Accurate Measurement of Shaft Speed

Measuring or controlling the speed of most rotating shafts is straightforward but significant problems can arise as soon as we start to talk about accuracy. Mark Howard from Zettlex Ltd. examines the issues involved and suggests some simple solutions to longstanding problems.

There is a famous question that sits at the crossroads of geography, mathematics and philosophy: "How long is the coastline of Britain?" At first glance, a pretty straightforward question - just think back to your geography textbook and dredge up the correct number. Most people say it's around 11,000 miles - estimated from the perimeter of a triangle formed by Dover, John O'Groats and Land's End with a 'fudge factor' added on for Wales.

For pub quizzes, this answer might be good enough, but is it accurate? Well, you could get all the ordnance survey maps and trace the coastline with a piece of string and eventually produce a second estimate. Certainly, the answer will be different to the first, but is it more accurate? The snag is how do you measure? Where do you trace the string for an accurate measurement? How far up the rivers do you go? Do you trace all the way to the source of the river and then back along the other bank? Do you measure the coastline at high tide or low tide? Do you trace in and out of every rock? At an extreme you would be measuring around every pebble and every grain of sand, to such an extent that the answer is much larger than first expected. So the truly accurate answer to the question is - it depends on how you measure it.

The same kind of bizarre logic applies to shaft speed measurement. If you simply need a rough idea of how many revolutions a shaft completes in one minute, it's pretty straightforward and even the crudest of measuring systems should deliver a fair answer.

But what happens when you need to know what the speed is, say, every millisecond and then control the actual speed so that this corresponds to a tightly toleranced set point?

To borrow some terminology from the old vinyl record turntables, speed variations can be described as 'wow' and 'flutter'. Wow typically refers to speed variations over relatively long periods and flutter refers to speed variations over relatively short periods, typically less than once per revolution. Often, both are important and the requirements to tightly control both wow and flutter are common in many sectors of industry: CNC machine tool motion control; aircraft flight controls; radar antennae; and weapons control systems. Such control engineering issues do not just relate to complex motion control but also to driving shafts at a constant speed (especially when there are variations in load). When one approaches the issue of a shaft that must rotate at truly constant speed, then the same logic that has us measuring around the grain of sand will quickly tell you that to get a shaft to rotate at a perfectly constant speed is - in the extreme - impossible.

One common problem is that the shaft speed is not measured directly but indirectly. A typical drive system will include a shaft, driven by a gearbox or pulleys, a motor and a motor encoder. Readings from the motor encoder - usually a stream of pulses - are used to calculate the speed of the shaft itself. In turn, the output from the encoder is fed back (in a servo loop) to the motion controller, which adjusts the power fed to the motor to increase or decrease speed. A significant problem is that although the system may be somewhat mechanically coupled, what's going on at the motor is not the same as what's going on at the shaft. For example, any gears in between the encoder and shaft will not be perfect and are subject to wear, backlash, thermal expansion/contraction, mechanical tolerances and clearances. Further effects include mechanical friction (especially 'stiction' at lower speeds), variations in lubricant properties, mechanical twist due to torque, shaft bending, shaft concentricity and so on.

To determine what is actually going on at the output shaft needs an encoder on the actual shaft itself. In reality, this can prove both difficult and expensive, especially if the shaft is large or where space constraints are tight. A further difficulty arises when the shaft speed is low, since accurate speed control will depend on sufficient measurement information being produced per revolution, to give sensible or timely control of the motor. A measuring device on the shaft with, say, 100 counts per revolution is not going to permit accurate speed control of a shaft that is rotating at 1 rpm, since the measurement will only be updated every couple of seconds. To control shaft speed accurately at low speeds, the greater the need for high resolution angle information at the output shaft.

There are 3 main methods for measuring shaft position or speed: magnetic, optical and inductive. The most common is magnetic - usually Hall effect - but this is not used in high accuracy or low speed applications since it lacks the resolution, whilst magnetic hysteresis or temperature effects will degrade measurement performance.

Optical encoders offer good measurement performance but are delicate and unreliable in harsh conditions. Optical sensors are typically rated to only modest temperature ranges (-20 to +70 Celsius is typical); they can fail because of foreign matter and harsh mechanical vibration or shocks can damage the optical grating.

Inductive devices such as resolvers and synchros are the traditional choice for high reliability or harsh environments, including military, oil and gas, aerospace, and heavy industrial applications. Whilst the reputation of inductive devices for reliability and accuracy is well founded, they are bulky, heavy and expensive, especially in the larger sizes or 'A class' measurement performance. 

A new generation of inductive technique now enables more and more people to choose inductive devices for mainstream control applications. Rather than the traditional wire windings or spools, this new generation of devices uses printed, laminar constructions, which dramatically reduces the bulk, weight and cost compared to traditional devices.  At the same time, accuracy increases and the possibility of a wide range of shapes and sizes of sensor opens up. In particular, large bore devices can be provided without massive increases in cost. In turn, this makes direct mounting to the shaft more practical and hence more accurate. Furthermore, the need for high precision gearboxes is eradicated and generally the gearbox can be derated - allowing additional cost reductions.

Zettlex of Cambridge is a world leader in this new technique and has produced the IncOder ('Inductive Encoder'). These are purposely designed to easily fit to shafts or around slip rings.  Supplied as two rings - a stator and a rotor - the shaft passes through the IncOder's large bore and the rotor simply screws onto the shaft, either directly or by using a grub-screw style collar. IncOder offers absolute angle measurement, combined with a re-settable zero position and a high number of counts per revolution (256k to 512k counts per revolution as standard with 4 million counts possible). As the devices are inductive, there are no problems if they get wet or dirty. The absence of any bearings or seals means that the devices will operate for long periods without requiring maintenance or servicing. Fitting such devices directly to the shaft enables measurement of much more representative conditions for speed and position control.

