Honeywell Switches and Sensors manual Environmental influences, Repeatability, Mutual interference

ULTRASONIC DISTANCE SENSORS

< 5°	10 cm
< 5°
Sn	10 cm
< 5°	10 cm
< 5°
Sn	10 cm

Figure 2: Effect of target inclination on the measurement

Environmental influences

The velocity of sound in air is temperature-dependent, and increases at a rate of 0.18 %/°C. Honeywell ultrasonic distance sensors have their own temperature transducer, which adjusts both the clock frequency of the elapsed time counter and the carrier frequency. Major temperature fluctuations within the measuring path can, however, lead to sound dispersion and refraction, which disturb the measuring result and limit the stability of the measurement (Figure 3). Air streams, turbulence and air layers of different densities can, in certain conditions, attenuate or deflect the echo to such an extent that the sensor cannot detect it. On the other hand, air humidity and normal atmospheric air pressure fluctuations have virtually no influence on the measurements.

	cold
	10 cm
Sn	10 cm
	warm
	10 cm
Sn	10 cm

Figure 3: Effect of warm air turbulence on the measurement

Repeatability

All information concerning repeatability and hysteresis in this data sheet is valid for axial target movements (Figure 4). If a target approaches the sensor from a distance, the output switches at the set value ± the given repeatability. If the target moves further away from the sensor, the output switches back into its original condition, at a distance which is equal to the sum of the setpoint and the given hysteresis ± the repeatability. If a target moves laterally into the acoustic beam, the echo energy increases. If the measurement threshold of the sensor is reached, the output becomes active. This threshold depends on the target properties and its distance from the sensor. The position can only be determined experimentally.

(cm)	5						Target
	3
	1
	-1
	-3
	-5	200	400	600	800	1000	1200	1400	(mm)
	0
					Repeatability
					Switching point in
	Output				forwards direction
					Switching point in
					backwards direction
			Hysteresis

Figure 4: Repeatability and hysteresis

Mutual interference

Despite pulse coding, if several sensors are used simultaneously in a single application, mutual interference can occur. This phenomenon will, however, only arise if, as a result of the inclination of the object, or the positioning of two sensors opposite one another, false echo signals can be received. By using the inhibitor input, maintaining minimum distances or restricting the beam angle with a focusing reflector, the problem can be almost entirely avoided.

Synchronisation

The majority of Honeywell ultrasonic distance sensors can be very easily synchronised by interconnecting the appropriate inputs or connecting them with an external synchronisation unit. The transmission of the acoustic pulses then occurs simultaneously. This makes it possible to use the sensors for applications in which the ultrasonic transducers are facing each other, while still avoiding mutual interference.

Protective measures

All sensors are protected against water and dust, according to the DIN standard IP 65. The transducer is coated with silicone rubber or epoxy, but it can be attacked by aggressive acid or caustic atmospheres. It is also necessary to ensure that the transducer face remains clear of liquid or solid deposits, which could limit the performance of the sensor. Drops of water may be deposited on the transducer surface, as a result of condensation. These could severely reduce the sensor range. Also because of the risk of icing up, and because sensors detect raindrops, the suitability of these sensors for outdoor use, despite the protective measures, is limited.

Electrical interference

All Honeywell ultrasonic sensors are protected against reverse polarity, short circuits, overloads and voltage spikes. Special protective circuitry makes the sensor almost entirely immune to electromagnetic and radio frequency interference. However, unstable measurements may arise if the sensor is placed in the vicinity of strong electrical fields. In such cases, the interconnection cables should be screened as far as possible, or separated from power cables. The use of regulated power supplies with mains filters, and limiting the maximum cable length to 50 metres can also offer possible solutions. All sensors are CE marked.

Alignment aid

The majority of Honeywell ultrasonic distance sensors have an LED, the output intensity of which is proportional to the ultrasonic echo received. The brighter the LED, the better aligned the sensor.

Please contact your nearest Honeywell office for details of other models available.

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