TechArchive

This article was originally written in the period 1995-2000

Design engineers with applications which include linear motion have never had it so good! Today, they have a number of well proven technologies to choose from, that offer them a reliable means of providing linear motion from point A to point B. In many cases they require to move from point A to point B via a number of intermediate points at speeds and accelerations to suit the job in hand.

One technology that has experienced an enormous growth over the last few years is the electromechanical actuator, or ‘electric cylinder’, as it is more often called.

This growth has been brought about in part by the advances in the field of electronic controls. The ‘mighty microchip’ has revolutionised all aspects of our businesses: motion control being just one of them. Advances in manufacturing techniques has also enabled the mechanics of the actuator to be engineered into a very cost-effective piece of machinery. Onto this is added the electric motor; not one, but three, basic types, all with specific talents for power, speed and controllability. As a result, the electric cylinder is increasingly being chosen over other forms of linear motion alternatives, including hydraulics, pneumatics, rack and pinion, linear motors and table/slide assembly.

Another stage in the evolution of linear motion has come about through the use of toothed drive belts in the electric cylinder, Belt technology itself has also advanced in leaps and bound during the last decade. This technology will not replace the traditional leadscrew, but will complement it, rather than compete with it. This is a much more arduous duty than the clean dry conditions inside an electric cylinder. The belt drive has made available much higher speeds and lengths hitherto unattainable with leadscrew drives.

Before deciding which linear motion route we should take, let us first compare the electric cylinder with other technologies available.

Hydraulics and pneumatics

The negative aspects of hydraulic and pneumatics are well known: continuously operating pumps or noisy compressors and continued maintenance requirements.

Hydraulic cylinders have very much higher load capacity over the electric cylinder. Where high thrust and low speed is required the hydraulic systems is still supreme. However, hydraulic oil is proving to be environmentally unfriendly. Leakage and disposal of the oil presents a major problem to users Contamination of buildings and machinery with this searching liquid is no longer acceptable in many cases.

Pneumatics have made vast strides in recent years with improved control systems to provide high speeds with greater accuracy. Electronic control valves can be very sensitive and require filters and regular maintenance.

In the case of hydraulic and pneumatic systems running costs are also high, the power packs may run continuously, absorbing electrical energy. One point that is often not realised is that an electric motor running ‘light’ has poor power factor and efficiency. This is a hidden cost not easy to calculate without expert knowledge.

The electric cylinder only uses power as and when it is required. Energy is not used to feed losses in pumps, valves and pipes, and have extremely low standby power consumption.

Rack and pinion

Where positional accuracy is not too important, the rack and pinion still has a role to play. For light weight or even heavy loads eg railways, this principle is still used. Its drawbacks are obvious: open construction is subject to contamination from dust and liquids. This means costly maintenance. Inherent backlash means it is not suitable for close accuracy.

The electric cylinder on the other hand is well protected and virtually maintenance free. Protected to a minimum of IP44.

Linear motors

There are two drive units that can be categorised as linear motors.

One is the linear induction motor which is in effect an induction motor opened out to form an indefinite length. The second is the Direct Thrust Linear Drive Unit which utilises magnets enclosed in a tube along which the carriage is propelled by magnetic coils electronically commutated.

The original linear motor, when combined with magnetic levitation was expected to revolutionise transportation systems world-wide. The DTLDU is a very efficient high speed, and very accurate, linear actuator. However by the nature of its design it has limited thrust. In a clean environment it has many application to move masses of over 20kg at speed over 3 m/second.

By virtue of its trailing cables and exposed magnets (inside the linear thrust tubes) it is prone to problems from pollution. Any ferrous particle is attracted to the magnetic tubes like a moth to a flame, and with similar results!

Trailing cables are open to the ingress of dust and dirt, even in dry conditions, add to this a little moisture and the problem is compounded.

New belt driven electric cylinder can match the DTLDU in speed and accuracy.

Tables and slides

Many manufactures offer a comprehensive range of tables and slides which have high carrying capacities. In many cases they are available without the drive motor and control system. The mechanics are easily ’sized’, but the drive is provided by the customer which means the overall system performance is their responsibility. If the performance expectations are not met, then there is a divided responsibility. Often the machine builder or customer looses out.

With the electric cylinder, the actuator, motor and controller are from a single source and proven performance is assured.

The principle of the electric cylinder

Electric cylinders have a very simple principle of operation and few moving parts. When power is provided from the motor through a timing belt or gear drive, the leadscrew rotates. This causes the drive nut to translate rotary motion into linear motion. This in turn causes the cylinder to extend or retract, as it is directly attached to the nut.

The thrust and speed will be a function of the type, pitch, speed of the screw, which is in turn driven by the motor.

The specific manufactures design accounts for the method to retain the nut and cylinder to prevent it rotating. This often takes the form of internal guides with Teflon or similar low friction material inserts. It is always wise to fit end of travel limit switches to disable the drive before it hits the end. If however, this does happen, it is not disastrous, because the cylinder is fitted with elastomeric spring cushions to absorb any impact at the end of the stroke, preventing mechanical damage. Contaminates are prevented from entering into the cylinder by a Polyurethane material, rather like an oil seal, around the shaft. This principle enables rotary energy from the motor to be converted very efficiently into the linear energy of the actuator.

