Servo motor and high torque: what’s the big deal?

Expressed as a formula, torque is the twisting action applied perpendicular to a distance vector times the vector length from the reference datum of impact to the pivot point. The equation has a positive result if rotation is in the anticlockwise direction or a negative one when an object turns clockwise.

In regard to a servo motor,the technical term stands for the mechanical work produced by its shaft spinning, typically measured in Newton-meters (Nm). However, mind that the parameter is always about rotation, and you will not find it in the specifications of linear models. Instead, their ability to move objects is defined by force.

torque action

Servo motors: what high torque means

For a servo motor, high torque means the mechanism is able to handle heavier loads. How much turning force a servomechanism can actually output depends on design factors, such as supply voltage, shaft speed, etc.

There are several ways to measure the turning force:

  • One option is to fix a gauge onto the shaft and read it while running the servo. In this case, measurement accuracy depends on that of the instrument. Additionally, you have to count in errors due to inaccurate coupling alignment.

  • It is also possible to derive the rating from the position or velocity sensor readouts. For the purpose, couple a large load with known inertia moment to the motor. Relying on the measurement results, calculate velocity and acceleration and then use the inertia moment to find out the moment of force. The advantage of the method is that it enables quantifying torque against speed.

  • A third technique is to determine it based on the force sensor measurements taken at an established distance. Known distance times force minus any inaccuracies gives you the needed rating.

A possible remedy to get a servo motor with high torque is to add a gearhead—an arrangement of gears that provide proportional speed-to-torque conversion. The gear ratio is the measure to which the device amplifies rotational force, while decreasing revolution rates.


Servo’s torque: technical details

In the manufacturer specifications for a servo motor, torque ratings, whether high or low, are numerous. Let us see what each of them means:

  • Rated—how much a servo can produce at given operating conditions, such as rated speed and ambient temperature. When the temperature at the customer premises is beyond the recommended maximum, torque tends to derate faster in proportion to RPM. If the situation is reverse, derating slows down.

  • Peak—the highest level at which a motor can function stably. The value is usually calculated as a sum of three rotational forces—due to acceleration, due to friction, and due to load. It should be brief in duration (e.g., as long as needed to overcome mechanical friction as the servomechanism ramps up) and within the intermittent segment.

  • RMS—takes into account various magnitudes of the parameter that can occur throughout a duty cycle of a servo motor, as well as their required duration. It reflects how variations of the turning force can affect motor performance during acceleration, deceleration, and at constant velocity. The RMS values should lie within the continuous section of the curve.

Additionally, if an application works with static loads that are more significant than dynamic ones, you may need to know the stall rating—how high is the rotational force produced at zero velocity. Though engine is not moving in this state, current continues to flow, which may lead to extra high temperatures. So, the value is the limit, keeping to which a stalled motor can operate safely.

In regard to a servo motor, the technical term stands for the mechanical work produced by its shaft spinning, typically measured in Newton meters. However, mind that the parameter is always about rotation, and you will not find it in the specifications of linear models. Instead, their ability to move objects is defined by force.


Other critical metrics

When you are looking for a servo motor, high torque is a sizing criterion of first priority. However, it is also essential to account for how the rating changes depending on velocity variations and the inertia ratio.

Inertia ratio is the relation of the motor inertia to that of its driven load. The parameter affects the mechanical component in the overall efficiency formula. When matched incorrectly to your system, it can degrade positioning precision by impairing operation of control loop elements.

For the motor inertia, refer to manufacturer specifications. To know the other value reflecting resistance of the attached load to changes of its position, certain calculations are required. These should account for all application components involved in motion (e.g., couplings, driving elements).  

Ideally, both estimates should be identical, which is infeasible, though, and even inappropriate under certain conditions. It is hard to determine conclusively what is the perfect inertia ratio, but one thing is for sure—it should neither be super high or unreasonably low.

At excessive values, current consumption increases and so do operating costs. There can also occur resonance or vibration issues. At the same time, a minimum mismatch (close to 1:1) is often an indication that a drive is oversized for your application, which leads to extra high expenses.  

To obtain the adequate match, try to experiment with servomechanism dimensions, making them bigger or smaller, or have a go at a geared actuator. A gearing set allows to adjust the inertia ratio to prerequisites, while optimizing turning force and keeping revolution rates at desired levels. Whichever of the variants you go for, the baseline is the profile of the designed application.

Servo speed is critical for sizing calculations in how it relates to torque variations over time. The general correlation: when the motor RPM is high, the other rating is low and vice versa. The dependence is evident from a torque-speed curve, which is unique for each engine model and has two regions—continuous and intermittent duty.

servo motor torque speed curve

If your target velocity and torque pair falls within the continuous duty region, this means a drive can produce this output for unlimited time span without thermal or any other damage. Value combinations in the intermittent region are permissible only for a short period, such as during acceleration or deceleration. After the interval expires, the servomechanism starts to overheat.                   

The RDrive series servo motors, high torque by design, can provide you with the right solution to achieve your targets. Gearing eliminates the inertia matching issue, while allowing to obtain high torque output—up to 216 Nm in the continuous duty region and 333 Nm in the intermittent one.

The line-up includes an array of models in five sizes, available in both geared and gearless versions. Without a gearbox, you get increased RPM and reduced housing length. For more information about their torque and other ratings, look through the specifications. 


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