Linear actuators replace multiple pneumatic bellows

Based in Indianapolis, USA, Goulding & Wood designs and builds pipe organs for churches across America. Installing one to three organs each year, the highly skilled team combines woodworking skills, acoustics knowledge and experience in their work. Every organ has a custom nature because it forms an integral part of a church and must suit the acoustic, aesthetic and musical requirements of the congregation.

The basic operation of a pipe organ is simple. A blower is used to create 'wind', which is sent through a series of reservoirs and regulators that stabilise it at a pressure at anywhere from 1.25inch to as much as 10inches of water column (this unit of pressure reflects historic design). The wind is then supplied to airtight windchests that contain the mechanism that admits the wind to the pipes.

Each note on the organ keyboard corresponds to a note action in the organ. When the organist plays a note, the mechanism allows the corresponding note to play. There are many, many ways in which this may be accomplished. Some windchests have direct mechanical connections between the key and the valve, whereas others are fitted with a remote action. The volume of certain divisions of an organ can be controlled with a pedal at the console, which controls a series of parallel wooden shutters between the pipework and the audience.

There has always been a tradition of innovation at Goulding & Wood. One of the company's founders, John Goulding, was instrumental in the design of the company's exclusive electro-pneumatic slider and pallet windchest, which allows the pipes to be voiced on a wide range of wind pressures as dictated by musical and acoustic requirements, rather than mechanical requirements. He also developed the butterfly schwimmer valve, which allows for higher differential wind pressures in the regulator design. Goulding & Wood has also embraced solid-state note action switching to provide flexibility for the organist while reducing mechanical complexity.

Remote control concepts

One design challenge that has remained for organbuilders is controlling the expressive divisions remotely. Traditionally in mechanical action organs, or 'Trackers', as they are more commonly known, the linkage is direct from the pedal to the shades. At the advent of using electricity for remote organ action, organbuilders found it advantageous to be able to locate the organ console where it was convenient for the organist. This made a direct connection between the console and the expression shades impractical. Therefore many devices have been invented to remotely control the expression shade. Many of these devices are of an electro-pneumatic variety. However, it is difficult to generate any amount of force from a pneumatic actuator with 5-15inches of water column pressure (0.01-0.04bar). This made for some quite sizeable devices.

To clarify, the actuator is in fact made up of 16 pneumatic bags sandwiched between a series of plywood separators. Electrically operated solenoid valves control the larger valves that admit or exhaust the wind to each pneumatic. As each valve is turned on, air is allowed to inflate the bag that pushes on the next to extend the actuator. The device is only single-acting, as it makes use of compass (torsion) springs to return the actuator to the open position. Although elaborate, this system does provide the required smooth operation and makes use of existing power from the wind available in the organ.

However, the huge size of the pneumatic actuator is undesirable, not least because it restricts floor space, but it also places restrictions on the layout. The linkages must be carefully constructed in order to actuate them in concert. Another issue is the noise produced by the moving air, not to mention the limitations on dynamics. This led Organ builder Charlie Dickson to consider industrial linear actuators as an alternative.

Performance specification

The required actuation had to be smooth, providing the desired level of control for the organist, and whisper quiet, because any sound produced could spoil the musical experience. Good repeatability was also important, to avoid potential clattering or damage to the shades caused by over-extension.

One of the first actuators evaluated was a low-cost, stepper-based ACME screw actuator. Dickson was unimpressed by the crude actuator: "I put it in a 7/8inch mahogany box, with 0.5inch wool insulation and it wailed like a Banshee." Goulding & Wood also expressed concerns about reliability and maintenance requirements.

A church organ's mechanical systems are expected to last 50-100 years in the non-technical environment of a church, which is a far cry from the busy factory. Yet the device could be expected to complete 300-500 cycles per week. To prevail, the installed actuator had to be reliable and, if possible, maintenance-free.

Determined to find a solution, Dickson conducted a number of internet searches as well as working with a number of distributors and discovered that UK-based company Copley Motion Systems manufactures linear motors. The new ServoTube actuator can be used like a ball screw or ACME linear actuator, but is based on quiet linear motor technology.

Integration

The existing expression pedal produced an 8-bit binary signal that was multiplexed with the note action switching, so a small interface board with a D/A converter was installed to provide the analogue demand to a Copley Controls Xenus servo amplifier that controls the ServoTube. The design of the ServoTube means that it can be used with any servo amplifier providing sinusoidal commutation, but, combined with the Xenus servo amplifier, the result is an excellent linear motion system from both a technical and single supplier perspective.

The load was small, approximately 10lb (4.35kg) because it was well balanced with little friction, but awkward due to potential side-loading. Igus pillow block bearings were used to reduce any misalignment or deflection, forming an elegant and space-saving configuration for organ volume control.

The ServoTube lived up to Goulding & Wood's expectations; it operates silently and has 12-micron repeatability. The actuator is reliable and requires no maintenance or lubrication, the zero-cogging design ensuring smooth operation. Used in combination with Copley's Xenus servo amplifier, Goulding & Wood had a complete, if not a little over-specified, linear motion system.