ELECTRO-MECHANICAL TRANSDUCERS

September 25, 2016


 

The following post is taken from a paper written by this author regarding encoders.  The paper is published through http://www.PDHonline.org.

INTRODUCTION:

The use of motion sensors has become commonplace and increasingly important to motion control system designers in all sectors of manufacturing and medical endeavors.    Rapid advances in size, accuracy, resolution, and application of sensitive motion control sensor systems have quickly become more attractive to design engineers. The broad range of devices currently available can offer design engineers multiple solutions to their motion control needs.

Measurement and control often involve monitoring rotary and linear motion. Both are multi-stage processes with the first stage being the generation of an electrical signal to represent desired motion. When measurement is the objective, this signal is used to quantify the desired property (i.e., displacement, velocity, etc.).  The second stage is data translated into a readable format that can be understood by the end user. When control is the objective, the signal is used directly by the associated controller.

Whether measurement or control is required, generation of the electrical signal to represent the motion is accomplished with transducers. The design and selection of a transducer is determined through the evaluation of application considerations.  An electrical transducer is a device capable of converting a physical quantity into a proportional electrical quantity such as voltage or electric current.  A transducer converts any quantity to be measured into usable electrical signal.  (Note that any device which is able convert one form of energy into another form is called as a transducer. For example, even a speaker can be called as a transducer as it converts electrical signal to pressure waves (sound).   An electrical transducer will convert a physical quantity to an electrical quantity.

There are several types of transducers as follows:

TYPES OF TRANSDUCERS:

  • Proximity Switches: Proximity switches open or close an electrical circuit when they make contact with or come within a certain distance of an object. They are most commonly used in manufacturing equipment, robotics, and security systems. There are four basic types: infrared, acoustic, capacitive, and inductive. These switches, probably the oldest of the control elements, are basically location sensing devices. They include true mechanical switches, photo sensors, magnetic pickups, pressure sensors, etc.  Proximity switches have historically been the primary location indicating device in control systems, but rarely used for measurement except in “go-no-go” gauging. Output is a discrete change in signal level, and these devices are easily interfaced with both custom controllers and computers. Response of mechanical switches is relatively slow. Precautions must be taken to ignore the multiple signals generated by contact bounce when these switches are monitored with control devices such as computers that are capable of rapid response
  • Potentiometers: A potentiometer is a manually adjustable electrical resistor that uses three terminals. In many electrical devices, potentiometers are what establish the levels of output. For example, in a loudspeaker, a potentiometer is used to adjust the volume. In a television set, computer monitor or light dimmer, it can be used to control the brightness of the screen or light bulb. Potentiometer outputs, from both rotary and linear devices, depend on the position of a sliding contact on a resistive element.   Normally operated as a voltage divider, output is analog, and analog to digital hardware is required for digital output applications. Potentiometers are often used to measure displacement as opposed to proximity switches whose chief function is control safety or limiting. Potentiometers are moderately accurate devices when properly calibrated, but are susceptible to degradation due to wear. Resolution may be limited, but is often adequate for many applications. Potentiometers are susceptible to many environmental constraints. Essentially mechanical contact devices, they must be protected from shock, vibration, and foreign matter contamination.
  • Analog Inductive Components: Inductive analogue distance sensors provide an output signal that is proportional to the distance between the trigger surface and the object. In many applications you also need to generate a switching signal at specific points along the output curve. These switching signals enable systems to recognize when a particular position (distance from a machine part) has been reached. In the past this required an additional external analogue switching amplifier. This component is no longer required. Inductive transducers are widely used devices for both rotary and linear applications. Similar to the trans-former, alternating current in one coil (primary) induces alternating current in an adjacent coil (secondary), the principle of operation is electromagnetic coupling between parallel conductors. Position can be deduced accurately with external electronics and output is sinusoidal.

There are many variations of inductive transducers. Some of the most common are synchros, resolvers, induction potentiometers, and linear variable differential transformers (LVDTs).

A true synchro resembles a three-phase motor, but produces an electrical output corresponding to the an-gular position of its shaft. The output is analog and its position can be interpreted from the relative volt-age, amplitude or phase. The synchros can be connected so that the output shaft assumes the same relative position as the input shaft.

  • Encoders: An encoderis a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed or compressions. Encoders can be categorized into two broad types: contacting and non-contacting. The contacting type requires brushes or finger sensors that electrically transmit a signal to indicate a change in position. Non-contacting encoders rely on magnetic, capacitive or optical phenomena to sense the motion. Outputs can be either absolute, a digital coded word that indicates absolute position, or incremental, with repetitive pulses that are counted to accumulate total motion.

Rotary position sensing, either absolute or incremental, indicate the rotation of a shaft. The encoding disc is patterned with radial lines that are sensed as the input shaft is rotated. Mechanical packaging varies greatly depending on application requirements.

Linear position sensing depends upon a moving head whose motion is sensed along a linear track and a scale. Principles of operation and output types are similar for rotary devices. Mechanical packaging accommodates a wide spectrum of application requirements.

OPERATIONAL CHARACTERISTICS OF TRANSDUCER TYPES:

The transducer that should be used in a given application depends on the performance requirements, environmental constraints, and other factors such as cost, space requirements, etc. Some of the more important design considerations are listed in Table One shown below.  Potentiometers, encoders, and inductive transducers are used for both measurement and control. The wide variety of these devices creates a considerable overlap in their application. The proximity switch is technically a measurement and control device but because of its limited two-stage output, it is not normally performance competitive with other devices. The following table will give some basic idea as to those considerations needing to be made prior to selecting a transducer type.

encoder-types

CONCLUSIONS:  As you might expect, a great deal of care is needed before specifying a transducer type. The table above is a very brief guideline as to those considerations necessary.  I certainly hope you enjoy this one.  The remainder of the course is a work in progress.

 

 

 

 

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