FREQUENCY INPUT

Several sources create signals where the frequency carries the desired data. The measurement of rotating parts speed is one of the largest sources of frequency related data. Flow meters are also a large data source.

SIGNAL WAVE SHAPE

Frequency signals generally fall into 3 basic categories that cover most applications.

Magnetic Pickup
Magnetic pickups create a pulse or a sinusoidal wave shape depending of the duty cycle of the total time for 1 cycle of measurement versus the actual time the magnetic sensor sees the sensed piece in its area of influence.

Rotating Gear Sensing
Many rotating parts move at a relatively slow speed and would produce a low frequency if only 1 sample were taken per revolution.

It is common practice to mount a gear on a shaft that rotates and use the magnetic pickup to sense the teeth of the gear as they move past the pickup. A 60 tooth gear will produce a frequency in Hz that equals the rpm of the shaft.

The even spacing of the teeth produces a sinewave output from the magnetic pickup. The amplitude of the sinewave varies linearily with frequency and doubles in amplitude for a doubling of frequency.

The amplitude from the magnetic pickup may be in the range of 50 mV to 100 volts, depending on the velocity of the target and the inductance of the pickup.

The sinusoidal nature of the symmetrical signal allows simple filtering, inside the signal conditioner, to prevent noise from upsetting the measurement.

Single Point Sensing
Often a magnetic sensor may be required to sense a single point such as a set screw head or some other protuberation on the rotating part. This type sensing produces a single bipolar(+ and - swing) output per revolution of the shaft.

The shaft must rotate fast enough to give an output frequency high enough to allow the measurement to be made accurately.

Digital Level Inputs
Several electronic products produce outputs of a square wave nature that are equal in amplitude to the digital circuit's power supply (usually 5 V or greater). Flowmeters and proximity detectors are available that have built in sensors to produce logic level outputs.

Inputs that are symmetrical square waves can be treated as sinusoidal signals with the added benefit of constant high amplitude.

Some digital sensors or products that produce a frequency output create a narrow constant width pulse of only 100 microseconds or so width. The rate of these pulses varies with the frequency data, but the width remains constant. The signal conditioner used to process this type input has a limited ability to filter the incoming data. The frequency may be quite low, but the filter is dictated by the narrow pulse width.

There are digital sources that provide an open collector transistor as its output. This transistor is made to turn on and off, like a switch, by the source's input frequency.

Reed relays are also sometimes used as an input device.

The open collector transistor and reed switch are used to develop a square wave in the signal conditioner. Open collector transistors are fast and can pass signals to many KHz in frequency. Reed relays are limited to about 2 or 3 Khz and have a relatively short life when compared to the transistor.

FILTERING

A straight analog signal conditioner requires a filter to remove the measured frequency component from the DC output. The time constant of this filter is 100 times or more longer than the period of a single cycle of the measured frequency. The desired response time dictates the frequency of the input signal.

Circuits inside the signal conditioner can be used to multiply the frequency. The amount of multiplication is dependent on the characteristics of the input signal - primarily the rate of change of the input signal.

THRESHOLD SENSITIVITY

A constant source of noise, especially with low level magnetic pickups, is the AC power mains. The magnetic pickup is the equivalent of half a transformer and is sensitive to magnetic fields.

Many frequency input conditioners have a fixed or adjustable input amplitude threshold. Until the amplitude of the input exceeds the threshold, the output of the conditioner stays at its zero scale value.

If the contemplated product is consistently wired the same way, wiring can usually be controlled so a threshold sensitivity is not required.

If the sensor-to-conditioner wiring is different for every installation, a threshold should probably be provided.

Threshold sensitive inputs eliminate the problem of having the conditioner see the power line frequency( or multiplies) as an input when the normal input is supposed to be at zero frequency.

COST FACTORS OF FREQUENCY INPUT SIGNAL CONDITIONER

1. Absolute accuracy of frequency measurement.
2. Stability versus ambient temperature.
3. Type of input. Low level input (mV levels) most expensive.
4. Linearity of output versus frequency.
5. Environment (hazardous, humid, corrosive, etc.).
6. Span of input. Very narrow spans require more stable design.