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30 January/February 2021 Tablets & Capsules to the shaft, the shaft material twists or "stretches" a very small amount in a direction 45 degrees to the axis. Torque transducers measure changes in the shaft stretch- ing to calculate the torque. This measurement is con- ducted using strain gauges bonded to the shaft, which measure the strain induced in the shaft by the applied torque or force. The direct torque measurement approach requires the following tools: • A direct torque measuring device with a con- tact-free sender and receiver integrated into the impeller shaft; • A pre-calibrated measuring device; • A display of measured values on an HMI; • Granulation endpoint determination function; and • Set points for granulation endpoint determination on an HMI. Figure 2 shows a comparison between the impeller current and the impeller direct torque using a wireless telemetry system for an ACG 1200 L HSM mixer. The left vertical axis denotes the current in amperes, and the right vertical axis denotes the impeller direct torque in newton meters. The red line indicates the pattern of the impeller current, and the blue line indicates the pattern of the impeller direct torque. The graph clearly shows that fluctuations in the impeller current are more frequent than fluctuations in the impeller torque even though the values of torque range from 0 to 4,000, while the values of impeller current range from 0 to 90. This precision in torque, so faulty ampere readings will impact the impel- ler torque calculations. Reaction torque. When an impeller shaft rotates, the motor attempts to move in the opposite direction but can't because it is bolted in place. This tension in the stationary motor base can be measured using a reaction torque transducer. However, this method does not pro- vide very accurate readings owing to variations during the wet mixing process, so it leads to inaccurate end- point prediction. Torque rheometer. A torque rheometer provides an offline measurement of the torque required to rotate the mixer blades and can be used to assess the rheologi- cal properties of wet granules. The disadvantage of this method is its offline measurement. Acoustic emission. The acoustic emission method uses piezo-electric acoustic emission sensors to determine the granulation endpoint. The granulation process indicators obtained with an acoustic transducer can be used to mon- itor changes in particle size, flow, and compression prop- erties. It is a non-invasive, sensitive technique but is still relatively expensive. Near-Infrared (NIR) spectroscopy. NIR sensors can be used to determine the moisture content during granu- lation, but this method does not accurately determine the endpoint of the process. Focused beam reflectance measurement (FBRM). FBRM is a particle size determination technique that uses a laser beam focused in the vicinity of a sapphire window of a probe to determine the chord length dis- tribution (CLD) of the particles. The CLD is a measure of the particles' size, shape and concentration and can be recalculated to represent either the number of parti- cles or the volume-weighted particle size distribution. In many cases, CLD measurements are adequate to monitor dynamic changes in process parameters related to parti- cle size and shape, concentration, and rheology, but the method is expensive. Direct torque measurement Direct torque measurement provides the most accu- rate, state-of-the-art process control available by mea- suring the impeller torque in newton meters in real time using a device mounted onto the impeller drive shaft. In comparison to standard torque measurement techniques, which calculate a theoretical impeller torque value based on the impeller drive's current consumption, this tech- nique can eliminate all factors that may influence current consumption, including wear and tear of bearings and gears, oil viscosity, and temperature fluctuations. Torque sensors use strain gauges that are applied to rotating parts. This can be accomplished using rings, wireless telemetry, or rotary transformers. Figure 1 shows the arrangement of a direct torque measurement device on an impeller hub. The underlying principle of direct torque measure- ment is based on measuring the force being used to turn the impeller shaft. When a force or torque is applied Figure 1 Direct torque measurement device mounted on an impeller hub Torque sensor Impeller shaft