Page 132 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors
• When upstream sensors are not conducting, the downstream 93
sensors are disconnected from their power source and are
incapable of responding to a target until the upstream sen-
sors are activated (damped). Time before availability will be
increased due to the response in series.
Series and parallel connections that perform logic functions with
connection to a PLC are not common practice. Utilizing sensors this
way involves the preceding considerations. It is usually easier to con-
nect directly to the PLC inputs and perform the desired logic function
through the PLC program.

2.9.7 Inductive and Capacitive Sensor Response
Time—Speed of Operation
When a sensor receives initial power on system power-up, the sensor
cannot operate. The sensor operates only after a delay called time
delay before availability (Fig. 2.87).
In AC sensors, this delay is typically 35 ms. It can be as high as
100 ms in AC circuits with very low residual current and high noise
immunity. In DC sensors, the time delay is typically 30 ms.

2.9.7.1 Response and Release Time
A target entering the sensing field of either an inductive or a capaci-
tive sensor will cause the detector circuit to change state and initiate
an output. This process takes a certain amount of time, called response
time (Fig. 2.88).
Response time for an AC sensor is typically less than 10 ms. DC
devices respond in microseconds. Similarly, when a target leaves

FIGURE 2.87 Time delay prior to availability.