Automated lab equipment needs precise and dependable subsystems to meet demands for accuracy, speed, and lower cost. Direct drives for linear and torque motors are one way to deliver on these demands. In addition to higher productivity, direct-drive technology reduces wear and maintenance. The drives are becoming more important in dynamic equipment such as cancer treatment, imaging techniques, slide digitizing, and DNA-protein analysis.

However, a productivity increase is possible only when the control, motor, machine frame, and position encoder are adjusted or optimized to one another. Direct drives put rigorous demands on the quality of measurement signals from position encoders. A measuring signal can be called optimum when the machine it's on delivers these characteristics:

• Fewer vibrations in the machine frame. Smooth operation improves the equipment's overall performance and dependability.

• Precise constant motion. This is critical to medical applications, especially imaging.

• Low noise and velocity-dependent motor sounds. This produces quieter environments.

• Minimum heat generation. It reduces thermal growth in the machine for better overall accuracy.

• The motor reaches maximum mechanical power rating, a useful trait for larger or high-speed medical equipment.

Consequently, position encoders influence the efficiency of linear motors with regard to accuracy, speed stability, and heat.

Direct-drive designs

A direct drive provides a stiff coupling between drive and moving component. The drive eliminates mechanical-transfer elements such as speed reducers and shaft couplings. The direct connection allows a significantly higher gain in control loops, up to 40% more than in conventional drives.

The position encoder measures position and speed so there is no additional need for separate speed encoders. Linear encoders work on linear motors and angle encoders on torque motors often on rotating medical equipment. Without a mechanical transmission between the speed encoder and the moving unit (that might be a camera, x-ray source, or array of pipettes), the position encoder must have a correspondingly high resolution for exact velocity control at slow or constant speeds. Velocity is, of course, calculated from the distance traversed per unit of time. This calculation also applies to conventional axes such as a stepper motor driving a ball screw. The calculation represents a mathematical differentiation that also amplifies periodic disturbances or noise in the signal. Drive performance often drops when combining a significantly higher control-loop gain (particularly with direct drives) with a noisy encoder signal.

Signal quality of position encoders

Encoders measure position by either an incremental (non-unique count) or an absolute method in which all positions have a unique value. The units transform motion-path information into two sinusoidal signals with a 90° phase shift. Both methods require interpolating sinusoidal scanning signals for a sufficiently high resolution to control speed and position. A precise resolution is application specific but it's often 10 times the advertised positioning accuracy. So if a machine is said to have a position accuracy of 100 µm, measuring standard (encoder glass with the line counts or grating) should have a 10µm pitch.

Inadequate scanning, contamination on the measuring standard, or insufficient signal processing can lead to deviations from the ideal sinusoid. As a consequence, periodic position errors occur during interpolation within one signal period of the encoder's output. This type of error is referred to as interpolation error. It's typically 1 to 2% of the signal period on high-quality encoders. All encoders have some level of interpolation error.

When the frequency of the interpolation error increases, the drive no longer follows the error curve due to timing issues, and motion becomes minutely erratic. Furthermore, spikes in the electric current generated by the interpolation error increases motor noise making the motor run hotter.

Comparing the effects of linear encoders with low and high-interpolation error on a direct drive illustrates the significance of high-quality position signals. An 8µm-grating pitch on a linear encoder generates barely noticeable disturbances in the motor current. The motor operates normally and develops little heat. But if interpolation errors of the same encoder are increased by poor mounting, the same controller setting produces significant noise in the motor current and the motor runs hotter. Poor mounting is most anything that induces a roll, pitch, or yaw in the scanning unit, or takes the scanner too close or too far from the standard.

Digital filters used with direct drives can smooth position signals. However, the loss of phase association due to filtering in the speed-control loop must be kept to a minimum, otherwise dynamic accuracy decreases. Position encoders with the best signal quality can maintain the control bandwidth and thereby reduce the use of filters.

Linear encoders that generate a high-quality (low-noise) position signal with low interpolation errors are essential for the best performance of direct drives in medical and biochemical machines. Encoders that use photoelectric scanning are ideal for this task because they scan fine graduations.