Control torque

In another elongational rheometer (221), the specimen is suspended on the end of a flexible tape, which is wound onto a wheel turned by a servo-controlled torque motor. This design is the basis for the Gottfert Rheostrain instrument, with which strain rates of 5 x 10 3 to 2 s-1 are possible. 

The prevalent design is a microprocessor-controlled torque rheometer. The system consists of two basic units an electromechanical drive unit and a microprocessor unit. The basic system is shown in Figure 7. 

Prior to the invention of screw closures, corks, glass stoppers, pulpboard discs and parchment covers were the main closures. Historically, prethreaded screw closures have consisted of a threaded shell containing a wad or liner and a facing. The wad or liner may be made from cork agglomerate (gelatin or resin bonded), pulpboard, feltboard, plastic or rubber. It provides a resilient backing which receives the compression. In the case of screw closures the compression is due to the application of a controlled torque. Other closures, e.g. roll on, clinch, press on, may achieve this by the simple application of top pressure. Under the normally recommended torques, composition cork compresses by about 50% and pulpboard by approximately 20%. 

The test geometries listed in Table 2-2 are those commonly used for instruments that can apply rotary motion, either in the form of a controlled torque or a controlled amount of rotation. The listed examples can as a set handle samples from stiff viscoelastic solids to freely flowing liquids. All are... 

To implement a motor controller, a servo-amplifier is required to convert signal level power (volts at milliamps) to motor power (volts at amps). Typically, a design requirement for series, shunt, and brushless motor drives is to control torque and speed, and hence, power. Voltage control can often be used to control speed for both shunt and series motors. Series motors require feedback to achieve accurate control. 

Heart-valve housing and struts and stents, housings for artificial heart, composites to control torque and steering of catheters. Radiopaque fillers in polymers to identify location on x-ray... 

Clutch Type Controller. Torque T on a radial rotor element at a general radius r is given by... 

The first step is to determine the rheological characteristics of the material, which has been sampled. The material is inserted into a circular cylindrical rheometer of height H=50 mm, iimer radius Z i= 19.5 mm, and outer radius R2= 20.5 mm. The outer cylinder is stationary, while the iimer cylinder can rotate about its symmetrical axis. The rheometer operates by imposing a known and controlled torque, M, to the iimer cyhnder and measuring its angular rotational speed, Q, in a steady-state regime. The density of the material is /> = 2000 kg/m. ... 

Fig. 8. — Newcomen s steam engine as the primary milestone in the novel exploitation of heat and its alternative use other than for effortless getting warmness with the original sketch of cylinder with piston (middle), which can be acquainted with the shared employment of all four elements (where the earth-based construction represents the joining in-form = information transducer). Right it is added by illustrative drawing of the mechanical control torque, factually the first automaton realized through the Watt s innovative governor. ...

Both controlled-angular displacement (strain-controlled) and controlled-torque (stress-controlled) rotational rheometers are used, with the former giving superior performance at high frequencies and the latter better precision at low frequencies. There have also recently appeared on the market instruments said to be capable of operating in both modes. Controlled torque instruments can also be used to make creep and creep recovery tests, which are described in the next section. In order to obtain a linear viscoelastic characterization that includes the terminal zone, it is sometimes useful to combine data from oscillatory flow with those from a creep experiment, and this is also discussed in the following section. Rheometrical methods are described in some detail in several books [3,8,9]... 

Since it is the long-time behavior that is closely related to molecular structure, this is the information that is most interesting in the present context. For example, the zero-shear viscosity describes behavior in the limit of zero frequency and is very sensitive to molecular weight. However, for a material whose longest relaxation time is quite large, neither step-strain nor oscillatory shear experiments are useful to probe the behavior at very long times or very low frequencies. The main problem is that the stress is so small that it is not possible to measure it precisely. It is in this region that creep measurements are most useful. This is because it is possible to make a very precise measurement of a displacement, and it is also possible to apply a very small controlled stress. Controlled-torque (controlled-stress) rheometers are available from several manufacturers. 

Key application points must be defined in order to properly apply an induction motor to a solid-state adjustable-firequency controller torque, speed range, motor description, and environment. In order to ensure that adequate torque is available to drive the load and adequate current is available to produce the required torque, the starting torque, the peak running torque, and the continuous torque requirements must be defined. The continuous torque is usually defined, but the peak and starting torques are more difficult to define. For the case of retrofit applications, the speed-torque curve of the existing motor mig t be used as... 

Thanks to their relatively lower viscosity, food and cosmetic products allow this type of behavior to be easily documented, when performing experiments with controlled stress rheometers. There are commercial versions of such instruments, essentially rotating systems (parallel disks or cone-and-plate), which have the capability to measure extremely small rotation rates (in the 10 rad/s, i.e., one revolution in 20 years ). Experiments performed in such conditions are called creep testing using controlled torques as low as 10 Nm with a resolution of 10 Nm. Figure 5.5 shows an example of the shear viscosity function (vs. shear stress) measured on a typical cosmetic product (body cream), using such a controlled stress rheometer. 

---