Machine Tool Control

TEMPERATURE TRANSDUCERS 

When two wires of dissimilar metals are joined together, a voltage is generated if the junctions are at different temperatures. If the reference junction is kept at a fixed known temperature, the thermocouple can be calibrated to indicate the temperature at the other junction in terms of the voltage v. Electrical resistance change with temperature. Platinum gives a linear relation between resistance and temperature, while nickel is less expensive and gives a large resistance change for a given temperature changes. Semiconductors designed with this property are called thermistors. Different metals expand at different rates when the temperature is increased. This fact is used in the bimetallic strip transducer found in most home thermostats. Two dissimilar metals are bonded together to form the strip. As the temperature rises the strip curls, breaking contact and shutting off the furnace. The temperature gap can be adjusted by changing the distance between the contacts. The motion also moves a pointer on the temperature scale of the thermostat. Finally, the pressure of a fluid inside a bulb will change as its temperature changes. If the bulb fluid is air, the device is suitable for use in pneumatic temperature controllers. Machine tools are usually controlled for temperature readings using the application of temperature transducer.    

FLOW TRANSDUCER 

Flow rates can be measured by introducing a flow restriction, such as an orifice plate, and measuring the pressure drop across the restriction. The flow- rate- pressure relation is ∆p = Rc2 where R can be found from calibration of the device. The pressure drop can be sensed by converting it into the motion of a diaphragm. Figure 2.1 illustrates a related technique. The venturi- type flow meter measures the static pressures in the constricted and unconstricted flow regions. Bernoulli’s principle relates the pressure difference to the flow rate. This pressure difference is converted into displacement by the diaphragm.

The error detector is simply a device for finding the difference between two signals. The Function is something an integral feature of sensors, such as the synch transmitter- transformer combination. A beam on a pivot provides a way of comparing displacement, forces, or pressures, as was done with the pneumatic level controller (Fig 1.8). A similar concept is used with the diaphragm element shown in fig 2.1 A detector for voltage difference can be obtained as with the position-control system shown in fig. 1.5 An amplifier intended for this purpose is a differential amplifier. Its output is proportional to the difference between the two inputs. In order to detect differences in other types of signals, such as temperature, they are usually converted to a displacement or pressure. One of the detectors mentioned previously can then be used. This is simply applied in the flow systems of flow coolants/ fluids used in lathe, turret lathe, drilling machines etc.

The usual transducer and detector models are static models, and as such imply that the components respond instantaneously to the variable being sensed.  Of course, any real component has a dynamic response time must be considered in relation the controlled process when a sensor is selected. For example, the time constant of a thermocouple in air typically is between 10 and 100 sec. if the thermocouple is in a liquid, the thermal resistance is different, and so will be the time constant (it will be smaller). If the controlled process has a time constant at least 10 times greater, we probably would be justified in using a static sensor model.