For further information, view website: www.zettlex.com or email: mark.howard@zettlex.com 

Laser scanner provides 3D profiles of rotary pistons

A manufacturer of universal 3D measuring machines is using high precision laser profile scanners and laser displacement sensors from Micro-Epsilon to measure the surface profiles and position of rotary pistons.

In rotary piston pumps, as the rotary pistons are responsible for the flow of the delivery medium, the specified geometrical shape of the pistons must be extremely precise and accurate. Two rotary pistons need to mesh exactly with one another as the pump rotates, with movement of the pistons terminating just before the wall housing. The required service life of a rotary piston pump can therefore only be maintained if the piston surface fits precisely with the pump housing. This means that the whole of the piston surface needs to be measured and compared with the target shape to very strict tolerances.

Qsigma GmbH based in Kassel, Germany, has developed a universal 3D measuring machine for these types of applications. The piston is placed on the rotating axis of the machine. Micro-Epsilon's scanCONTROL 2700 laser profile scanner with a 50mm measuring range is used to record the shape or profile of the piston's surface. For larger components, the profile scanner can be traversed vertically using a linear motion system.

In addition, an optoNCDT 2200 laser displacement sensor from Micro-Epsilon with a 200mm measuring range is integrated to the Qsigma measuring machine in order to determine the precise position of the component. The optoNCDT 2200 sensor acquires 10,000 distance values every second, operating at a resolution of 3µm.

The piston rotates during measurements. Only five seconds are required for an automatic inspection of the piston. After successfully scanning the surface geometries, the data is compared with those of the target object. In addition to rotary pistons, the system can also be used to measure other rotationally symmetrical objects.

Micro-Epsilon's optoNCDT 2200 laser displacement sensor offers extremely high accuracy, high measuring rates and excellent signal stability, without any signal averaging. These sensors are world leaders in terms of their technical capabilities, enabling them to solve the most demanding measurement applications. The digital output signal can be combined with the IF2008 PCI card (also designed and supplied by Micro-Epsilon) to synchronise multiple sensors at high measuring speeds for easy data acquisition direct to a PC.

For further information, email: info@micro-epsilon.co.uk   Refer to page 74 

The new Intertronics DYMAX BlueWave® LED DX-1000

The new DYMAX BlueWave® LED DX-1000 is a unique and flexible 385 nm LED curing system that can be configured to operate as either a small-area flood or spot-cure system. In flood mode, up to 1 W/cm² can be delivered over a 1" x 1" (2.5cm x 2.5cm) area. In spot mode, a single or multi-pole lightguide can be installed into an optional adapter to deliver up to 15 W/cm² in a high-intensity spot.

This makes it ideal for production, development or R&D facilities, as well as manufacturing cells where rapid re-organisation is often required, especially since the flood light unit may be demounted from the base stand for separate mounting, e.g. on a jig or conveyor system up to 3m away from the controller. This enhanced flexibility, coupled with the advantages of LED UV, make the DX-1000 highly suited to industries as diverse as electronics, medical, automotive, optics, displays, LEDs etc., especially where rapidly adaptable manufacturing patterns are demanded.

Its shutterless operation, adjustable intensity, long life, low power consumption and consistency of output are major advantages over other systems while bringing the benefits of UV cure adhesives within the reach of even small businesses.

Appropriate adhesives include ultra-fast cure clear formulations that are tack-free in under 2 seconds, through electronics potting to catheter bonding and clear fluorescing adhesives with cure times under 30 seconds. Wider industrial applications are catered for with an extensive range of substrates from ceramic, glass, to many plastics, silicone etc.  For further information, e-mail: info@intertronics.co.uk or view website: www.intertronics.co.uk  Refer to page 198

Stopfreeze protection from Intertronics

The winters, they tell us, are getting colder and trace heating systems for vulnerable water pipes are finding greater demand. As part of their development process STOPFREEZE were looking for a superior way of protecting their in-line thermostat from water ingress and physical damage in its outdoor situation. Their solution came in the shape of a polyurethane potting compound from Intertronics, together with appropriate metering and mixing equipment for production filling of the thermostat housing.

Although primarily focused on  boiler condensate discharge lines, both the STOPFREEZE KIT and STOPFREEZE COMBI, which feature an IP67 weatherproof in-line thermostat with integral LED indicators, can both be used for a number of pipe frost protection applications such as farms, caravan parks and camp sites, stables, lofts, garages, holiday homes and lodges, nurseries, outbuildings and sheds etc.

Given the often arduous environments the engineers at Intertronics recommended their IRS3071 which is a semi-rigid, room temperature curing, flame retardant polyurethane resin system. It is specifically designed for the cost effective encapsulation of a variety of low to medium voltage electrical and electronic applications. The system offers medium viscosity, flame retardation to UL94 V-O at 6mm and excellent adhesion to a wide range of substrates. IRS 3071 is resistant to UV, water based cleaning chemicals, motor oil, lubricants and most dilute acids and alkalis.

Paul Whitehead, Technical Manager at Intertronics, described their thoughts: "IRS3071 was suggested because it is a cost-effective way of environmental protection and void free encapsulation. It has the added benefit of adhering to the enclosure making it difficult to disassemble the enclosure without damage."

Explained Neil Fairburn of STOPFREEZE manufacturers Eltrace: "We were looking for a potting system that was going to be superior to others used in the industry while being easy to use so that we could ensure consistently high quality, performance and reliability. The Intertronics system has been key to market acceptance of our STOPFREEZE in-line thermostat kits and allowed us to offer a simple and inexpensive solution especially suited to the residential market."

For further information, view website: www.intertronics.co.uk/pottingcompounds or visit their blog at: www.adhere.uk.com   Refer to page 198