Engineering Insight

By: J. Antille and Y. Meyer, Sonceboz SA, Sonceboz, Switzerland

Hybrid stepper motors can produce a tremendous number of steps—often as many as 200—per revolution. Because of this capability, hybrid motors offer the technical advantage of high torque at start and at low speeds. Resolution can be increased by driving the motor in micro-steps, resulting in smoother movement and more precise positioning. These characteristics enable engineers to reduce the size of the gearbox or even eliminate it entirely, which can represent a considerable gain in the end system’s reliability and compactness. Additionally, hybrid steppers are based on brushless technology. Parts do not exhibit wear, resulting in a longer service life than alternative technologies. Because of these features, hybrid stepper motors are widely used in medical automation applications requiring less than 50-W mechanical power.

 

Hybrid steppers need to be electronically commutated. Traditionally, these motors have been driven in open-loop mode, which results in a size handicap: this type of motor typically is “supersized” to ensure correct operation with no step loss or stalling. Even though this is a cost-effective solution that is widely used and has been proven in many applications, it still carries an inherent risk of malfunction. The use of a physically larger motor does not guarantee that the machine will not occasionally block for unknown reasons. When a motor runs in open loop, this malfunction will not be detected and could cause severe damage to the load or the machine itself. For this reason, most hybrid stepper motors used in medical or laboratory devices are linked to an encoder, which enables permanent and precise angle measurements.

 

The encoder is used by the motor driver or machine control unit to check the actual operation of the motor. Depending on the sensitivity of the application, different types of encoders may be specified. For critical applications, the encoder resolution has to be quite high and accurate, especially when the motor operates in direct drive with no reduction factor. Optical encoders are often back-mounted on the motors for these types of applications. Standard motors are also available with a double shaft to permit different types of assembly. It should be noted that expertise and attention to detail are required when performing the final assembly of such a delicate component.

 

Encoders typically are quite expensive, difficult to assemble and tend to be unprotected. Moreover, they require open space along the length of the drive axis. To minimise these technical drawbacks, a hybrid stepper motor with integrated encoder has been developed. The encoder—a rotary sensor—can replace many optical encoders in most automation devices advantageously whilst helping customers to reduce costs. Its technical characteristics can fulfill many requirements of the medtech industry. This newly developed system offers high resolution, 5920 counts per revolution—1480 pulses × 2 channels—as well as an index signal mechanically referenced to the motor shaft. The system not only provides fine resolution accuracy but also exhibits less than 0.3° linearity and costs less than a comparable optical solution. Highlights include:

 

Ruggedised engineering—vibration and wide temperature ranges will not affect operation.

Wear-free operation that guarantees a long service life.

 

Encoders are directly mounted on the motor and fully tested on the production line. The encoder is encapsulated in a metal and plastic housing that protects sensitive elements.

 

Component integration that saves space—the actual design is suitable for NEMA 23 and 34 motors but is small enough to be integrated in a NEMA 17 upon customer request.

 

This electromagnetic system also can run in a true closed loop using the latest generation of drivers from Sonceboz, resulting in a fully integrated motion system. The LoadSense driver estimates the payload, i.e. the torque required by the motor in real time. Driver parameters adapt instantly, thus ensuring that the hybrid stepper is free of any step loss. If required for security reasons, the encoder signals still can be output independently.

 

When motion is automated, the motor controls its acceleration ramps in relation to the actual forces in play—and, thus, for every single movement—in real time. This technology does not require a safety margin: instantly knowing the payload that is being applied to the motor makes it possible to achieve optimal performance. Under these circumstances, the maximum speed depends on the real forces affecting the motor, and not simply on a theoretical payload. The system enables permanent controls in real time and produces acceleration without an increase in inertia. In most cases, the technology can be a key driver in increasing productivity of an overall system. Gains of nearly 30% have been observed. The closed-loop technology is currently available for NEMA 23 and 34 motors but is adaptable for NEMA 17 sizes upon request.

 

Hybrid stepper motors with integrated encoders come into their own where torque at low speed, positioning and accuracy are determining factors. Their intrinsic electromagnetic attributes make them suitable for a range of medical and laboratory automation applications.

 

is Key Account Manager and
Yvan Meyer

is Business Unit Manager at Sonceboz SA, 5, Rue Rosselet Challandes, CH-2605 Sonceboz, Switzerland

tel. +41 32 488 1317

* to whom all correspondence should be addressed