TechArchive

Original article date: October 1997

A new family of microstepping drives has just been launched by Parker Hannifin’s Electromechanical Division. Known as the Zeta Series, the drives incorporate two techniques for which patents are pending. They are “active damping” and “electronic viscosity”. The combination of active damping and electronic viscosity gives much tighter control of step motor response and a significant improvement in overall performance.

The drives provide significant performance gains over conventional stepper systems, including faster acceleration, increased shaft power and reduced settling time – all without the cost of any additional components, such as mechanical dampers.

The Zeta drives offer a choice of two configurations: a drive-only unit known as Zeta4, and an indexer/drive package known as Zeta6104. Both models offer torque outputs from 0.5 to 3.4Nm, with user-selectable motor resolution from 200 to 50 800steps/rev, and feature active damping and electronic viscosity.

Step motor systems often require some form of damping to minimise oscillation at their resonant frequency, to prevent stalling. Until now, this was usually accomplished with a mechanical damper – but these are expensive, need to be matched to the load, and reduce acceleration rates by adding inertia. Active damping overcomes all these problems; offering fully adjustable damping ratios as high as 0.5, the technique enables much higher acceleration and torque figures to be realised for a given size of motor, reduces shaft vibration, and significantly shortens settling time. The effect is particularly dramatic following a speed change – compared to an undamped step motor, which can over a second to settle at the new speed, a Zeta drive will typically take just 20ms. On the Zeta6104, the damping ratio is programmable, so that it can even be altered during the machine cycle, if required.

At shaft speeds below 3rps, the Zeta drive’s electronic viscosity feature takes over from active damping, to reduce ringing of the motor at the end of a move. This results in faster settling times – facilitating higher system throughput – and also significantly reduces velocity ripple, giving very smooth performance at low speeds.

Both Zeta products employ the same drive electronics, making extensive use of ASICs and FPGAs (Field Programmable Gate Arrays). The power output stage is a high efficiency pulse width modulated design, with a peak output of 4A. Operating from any 50/60Hz AC supply in the range 90-130V, the drives are protected against phase-to-phase and phase-to-ground short-circuits, as well as under-voltage and over-temperature conditions; any fault will cause the drive to automatically shut down.

The Zeta4 drive accepts step and direction signals from any standard indexer, and is ideal for multi-axis applications. The Zeta6104 indexer/drive package is intended primarily for standalone use in single-axis systems. Fitted with 40K bytes of non-volatile memory as standard, the indexer can operate autonomously or under PC or PLC control – a configurable serial port handles RS232C, RS422 or RS485 communications. All I/O functions are optically isolated, and include 16 programmable inputs, nine programmable outputs, and two fast trigger inputs for registration or position capture signals. The package is supplied complete with Parker’s Motion Architect software, an easy-to-use development package for motion control programs, which runs under Microsoft Windows.

• Sharon Beale
• Parker Hannifin Electromechanical Division – Digiplan
• Tel: 01202 699000

Controlling step motor response

Step motors often require some form of damping to minimise the likelihood of stalling caused by oscillation at the resonant frequency. The higher the degree of damping, the quicker the oscillation will decay. A well-damped step motor system will be able to achieve the highest overall performance.

Previously, the usual way to increase the damping of a step motor system was by mechanical means. Mechanical dampers are mounted on the back of the motor and come in a variety of types. The most common and effective type of damper consists of a seismic mass suspended in a viscous fluid.

However, mechanical dampers do not always provide a perfect solution. They need to be sized according to the load. If the load changes or mechanical wear occurs, the damper is no longer as effective. Furthermore, mechanical dampers can add significant inertia to the system, reducing the acceleration rate that can be attained.

The Zeta series drives provide electronic damping with no additional devices to connect. The damping effect is considerable, so it can change if the application changes. In the case of the Zeta6104, the damping is programmable by software, so it can be altered during the machine cycle if required.

Zeta’s active damping (patent pending) offers the following benefits:

• The likelihood of stalling is minimised without the additional expense and inertia of a damper.
• Usable torque is increased
• Higher acceleration rates can be attained

Mechanical dampers can be expensive; a good one may cost considerably more than the motor itself. The Zeta series provides adjustable electronic damping at no additional cost.

In conventional step motor systems, the speed-torque curve represents the maximum measurable torque rather than usable torque. A safety margin is always necessary to be able to control rotor oscillation, as well as to allow for changing load and friction conditions. As a result of active damping, Zeta systems require a smaller safety margin, resulting in higher usable torque at all speeds.

Ringing can result when an undamped motor is commanded to change velocity from 4rps to 7rps. The motor is driving a load inertia equal to six times the rotor inertia. In this undamped system, it takes almost two seconds for the motor to settle at the new speed. Using the Zeta drive, the settling time is reduced to 20ms. Actual ringing and settling times are application-specific and will depend on the move parameters, as well as the inertia of the load.

With conventional stepper system, the motor shaft oscillates around its commanded final position before settling at the end of a move. In many applications, this settling time represents wasted time, because the next operation must be delayed until the motor has settled.

October 1997