3.5.2 Experimental instruments
Variable temperature viscometer,ZKY-PID temperature controller,stopwatch,temperature meter,tens of small steel balls.
Brief introduction of the instruments used in this experiment.
1)Variable temperature viscometer
The schematic diagram of the variable temperature viscometer is shown in Figure 3.5.1.The target liquid is filled in a slender sample tube,making sure that the temperature of the target liquid will be quickly equal to that of the heating water surrounding the sample chamber.In addition,the scale lines are scratched on the wall of the sample tube,which is beneficial to determine the falling distance of the ball.The heating water chamber is connected to a temperature controller and heated by thermal circulating water.Furthermore,four adjustment screws under the foundation plate are used to tune the vertical state of the sample tube.
Figure 3.5.1 Structure diagram of variable temperature viscometer
2)ZKY-PID temperature controller
The temperature controller contains a water tank,water pump,heater,control and display circuit,whose front panel is shown in Figure 3.5.2.Here,proportional integral derivative(PID)controller,one of the most widely used temperature controllers in automatic control systems,is employed to control the temperature in this experiment.The ZKY-PID temperature controller contains many advantages,such as displaying the real-time values of temperature,storing the temperature curves during operation,and high control accuracy.(https://www.daowen.com)
Figure 3.5.2 Front panel of ZKY-PID temperature controller
Figure 3.5.3 shows the principle of PID automatic control system.The regulator is adjusted by PID and the executive unit is a heater controlled by silicon controlled rectifier.In addition,the controlled object is the water in the tank and the controlled quantity is the temperature of water.
Figure 3.5.3 Flow chart of automatic control system
Figure 3.5.4 displays the temperature of water as a function of time in the temperature control process.After reaching the setting value,the system cannot be stabilized immediately but will be stabilized after a certain transition time.System inertia is the main reason of the overshoot phenomenon.During the heating process,the temperature of the heater is always higher than the temperature of the controlled object.After reaching the setting value,the heat stored in the heater will still heat the system in a period of time even if the heating power is reduced or cut-off,and the cooling has a similar reverse process.The phenomena above are called the thermal of inertia system.In the actual control process,PID parameters must be reasonably adjusted according to the system characteristics to achieve a better control effect.
Figure 3.5.4 Transition process of PID control system
