Closed-loop motion systems can only run as fast as their sensors let them. And there are many factors that go into sensor speed. We asked the experts to explain what makes sensors fast, and slow, and how sensors can help or hurt motion system response. Here are the answers that will speed you on your way to a faster, more productive machine.

How do you define speed in a motion system?

Tom/Heidenhain: Speed has to do with reaching a predetermined point efficiently and safely. It is defined by the application and subject to various design criteria. The encoder's role in this formula is extremely important, determining drive and system performance as well as safety.

Bob/Optek: We define speed as the ability to detect the distance an object travels, rotationally or linearly, in a given time. It also implies the ability to precisely control motor speed by monitoring the motor shaft or object being moved.

Howard/Renishaw:: Speed, simply put, is distance over time. It is specified in meters per second (m/sec) for linear motion and revolutions per minute (rpm) for rotary.

Kees/Allied Motion: There are several facets to speed: absolute system speed, speed in obtaining information from a feedback device, and the feedback rate itself. Absolute speed can vary from a few microradians per second, in an optical spectrum analyzer, for example, to tens of thousands of rpm.

In general, to get good dynamic response from a motion system, the sample time should be between 50 and 600 µsec, and the delay between capture and availability should not exceed 25% of the sample time. For minimal dead time, the position and speed signals should be available to the control loop continuously and within a few microseconds.

The high speed end is usually the easy part. A system barreling along at 15,000 rpm with some inertia will be very stable even with a limited amount of information per revolution available; as from the Hall switches in a brushless dc motor based system. Inertia at that speed forgives just about everything in the way of perturbations in the motor leads, coil switching hiccups, and so on. Also, the feedback rate is high, as much as 4.5 kHz in a 6-pole BLDC system.

Things get more interesting at very low speeds. The same inertia that worked so well at 15,000 rpm is now practically useless. The easy way out is to add inertia by installing an inertia wheel or flywheel. In some cases, however, this doesn't work. In a machine tool, for example, a flywheel can totally ruin system response time. Imagine trying to accelerate to 15,000 rpm rapidly with an inertia wheel hanging on the load.

Brian/Avtron: For digital, rotary incremental encoders, speed is defined by three parameters: shaft speed (rpm) output (pulses/rev or ppr) and maximum output frequency (Hz).

Martin/Gurley Precision: For a rotary encoder equipped motion system, speed is measured in revolutions per minute, revolutions per second, degrees per second, radians per second, and so on. For linear encoders, common units of speed measurement are inches, feet, millimeters, or meters per second. Of greatest concern to motion system designers, however, is the behavior of the system in three different speed regimes: very high speed, very low speed, and zero speed.

At very high speed, the rotating or translating components must be well balanced and typically of low mass, shaped for an acceptable moment of inertia, constructed of adequately strong materials, and securely fastened. It is often advantageous to use a noncompliant coupling between rotating encoder components and the motion axis. This may be accomplished by using a kit or modular style encoder, or by tethering a self-contained encoder's frame compliantly.

Electronic circuit performance also sometimes comes into play at very high speeds. Certain rotating applications, especially those employing air bearing spindles, maintain angular velocities well in excess of 10,000 rpm. High linear speeds for machine tools and robotics typically start at 2 m/sec for high resolution scales, though coarse tape-style linears for freight elevators and similar applications can go significantly faster.

At very low speeds, bearing and lubricant friction and stiction combined with shaft coupling windup can result in undesirable motion system behavior. In linear systems, similar considerations apply. These mechanical factors can become more acute when aggravated by low system drive torque. Smooth motion at very low speeds also depends on high encoding resolution and linearity, and may sometimes be assisted by control system filtering algorithms. Precise low speed motion in robotics, optical tracking systems, and other specialized applications often require rotary resolutions above 1,000,000 measuring steps per revolution and linear resolutions in the submicron range, with near zero velocities.