MSD: What are the advantages of servopneumatic control over other alternatives?

JR • Enfield: Aside from the familiar pros and cons between hydraulics and pneumatics in general (such as force capabilities, cleanliness, and cost) there are very few, if any, additional comparisons to be made between servohydraulics and servopneumatics, and there is almost no head-to-head competition between the two. Whether to use one or the other typically becomes clear in the early stages of a design process.

However, comparing servopneumatics with servoeletrics is somewhat tricky and there are some grey areas. The biggest advantage of servopneumatics is its high energy and power densities, which can result in significant cost savings (both in terms of money and payload) in applications that simultaneously require high speeds and high forces. Generally speaking, the cost of a servopneumatic positioning system holds constant across the power spectrum: A small increase in valve and cylinder bore sizes results in significant power gains (related to the square of the bore), with very little added cost for the end user. In contrast, low-power stepper motors driving ballscrews may be affordable, but get increasingly bigger, heavier, and much more expensive as power demands increase. Additionally, unlike steppers or dc servomotors, pneumatic actuators can store energy, which allows them to hold a static force with virtually no energy consumption.

MSD: What are the roles of the special valves and controllers used in servopneumatic control systems?

JR • Enfield: Servopneumatic control systems have four main applications — position, force, pressure, and flow. The explanation here focuses on position control, though the other three are almost identical and differ mostly in the type of sensor employed.

Typical servopneumatic (and most servohydraulic) position control systems consist of a double-acting cylinder, full-scale linear position sensor, high-speed proportional 5/3 (5-port, 3-position) valve, and a suitable closed-loop controller. The controller receives a target setpoint command signal and a sensor feedback signal, and based on its algorithm, communicates a plan of action to the valve. From an end user standpoint, however, the controller and the valve can be treated as a “black box” component: The user simply inputs a command signal (target position, velocity, or trajectory) and the cylinder moves accordingly. Typically, this command signal comes from a PLC and is in the form of voltage (0 to 10 Vdc) or current (4 to 20 mA).

The proportional valve (or in some cases, servo valve) undoubtedly plays the central role in the overall system. Even though it has three discrete “positions,” it can infinitely vary its effective orifice size, essentially controlling the “extend” and “retract” cylinder speeds. The valve's high-speed feature means that it can change the size of the orifice almost instantly. By quickly and accurately changing valve positions (cylinder direction) and orifice sizes (cylinder speed), any cylinder position can be obtained and maintained. This is where the controller comes into play.

The controller needed for a servopneumatic system is comprised of two main stages: The control stage (i.e., PID) compares the command and feedback signals, while the drive stage takes the algorithmic output of the control stage (i.e., “control effort”) and turns it into a suitable amplified drive signal for the valve. The controller algorithm, regardless of its nature, always works to minimize the difference between the command and the feedback by generating a cylinder motion in the direction of the setpoint command. Note: For adequate closed-loop position control, the controller needs to be the fastest device in the system.

MSD: Any advice for engineers setting up a servopneumatic linear positioning system for the first time?

JR • Enfield: The most important aspect for correctly setting up a servopneumatic system is sizing. The cylinder bore size must be carefully selected for the required dynamics, and the valve must be sized according to the flow requirements of the system. The two most common mistakes typically made when setting up a system are undersizing the cylinder bore relative to the system dynamics, and oversizing the valve relative to the cylinder size and dynamics, either of which usually results in an uncontrollable system. Here is a good step-by-step example of how to size a position control system using servopneumatics:

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1. Determine the maximum controlled acceleration required.

2. Based on total moving mass, determine the force required to provide the acceleration (F = m•a).

3. Select a cylinder bore size that can provide at least twice the force required (ideally, much higher) based on the line pressure and bore area. The higher this force margin, the more controllable the system becomes, and as such it may pay off to approach a margin of 10 if the space permits it. It's useful to keep in mind that the force increases exponentially with respect to the cylinder bore: To double the available maximum force, one needs to increase the cylinder bore by only 41%. Similarly, by doubling the bore size, the available maximum force increases by a factor of four.

4. Based on the maximum target linear velocity, determine the maximum airflow that will be needed in SLPM (standard liters per minute - SI units) or SCFM (standard cubic feet per minute - Imperial units). Most valves in the U.S. and European markets list their flow specs in at least one of these two units.

5. Select a proportional valve that can handle the required flow rate calculated in step four. In general, it is better for controllability to undersize rather than oversize.