Manufacturer Norgren – which also makes pneumatic and hydraulic actuators – declares that in some industries “electric actuators offer optimal performance for precision control, better flexibility, guaranteed repeatability and easy connectivity”. They may also incorporate monitoring systems to transmit temperature and other operational data via an industrial network such as Ethernet or Profibus, helping with process control and maintenance scheduling.
A basic linear actuator uses an electric motor driving through a reduction gearbox to operate a leadscrew; this drives a nut with an internal thread, which moves in and out relative to the motor. The least sophisticated actuators might use a brushed DC motor and a lead screw with an Acme thread. Limit switches at each end stop the motion – adequate for applications which only need to stop in two positions.
More sophisticated actuators might use a for greater efficiency and load capacity (see sidebar), while high-speed units have a belt drive. A few actuators use a rack and pinion mechanism: relatively heavy and unbraked, but more easily-packaged in some situations. And some use harmonic or stress wave drives for very low gearing with zero backlash: these can combine a thrust of 12kN with a positional accuracy of less than a micron.
AC- or DC-powered linear actuators can be controlled by relays and limit switches, but low-cost control boxes are available for actuators equipped with potentiometers or Hall-effect sensors, to give proportional control.
DRIVE
The majority of actuators are driven by stepper motors or servo motors. Stepper motors are precise, inexpensive and easily controlled by pulses from an electric drive unit connected to a PLC or other control system. Servo motors are more expensive – they rely on a built-in encoder for positioning – but are more efficient than stepper motors (so run cooler) and can provide greater torque at high speed.
Norgren states that stepper- or servo-motor actuators ‘can provide the highest degree of precision control’ and quote accuracies of ±0.02mm, with a repeatability of ±0.01mm. The firm goes on to say “Multiple intermediate positions can be achieved and… in-built positioning flexibility allows several actuators to move in unison. Acceleration and deceleration control [lets actuators] glide into position without stopping abruptly” – ideal where vibration must be minimised.
You can eliminate the screw or belt transmission mechanism altogether with a direct-drive linear motor – essentially a stepper motor laid out flat. Advantages include compactness, precise control and minimal maintenance. Until recently, linear motors were expensive and small, but they can now be found in large precision machines such as industrial laser cutters. The range of motion is several metres.
There are two basic form factors for conventional electric actuators: rod-style and rodless. A rod-style actuator looks like a pneumatic ram, with a piston rod extending from a ‘cylinder’ – in fact the body containing the leadscrew mechanism – and the motor mounted on the end or side. Rodless actuators have a rectangular cross-section, with screw or belt drive and the motor mounted in the middle or at an end. They tend to be more compact, and can have stroke lengths of five metres or more; unlike rod-style designs they can withstand some side loading.
It may be easy to retrofit electric actuators where a pneumatic ram was used before – some rod-type actuators meet the ISO 15552 standard for pneumatic cylinder sizing – but electric actuators are not always the best option. While pneumatic actuators need an external air supply, they are relatively easy to control, compact and lightweight, and suitable for use in hazardous or hygiene-critical operations.
Hydraulic systems offer massive power density and work well in hostile conditions, but they need an outboard pump and fluid reservoir. Self-contained electrohydraulic actuators are available, which incorporate the pump and reservoir into the motor housing, but they tend to be large and expensive.
Electric alternatives such as the RISE Cylinder from RISE Robotics are interesting: this looks like a hydraulic ram, but the motor drives a piston via an internal ‘block and tackle’ system of flat belts: the firm claims it is ‘lighter, stronger and faster’ than hydraulics, and halves net energy consumption. It is not necessarily a drop-in replacement – the cylinder is fatter than a hydraulic unit – but requires only electricity and control.
SPECIFICATION
When specifying an electric actuator, some basic considerations apply: what is the maximum thrust required (depending on the geometry of the motion), and how does the dynamic thrust compare with the static thrust needed when the mechanism is at rest?
And the duty cycle is key: with more moving parts than a pneumatic or hydraulic cylinder, aggressive or high-vibration applications will shorten their life considerably. Norgren’s electric actuators are divided into two categories: ‘industrial grade’ for higher precision and a duty cycle of 70% or more, and ‘commercial grade’ for intermittent use. The cost difference is stark, with the former costing typically €1,000-€5,000, and the latter only around a tenth as much.
Electric actuators need to be installed so that there is no excess side force (a compliant bush or pivot bearing may be needed) and so they cannot overextend. While pneumatic and hydraulic cylinders have physical overload protection in the form of overpressure valves, most electric actuators are less resistant to shock loads: if these are likely, you should specify a model with a clutch in the driveline.
Electric actuators generally require a minimum of maintenance: many are advertised as ‘lubricated for life’, but even these usually have suggestions for lubrication if they are in a harsh environment. And as with any electromechanical device, regular inspection of moving parts and of cable entries is advisable, and limit switches should inspected or adjusted occasionally.
LEADSCREWS OR BALLSCREWS?
Screw drive is the most common way to convert a motor’s rotation into linear motion, and a trapezoidal (Acme) threaded lead screw is the best-known way of doing this. But Acme threads create plenty of friction: efficiency is between 20% and 80% depending on design and materials, with most less than 50% efficient. Simple leadscrews also suffer from backlash, introducing lost motion. Sprung anti-backlash drive nuts improve matters, but increase friction.
Acme screws have some advantages: they are inexpensive, and with the right materials can be quiet with minimal lubrication. Their simple design makes them resistant to harsh environments, and most are ‘self-braking’: they remain in place when the power is removed.
Ballscrews are more complicated: the leadscrew has a sophisticated thread profile, while the nut mechanism allows hardened balls to recirculate as it travels along the screw. But the result is much less friction – 90% efficiency is typical – a significantly higher load capacity, and minimal backlash. This high efficiency means that ballscrews can be backdriven, so they may need a brake mechanism. And they are not maintenance-free; in fact, screw manufacturer Thompson BSA says: “Proper and frequent lubrication must be provided for satisfactory service and life. A 90% reduction in ball bearing screw life should be allowed where dry operation is unavoidable.”