Precision motion control for laboratory equipment

Stepper motor systems have become a popular method of achieving controllable motion due to their unique feature of the output shaft rotating in a discrete number of steps.

This digital step feature makes them ideally suited for simple open-loop position control.

When combined with a suitable controller, a stepper-motor system can be tailored to meet the requirements of a wide variety of applications, including X, Y and Z positioning and rotary indexing tables.

The accuracy that can be achieved makes steppers particularly attractive for scientific and laboratory applications.

Among the benefits offered by stepper systems are their simplicity: they provide a simple, cost-effective open-loop technique for positioning with ease of configuration and no need for tuning.

The fact that the motors are brushless means that maintenance requirements are minimal, and the units offer robust design and construction.

A high torque can be produced by a compact motor, and the system is stable when stopped, with no 'hunting'.

Motor response is also fast, with no settling time.

Finally, the ability to implement microstepping leads to exceptionally smooth motion control.

The modern hybrid stepper motor and its driving electronics have been developed and refined to provide cost-effective combinations for precise positioning applications at modest power and speed. Stepper systems owe much of their success to the wide variety of applications addressed, typically in the power range from tens of watts in scientific equipment up to 500-600 W of shaft power in industrial applications.

Unlike DC or servo motors, stepper motors are purely synchronous devices.

They rely on receiving pulse after pulse to step the rotor through discrete angles.

Any torque developed by a stepper is as a result of the load applied; with no load there is no torque, but still precise speed.

A DC or servo motor, on the other hand, is a torque device: its speed reduces when a load is applied, and if overloaded it will stop, restarting when the load is reduced.

If a stepper system is overloaded, it will not slow down; instead, it will keep in synchronisation with the applied step pulse until such time that the load is too great - at which time it will stall and not recover.

It is therefore essential to select the correct motor and drive to ensure that the combination is capable of producing significantly greater torque than the maximum expected load.

If a step pulse frequency is switched straight into a stepper drive, the motor is expected to produce near-instant acceleration of the total inertia; not surprisingly, with an excess load, it cannot achieve this at anything other than low speeds, and will consequently stall.

In order to construct a robust stepper system that meets expectations, attention must be given to the individual components and to the overall system integration.

A stepper system must successfully integrate four essential components: controller, drive, motor and power supply.

Stepper controllers.

The controller is the unit responsible for producing the initial step frequency (square wave) which determines the exact motion of the motor.

It is vital that the step pulse stream presented to the drive is uniform, without jitter and with carefully controlled acceleration and deceleration profiles so that the motor is not subject to high acceleration demands which will otherwise cause it to desynchronise.

Controllers vary in sophistication from simple transistor switching to sophisticated microprocessor driven packages, but they all offer the user a front-end interface whereby the motion profile of the required move can be created.

Drive unit.

The drive unit takes step and direction signals provided by the controller and sequences current through the stepper- motor windings using chopped power MOSFETS.

It is therefore important that the power output of the drive and motor specification are compatible.

The drive is also responsible for setting the step angle.

Full-stepping drives lead to 200 steps per motor shaft revolution (step angle 1.8deg). Half-stepping results in 400 steps (step angle 0.9deg).

At these resolutions some shaft noise will occur, particularly at low speeds (typically below 100rpm) where resonance can set up in the motor.

These are inherent features of stepper systems, but need not present significant problems.

For significantly reduced noise and better position resolution, microstepping drives are ideal. These drives can increase the number of steps per revolution from 400 to many thousands by smoothing out the torque pulses into a pseudo sine wave.

When the sine wave is applied to a suitable motor, it can give very smooth resonance-free motion and greater positional resolution.

Using DSP (digital signal processing) technology, it is now possible to combine the traditionally separate controller and drive stages, so that the power devices can be directly controlled by the DSP and mathematical modelling can be used to control the winding current accurately.

This advanced technique means that low speed motor noise can be eliminated, and ultra smooth motion is possible with over three million steps per revolution.

Stepper motors.

Of the wide variety of stepper motors available today, the most common are of two-phase hybrid construction (sometimes also referred to as four-phase), which offer higher working torque than permanent-magnet types.

These motors are best suited to shaft speeds in the range 0-1200rpm.

One fundamental property of stepper motors is that the torque falls as the shaft speed increases.

The exact profile is directly related to the chopper drive supply voltage and the motor windings, and therefore stepper systems are best applied to lower-speed applications where positional accuracy is important.

Closed-loop systems.

For applications where positional feedback is required, encoders can be added.

Software can then be used to compare issued stepper pulses with encoder pulses received, and a 'following error' value can be set.

If this error value is exceeded, the controller can flag the event and embark on a pre-programmed course of action.

The addition of an encoder input will allow the system to follow the motion of another axis driven by a completely independent device (such as an invertor or servo).

The software can also allow the stepper axis to be driven as a mathematical ratio of the encoder axis, thereby creating a 'software gearbox'.

Where co-ordinated motion on multiple axes is needed for moves such as circles or complex profiles, a single controllers are available with multi-axis interpolation capabilities.

Application example.

A good illustration of the application of these principles is provided by an integrated control system for the Wasp spiral plater developed by Don Whitley Scientific.

The Wasp unit is used extensively in the food industry to test for safe levels of bacteria, as well as in the pharmaceutical, cosmetics and water industries.

The spiral plater method eliminates the requirement for serial dilutions, saving time, labour and laboratory materials.

To achieve these savings while upholding strict quality regulations, precise control of all motion axes is required, including axis interpolation.

In the spiral plater system, a stylus arm with a stepper-driven syringe is used to dispense liquid samples in an Archimedes spiral, either uniformly or as a continuously decreasing volume across a stepper-driven revolving plate.

Pre-programmed options allow liquid samples to be dispensed in a variety of ways, and self-clean motion can be initiated at the touch of a button.

For each Wasp plater, SmartDrive produces a unique 'black box' subsystem, the design of which was a joint effort with the customer's machine development engineers so as to provide the optimum integration and cost-effective production.

A simple folded steel housing contains all the electronic parts, including a toroidal transformer and EMC filtered IEC mains inlet, a four-axis motion controller card and a three-axis microstepping drive card.

The cards are plugged into a backplane incorporating power-supply components with a driver for a small DC servo and interfaces for the several sensors.

A specially developed RS232 serial connected membrane keypad interface circuit board, which also carries LEDs to illuminate program-selected key positions through the membrane, provides the operator interface to control the machine. All interconnections to devices in the machine are made by plug and socket to give rapid first-level servicing.

Within the housing, the electronics is modular for easy second-line servicing.

With all the electronics being built and tested by SmartDrive as a fully integrated subsystem with hardware and software support, Don Whitley Scientific has been able to concentrate on their prime expertise in the areas of laboratory equipment design and microbiology.

Conclusion.

Modern stepper motor drive systems are making use of the latest advances in electronic and electromechanical technology to offer unprecedented levels of precision and controllability.

Systems are now available to cater for a wide variety of application requirements, from simple single-axis units to complex interpolated multi-axis systems.

Industries ranging from precision laboratory instrument manufacture to large-scale production machinery are all benefiting from the advances in stepper drive technology.