiently to achieve a uniform dish, but not to waste time and energy by over-mixing. This can be done by monitoring the torque on the mixer's shaft, as it will move to a steady state (within the characteristics of the given recipe) once fluid uniformity is achieved.

Nuclear

Precision gearboxes supplied to the nuclear industry have to be guaranteed not to fail prematurely, so testing them off-line is a vital function for Centa Transmissions. A test rig has been developed in which a motor drives the test unit against a load created by an industrial disc brake. The test runs initially for three hours at the full working load, and is then increased to 300 per cent load for another hour. At the heart of the rig is a TorqSense that constantly monitors the torque in the gearbox, generating a performance profile that can be compared with the ideal performance standard.

The duty the gearboxes are destined for takes place in an environment where reliability has to be 100 per cent. They are used in completely automated scoop mechanisms that collect small amounts of 'high-activity liquor' from the reactor cooling systems. This is sealed into thick-walled ceramic flasks for long-term storage until the radioactivity has decayed to safe levels.

This is at the very core of the nuclear plant where a component or system breakdown would mean shutting down all operations for months, automated/unmanned removal of the faulty parts, sealing into a secure flask and automated installation of a replacement. The cost would be millions of pounds - at the very least. To avoid this, everything has to be lifetime guaranteed to demanding criteria.

Mixing

A TorqSense is helping analyse recipes' mixing properties in a project that could slash development costs in the food and plastics industry and help nanotechnology advances in the pharmaceuticals world.

Research and development is being carried out at the University of Bradford to develop a miniature mixer (5-25g batches) that incorporates a set of integral instruments to monitor the properties of materials as they are being mixed. The instruments work in real-time during the mixing process and their output is captured to a PC for analysis. Software is being written so that the analysis can be performed simultaneously with the mixing and perhaps even used to interactively control the mixer itself.

One of the key parameters to be measured is the torque of the mixing element, as this will become constant once mixing is complete. This is measured by a TorqSense non-contact sensor that offers the development team the great advantage of not requiring complex and delicate slip rings, making the mixer easier to build (and rebuild between trials) and far more robust in operation.

The Mini Mixer development and validation is the result of a three-year EPSRC-sponsored programme of research. The results of this research are expected to have a major impact on formulation of viscous mixes and scale-up of extruders. Traditionally recipes for formulating, say, specific coloured plastics for consumer products are developed in 25-50kg batches, mixed in an industrial-scale twin-screw compounder. Several batches may be required before the recipe is finalised, so the cost and time involved can be considerable.

Clearly the development of a smaller mixer is advantageous, but the laboratory device must be able to duplicate mixing in the larger scale and guide design and operation of large machines and that is what the research programme has achieved. That fact that the technology will transfer to the plastic industry and other soft solid sectors means it is likely to rapidly recoup development costs.

Washing

High-performance torque measurement is helping to improve the energy efficiency of industrial (and domestic) washing machines. Process plant manufactures are redesigning their machines to reduce power consumption.

In horizontal-axis washers, the load (wet laundry), is lifted on one side of the axis and falls on the other side. This is a dynamic where regenerative energy recovery is very attractive if it can be practically achieved.

A test rig has been built that subjects washing machine systems to extensive tests using an industry-standard inverter to simulate the various washing cycles. A critical element of the programme was the ability to make continuous accurate torque measurements, and for this TorqSense is suitable. The time saved in setting the transducer, compared to installing a slip ring-based sensor, over a big project is significant.

By measuring the torque change the exact moment when to switch the drive from power to regeneration and make the most of the potential energy released by the falling load could be defined. Given that the motor could be rotating at up to 1500rpm, this called for very accurate data collection and equally responsive control programmes.

This technique has proved so worthwhile that it will be built into next-generation washing machines. With industrial sized loads, energy savings of 20-30 per cent may be achievable.

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