The leadscrew

There are two basic form of leadscrew: the Acme thread and the ball screw threaded leadscrew. The Acme thread is in many ways similar to a normal nut-and-bolt thread. By rotating the ‘bolt’ the ‘nut’ is moved along the thread. It is not too efficient, but this is put to advantage! It is self-locking, which means it will not back-drive. Used within its design parameters, the Acme screw is low in cost and very effective in many situations.

The ball screw, as its name implies, consists of a semicircular groove machined into the ‘lead rod’ in which ball bearings run. The bearing housing allows the balls to recirculate. This provides a very smooth and efficient drive: so efficient that in many cases, a small force on the cylinder will cause the nut to back-drive, and the cylinder to extend or retract. This is far from being a bad feature, its low losses means very little friction which in turn means low heat generation. This means the cylinder can operate at very high speeds and loads for long periods without damage. If there is a requirement for high performance and to hold position when powered down, an electromagnetic brake can be fitted to the motor or on the end of the drive screw.

In all systems, there is a small amount of backlash in the drive. Contrary to general belief, this must be present to allow for thermal expansion and lubrication. Typical machined screw system backlash can be better than 0.3 mm. For special applications, precision ground screws are used which considerable reduce this figure.

Life of the electric cylinder can be defined in thousands of hours of km or travel. The load on the systems will also have dramatic effect on the life. A typical Acme screw will have life of 25,000 km at maximum load, however with the load only being 75%, the life is extended to 625,000 km; ie 25% reduction in load results in 250% increase in life. Life is not calculated as when the screw is worn out but when the backlash has increased by 50% (which may still be better than 0.6 mm).

Ballscrews are rated the same as ball bearings (B10 life) and retain almost constant backlash throughout their life.

Leadscrew vs belt drives

Up to now we have only considered the electric cylinder which is similar in many respects to the hydraulic or pneumatic cylinder. There are many applications where the rod type cylinder is not suitable. The rodless cylinder is similar in design to the rod cylinder, but in this case the motion is provided by a carriage that is supported on internal bearings. The carriage in this case can be powered by a leadscrew or endless belt. A seal is proved between the casing of the actuator and the carriage. This behaves in many ways like a zip fastener, opening as the carriage moves along and sealing up again when it is past. Protection to IP44 is standard.

Long rodless actuators with leadscrews, when rotated fast, will ‘whip’ at their critical speed (a function of length, speed and stiffness). This mechanical phenomena restricts the length and speed of the screwed actuator. The answer to this is to use an endless belt onto which the carriage is attached, the steel-reinforced polyurethane toothed belt which is fitted to most car engines today. It offers speed, efficiency and low noise level drive transmissions. Speeds of over 3 m/s and loads of 1800N are attainable. Actuator lengths in excess of 5m are available.

The electric drive

There are three types of electric motor available to us, the dc (or brushed) motor; Stepper motor (pulsed round in small increments from 200 per rev. to 25,000 per rev.) and brushless servo motor. When are they used?

The low voltage dc motor is normally used when single or dual speed limit switch control is required. e.g. move between two fixed points and stop at limit switches pre-set at installation. Moderate speeds and powers, and repeatability of typically +/-0.025mm available.

This is fine if the process is fixed, and speed of action is the same for all conditions. Where machines are used for multiple operations, and where the ability to pre-set the speed and distance together with their acceleration and deceleration is paramount in many cases, then the stepper or servo is used.

Stepper motor

Stepper motors are normally sized type 17, 23, 32 and 42, based on the NEMA specification which denotes the diameter in inches. The motors are made up of magnet stacks and can be 1, 2, 3, or even 4 stack lengths. A stepper motor, when energised from the amplifier, has ‘holding torque’ or stalled torque. This is its maximum torque, as pulses are applied, the motor starts to rotate and the torque falls off in typically an exponential curve. So it can be seen that the stepper motor has high torque at low speed and low torque at high speed. Providing these characteristics are known and the drive operated within the speed torque envelope, all is fine.

Stepper motors may therefore not be overloaded, as many engineers have found to their dismay; they are unforgiving. Outside this curve they are in trouble.

Control of the stepper motor can be provided by any controller than can input ’speed and direction’ pulses into the control amplifier. A stepper motor in single-step mode will require 200 pulses to make one revolution of the shaft.

Servo motor

The brushless servo motor is the ‘GTI’ of electric motors. Brushless design means it has not commutators or brushgear to maintain. Small diameter rotors have low inertia, which allows fast reposes. Servo motors are fitted with speed-and-position feedback devices ie encoder or resolvers. Typically these feed signals to the amplifier which are translated into position and speed related information. The motion controller quadratures the pulses so a 2500 line encoder will be used at 10,000 pulses per rev. This means that drives and actuators can be positioned to one pulse, or 0.0001/revolution of the motor shaft. The servo motor has the ability to produce over 200% torque for short periods (2s) for acceleration and deceleration. This gives phenomenal performance and accuracy.

Controls

The stepper and servo drive are powered by a whole range of digital controllers. The engineer can provide speed and direction signals from his plc or motion controller.

A more elegant way is to have combined amplifier and motion controller in a single panel mounting unit. This allows up to 100 motion control programs to be stored in its memory, one to suit each component or process. Self tuning and easily-understood programming languages makes light work of the most complex motion control systems. It is also possible within these programs to take into account the system backlash. Backlash is measured and translated into position pulses; simple mathematics can add or subtract this in the move calculation. It is as if backlash has been eliminated.

• Industrial Devices
• Tel: 01788 812951
• Fax: 01788 522633