Designers are offered a greater choice of linear motion solutions than ever before. For example, the latest pneumatic technology offers features traditionally associated with electric drives. Point-to-point electric drives are becoming more cost-effective and can replace some movements traditionally undertaken by pneumatic cylinders. With new servo and digital pneumatic technologies added to the mix, picking the solution that will offer the best performance, cost and energy efficiency for your application can be challenging.
There are many application requirements to consider, such as:
- Payload: what mass is to be moved e.g., handling eggs or car engines?
- Force: does the application require a defined pressing force, and does it vary based on the product?
- Repeatability: multiple positions and the repeatability needed?
- Footprint: is this a desktop machine or end of arm tooling on a 6-axis robot?
- Energy source: pneumatic supply pressure, single or 3 phase, etc.?
- Lifecycle costs: TCO or capital expenditure, which is more important?
- Energy efficiency: is there a carbon footprint target?
- Motion control: speed, synchronisation, or interpolated motion required?
- Expertise: does your team have the knowledge to select and support the desired technology?
Now let’s examine the main technologies to see which of these criteria they fit best:
A robust, simple to use technology with a high force density. Pneumatics offer best in class force output for the defined footprint and weight, together with high repeatability end-to-end positioning. Speed and position are defined with flow control and sensors. Force control is linked directly to the applied pressure. Pneumatics can be energy efficient when using best practice design and operation.
All the benefits of pneumatics, plus positioning capabilities. Servo pneumatic devices have a linear displacement encoder attached to the cylinder in question. The positioning commands and the encoder feedback create a closed loop control and the output is fed via a proportional directional control valve back to the actuator. The controller positions the cylinder with motion control to defined speeds and accelerations. This technology is specifically useful for applications not suitable (due to size, weight, energy source or environment) for electric drives.
Controlled pneumatics offers energy and diagnostics benefits. The Festo Motion Terminal (VTEM) combines fast switching and virtually silent piezo control valves with integrated sensors for closed loop control. The full flexibility of the smart terminal is accessed using the wide range of downloadable Apps (i.e. proportional control, soft-stop, pre-set travel time, leakage etc). Each valve position houses four piezo valves with boosters in a bridge format. The valve has both pressure and flow sensors for closed loop feedback of the pneumatic output at each port. This configuration enables a multitude of different functions, which are engaged by the motion apps. The function of a single valve can be changed. For example, from a directional control valve to a proportional valve in real time.
End-to-end electric actuators are available in an array of mechanical axes designs (gantry, ball screw cantilever or tooth belt and cantilever). The integrated motor is a stepper with an encoder and controller offering connections for both power supply and communications (digital IO or IO-Link). Simple to commission, these devices can be taught the desired end position and perform point to point positioning under a defined motion profile (set over the push buttons or IO link). Mid-positions are also possible with IO-Link control.
Advanced motion control with high dynamic motion. Common servo controllers range from 24 V DC power supply to 3 phase 480 V power supply. Based on the motor type selected for the application (stepper, BLDC, servo), designers can deploy an array of different feedback encoder types. Position, speed, and force can be controlled from the PLC via the servo controller. Multiple drives can be combined via motion controllers for complex handling solutions. Enhanced motion control can also be achieved, including synchronisation, flying saw and interpolated motion. Functions such as DC link can be enabled for improved energy efficiency.
A summary of the benefits and applications of each linear motion technology is given in Figure 1.