Reaction vs. Inline
Inline torque measurements are made by inserting a torque sensor between torque carrying components, much like inserting an extension between a socket and a socket wrench. The torque required to turn the socket will be carried directly by the socket extension. This method allows the torque sensor to be placed as close as possible to the torque of interest and avoid possible errors in the measurement such as parasitic torques (bearings, etc.), extraneous loads, and components that have large rotational inertias that would dampen any dynamic torques.
Figure 1: Simple torque measurement
From the previous example above, the dynamic torque produced by an engine would be measured by placing an inline torque sensor between the crankshaft and the flywheel, avoiding the rotational inertia of the flywheel and any losses from the transmission. To measure the nearly static, steady state torque that drives the wheels, an inline torque sensor could be placed between the rim and the hub of the vehicle, or in the drive shaft. Because of the rotational inertia of a typical torque drive line, and other related components, inline measurements are often the only way to properly measure dynamic torque.
A reaction torque sensor takes advantage of Newton’s third law: ‘for every action there is an equal and opposite reaction’. To measure the torque produced by a motor, we could measure it inline as described above, or we could measure how much torque is required to prevent the motor from turning, commonly called the reaction torque.
Figure 2: Placement of torque sensors is important
Measuring the reaction torque avoids the obvious problem of making the electrical connection to the sensor in a rotating application (discussed below), but does come with its own set of drawbacks. A reaction torque sensor is often required to carry significant extraneous loads, such as the weight of a motor, or at least some of the drive line. These loads can lead to crosstalk errors (a sensors response to loads other than those that are intended to be measured), and sometimes reduced sensitivity, as the sensor has to be oversized to carry the extraneous loads. Both of these methods, inline and reaction will yield identical results for static torque measurements.
Making inline measurements in a rotating application will nearly always present the user with the challenge of connecting the sensor from the rotating world to the stationary world. There are a number of options available to accomplish this, each with its own advantages and disadvantages.