Sensors keep pace with motion control needs

Sept. 1, 2000
To meet higher productivity requirements, motion systems offer increasingly faster responses. But this requires feedback devices that also respond more quickly. Here’s a look at two such devices

To get accurate position information in some applications, engineers used to double integrate accelerometer signals, because these were the only devices with sufficient frequency response. Well, the days of double integration of accelerometer signals are gone.

Two recent position transducers — enhanced versions of conductiveplastic position transducers and new versions of noncontact-inductive position transducers — provide reliable, continuous, and absolute position feedback for servo control systems in many applications. Monitoring position, they are used in plastic injection molding machines, medical infusion pumps, robots, hydraulic cylinders, crash-test dummies, and strip chart recorders.

Fast inductive position transducers

Noncontacting linear-displacement transducers, Figure 1, are the newest sensors capable of keeping pace with today’s motion control systems. (In this article, the term noncontacting describes the internal assembly of these sensors. The coil and core do not touch each other. Therefore, there is no degradation or wear from friction. These devices can last through billions of operations.)

Inductive position transducers came from the need for accurate position measurements in reciprocating heat engine research. The sensors handle the vibration, high temperature, and oscillating pressure common in engines.

These sensors measure displacement at frequencies to 15 kHz. Because of this high frequency response, they do not experience phase shifts between motion and output. And because they run on a power supply of 112 kHz, they are not subject to extraneous electrical fringe fields, such as those found near an ac generator.

In addition to engine research, the transducers are used to measure:

• Viscosity changes of oils for car engines. A loudspeaker is connected to a steel ball that oscillates on a drop of the oil. One of these transducers is mounted in the loudspeaker. As the speaker moves, the transducer measures the position displacement of the ball-loudspeaker connection.
• Speed that a chest cavity of a crash test dummy caves in after sudden impact. An inductive position transducer is mounted inside the dummy’s chest.
• Position of a robotic welder as it moves to the next weld spot in car frame assembly. The transducer provides feedback to a programmable controller on servo movement.

These transducers are also small enough to fit the tight space requirements found in many applications, like the heat engine research, and sense relatively short changes in position (0.001 in.). Some of these transducers have an extended linear range to deliver a standard 0.1% linearity, with the sensor length exceeding its linear range by only 30 to 35 mm. The result is a nearly 1:1 stroke-length-to-body length ratio.

The sensors are variable inductors in the form of a helical coil with a moveable aluminum tube core, Figure 2. The core attaches to the object whose position is to be measured. An ac voltage of 112 kHz, sent by a signal processor, excites the coil. The core, made of an electrically conducting material with low magnetic permeability, moves with the object. As the core moves axially, it expels the magnetic flux that surrounds the coil. This action changes the inductance of the coil. The relationship between the change in inductance and core position is nearly linear at 112 kHz.

Conductive plastics

The second sensor capable of matching the fast feedback needs of today’s motion control systems is known as a conductive plastic transducer. The internal components of this sensor are based on the components used in older potentiometric positioning sensors, which used a wire that was wrapped around a mandrel and that slides from one end to the other, Figure 3A. Voltage applied across the element indicated wiper position and varied linearly.

The life of these early wirewound position transducers was usually given in thousands of cycles, acceptable for a transducer that moved a few times a day. The contact forces between the wiper and the wire element determine the amount of wear and thus, the length of the sensor’s operating life.

However, components in today’s control systems must move almost continuously, limiting the life of a wirewound position sensor. In addition, the loops in the wrapped wire create jumps or steps in the output voltage as the wiper moves from one loop to the next. As control circuits become more accurate, these steps in output voltage are less acceptable.

Newer position transducer sensors capable of continuous output and with long life come in the form of conductive plastics. Many of these conductive plastics are a combination of carbon, silver, binders, and other additives that give each formula unique mechanical, electrical, and environmental attributes. Several combinations can measure displacement at speeds to 15 m/sec.

Conductive plastics look like thick black ink, Figure 4. Their consistency is controlled by the amount of the various components in the mixture.

The conductive plastic is the resistive element. The wiper is a piece of platinum alloy ribbon with “fingers” punched from it, Figure 3B. These parallel finger pieces are bent up (imagine the tines of a garden rake) so the tips touch the conductive plastic to make the voltage reading, which varies linearly.

Some conductive plastics can operate at temperatures to 500 F with mechanical flexibility while others are rigid and inflexible. The mixtures are not patented but held as corporate secrets.

Features include:

• Installation — Looking like ink, this sensor is adaptable to different configurations; it can even be placed on curved surfaces with tight clearances, such as in a prosthesis for an elbow joint or on motor shafts that rotate antenna or other non high-speed rotating applications.
• Operating life — Life is often quoted in the millions of operating cycles, because of the properties possible through various mixtures of materials.
• Linearity — Conductive plastics can be made to a linearity of 0.05%.
• Output — These transducers will give a 10-V output from a 10-V input. The signals can be generated for short strokes, such as 1/4 in. total, without signal amplification.
• Resolution — A plastic film’s grain structure determines the resolution. Conductive plastics are mixed to achieve small grain structure and thus, high resolution.
• Lubrication — These sensors contain additives that reduce the coefficient of friction between the element and the wiper. No further lubrication is required for conductive plastic transducers to meet their rated life.

Gary BriggsGary Briggs is transducer products manager of Data Instruments Inc., Acton, Mass.

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