TechArchive

Original article date: June 1995

The compressibility of air has limited pneumatic motion controls on programmable accuracy. Now MICHAEL BRISLAND describes Motron Dynamics’ air motor.

By its nature motion control involves frequent changes in position direction and velocity under load conditions. Despite high response features derived from low inertia characteristics the electric motor has limitations. However motion systems based on derivations of the electric motor currently dominate the market. Nevertheless pneumatics is suited to intermittent motion where frequent changes in speeds direction and positions are common.

One cause of premature electric motor failure is overheating which is primarily due to excessive current levels. Even if the motor is overheated by as little as 10Cdeg its life can be halved. This weakness inherent in all electric motors imposes restrictions on their use. Heat is generated when an electric motor is under stress usually during acceleration and deceleration phases of an operation. Suitable intervals must be allowed for the heat to dissipate. These periods occur when the motor is either stationary or running at a constantly high speed. An alternative is to install a much larger motor which though operating below peak capacity can tolerate the heat generated in doing so. In addition to extra capital cost the larger motor and amplifier system is a less energy efficient solution for the application.

A key factor in accelerating and decelerating or moving a load is torque. Compared with fluid power devices electric motors have poor torque density and are not renowned for their capacity for starting under load. To compensate a reduction gearbox is generally fitted to the motor which introduces additional inertia losses into a system.

Electric motors operate most effectively at maximum speed. The variable speed conditions of motion control means electric motors rarely operate at optimum efficiency. In acceleration sequences which requires high torque at low speed the electric motor consumes energy at a much greater level than when it is operating at a constant high speed.

Motron Motor

Drives in the Motron System are provided by a high torque air motor which is able to function at very low speeds under full load. The motor has been developed with the National Engineering Laboratory.

Design of the Motron motor has been influenced by research which indicates that over 80% of drive applications in industry have a final output speed of less than 100rpm. As a result the Motron motor has been developed as a low speed device which can operate at very low speeds under load without resonance. In most cases electric motors have optimum speeds exceeding this level and attain reduced speeds through a reduction gearbox. The ability to operate at low speeds allows the Motron motor to be directly coupled in its application without an intervening gearbox. The absence of a gearbox eliminates transmission inertia and avoids the effects of gearbox backlash.

Operating at low speeds the Motron motor consumes minor volumes of compressed air. Although regarded as a poor medium for transmitting power compressed air is an effective and efficient means of transmitting force or torque. Compressed air is produced by a compressor and stored as energy which is consumed only when the Motron motor is moving. Invariably the compressor is driven by an electric motor rotating at its maximum and optimum speed during which time it is in its most energy efficient condition.

At the centre of the Motron motor is a tri-lobed cam mounted on the motor’s output shaft. The balanced low inertia cam assembly is supported between sealed deep groove bearings. Radially positioned and acting on the cam are four pistons which reciprocate in fixed cylinder liners. Self lubricating bearing surfaces interface the pistons with their enclosing cylinders. Rolling contact with the cam is made by sealed needle roller bearings. The action of the pistons on the cam converts linear force to rotational motion.

Each of the pistons contains two slide valves to control the sequence of air flow through the motor. Each valve consists of a free floating self lubricating seal pad seating against the bore of its containing cylinder liner. As the piston reciprocates a channel in the face of the seal pad connects two ports formed in the liner.

A series of channels is formed in either flank of the motors cylinder block. The channels link the control port in the cylinder liners to the piston chamber of the adjacent piston. An annular manifold is also formed in each of flank faces of the cylinder block. The manifolds are connected to motor inlet/outlet ports. The pattern and form of channels and manifold are identical in both flanks of the motor. The Motron motor operates by linking either of the manifolds to a supply of compressed air.

A flow control valve is linked to the motor ports. The function of the valve is to control the direction and rate of airflow through the motor. Actuation of the valve is by a proportional solenoid. An optical encoder is mounted on an extension of the motors output shaft. The encoder transmits information which allows motor speed and direction to be detected.

This piston cam and piston valve arrangement results in a very compact unit which produces 12 power strokes for each revolution of the motors output shaft. By employing computer modelling techniques during the design of the motor the torque “ripple” amplitude is harmonised to achieve constant torque output and a high torque efficiency in excess of 83%.

Motron motors feature high torque output with a consistent level of starting torque irrespective of shaft position. Constant torque output enables the Motron motor to operate under load at slow speeds smoothly and without resonance or vibration. As a general comparison the Motron motor develops starting torques exceeding those of an electric motor of similar power by a factor of six.

The Motron motor also develops full torque at start. This feature enables the incremental movement of substantial payloads and a constant “holding” torque which can accommodate holding loads precisely in position or suspension indefinitely. Zero speed torque control system feature enables the Motron System to accurately hold position under changing load conditions even if the load is vertically suspended.

Should the roles of the motor ports be exchanged air flow will be reversed and the motors shaft will rotate in the opposite direction. The cam piston arrangement is a low inertia system resulting in the Motron motor being highly responsive enabling rapid changes in shaft speed and direction. The symmetrical layout of the air galleries and manifolds formed in the flanks of the motor produces identical gas flow characteristics and equal motor performance in either direction of shaft rotation.

As a positive displacement device the Motron motor is able to function as a dynamic brake by reversing the air flow to produce a retarding torque (dynamic braking). The Motron motor can also be stalled under load indefinitely without damage.

Up to eight Motron motors may form a multi axis system in which all the drives are linked to perform integrated movement. Applications include robotics dosing and metering particle dispensing smart conveyors and jack systems. The Motron is also used to control the speed of a gas turbine powered electricity generating station!

• Motron Dynamics
• Tel: 01226 204040
• Fax: 01226 207158

June 1995