Motion-Control Systems: Precision, Power and Performance: The Multispeed Accuracy Advantage

The Multispeed Accuracy Advantage

Because synchros can rotate continuously, they can be used in multispeed arrangements where full-scale system travel may be represented by 36 or 64 full rotations.

When reduced by gearing to a single, full-scale turn, the synchro's electrical inaccuracy becomes the typical 0.1° error divided by the gear ratio (36 or 64 or whatever ratio is used). This error is insignificant compared with the error of the gearing coupling the high-speed synchro and the single-speed output shaft.

The accuracy is dependable and stable using standard synchros and gearing — which is why synchros remain in service across military, aerospace, and industrial applications decades after their introduction.

Hydraulic and Pneumatic Systems: When Electrical Power Isn't Enough

Now the story shifts from the practitioner's transducer failure to a different kind of challenge — the one faced by the practitioner, a hydraulic systems engineer at a heavy equipment manufacturer, who had to choose between electromechanical and hydraulic control for a new 50-ton press brake.

The choice wasn't obvious. Here's why.

The Fundamental Hydraulic Principle

Hydraulic actuators that drive loads may be cylinders (for linear motion) or motors (for rotary motion). The load must be defined by its torque–speed characteristics and inertia, and a suitable hydraulic actuator selected before the remaining system components can be chosen.

The basic force-area relationship governs all hydraulic systems:

$$F = P \times A$$

$$A = 0.7854 \times d^2$$

Where:

• $F$ = force in lb
• $P$ = pressure in lbf/in²
• $A$ = piston area in in²
• $d$ = piston diameter in inches

Hydraulic Cylinder Operation

Both single- and double-acting cylinders are available. Pressure can be traded off against velocity by placing a different effective area at each side of the piston — the same pressure on a smaller area will move the piston at a higher speed but lower force for a given rate of fluid delivery.

Hydraulic Motor Calculations

The rotary equivalent of the cylinder is the hydraulic motor, defined by:

• Fluid displacement required to turn the output shaft through one revolution
• Output torque (proportional to fluid pressure)
• Output speed (defined by gallons per minute supplied)

Example calculation:

If 231 cu. in. = 1 gallon, an input of 6 gpm with a 5 cu. in. displacement gives a mean speed of:

$$\text{Speed} = \frac{6 \times 231}{5} = 277 \text{ rpm}$$

Power Formulas for Hydraulic Systems

Note: Theoretical hp output must be multiplied by the efficiency of the hydraulic circuits to determine actual output.

Why the practitioner Chose Hydraulic

Hydraulic pressures of up to 3,000 lbf/in² are readily obtained from hydraulic pumps. Cylinders can exert forces of hundreds of tons without speed-reducing transmission systems to increase the force. For the practitioner's 50-ton press brake, hydraulic was the clear choice — the hydraulic fluid also distributes heat, helping cool the system.

Hydraulic Pumps: The Pressure Source

The most-used hydraulic pump is the positive-displacement type, which delivers a fixed amount of fluid for every cycle. These are also called hydrostatic pumps because they deliver energy by static pressure rather than by the kinetic energy of a moving fluid.

Key Pump Ratings

Delivered GPM are reduced under load due to leakage — and this reduction is described by the volumetric efficiency. Always design for the actual output, not the theoretical rating.

Hydraulic Fluids: The Lifeblood of the System

The hydraulic fluid is the basic means of transmitting power, and it also provides:

• Lubrication (with special additives to minimize wear)
• Cooling (when passed through a heat exchanger)
• Corrosion prevention and pitting resistance

Compressibility and Resonance

The fluid must be minimally compressible to avoid springiness and delay in response. The total system inertia reacts with fluid compliance to generate a resonant frequency — much as inertia and mechanical compliance react in an electromechanical system.

Compliance must be low enough that:

• Resonances do not occur in the active bandwidth of the servomechanism
• Unacceptable transients do not occur under shock loads

Seal friction and fluid viscosity tend to damp out resonant vibrations. Shock-absorbing limit stops or cushions are usually located at travel limits.

Viscosity: The Critical Balancing Act

Viscosity falls very rapidly with increasing temperature — a critical design consideration. For hydraulic actuators operating at very low temperatures, the fluid pour point is important; below this temperature, the fluid will not flow.

Fire-resistant fluids are available for conditions such as die casting, where furnaces containing molten metal are often located near hydraulic systems.

The Contamination Problem

A problem with hydraulic systems that is absent in electromechanical systems is dirt, air bubbles, and contaminants in the fluid.

Contamination management requirements:

• Enclosed systems designed to keep out contaminants
• Suitable sealer in the reservoir to prevent corrosion
• Filter used during filling
• Coarse and fine filters to remove contaminants (rated to remove micron-sized particles; 1 micron = 0.00004 in.)
• Very fine filters in high-pressure lines where dirt might interfere with sensitive valves
• Only coarse filters on fluid inlet lines (fine filters might introduce excessive pressure drop)

Line Connections and Compliance

Expansion of line connections, fittings, and couplings under pressure increases the mechanical compliance of the system, reducing the frequencies of resonances and potentially interfering with the response of wide-band systems.

Hydraulic and Pneumatic Control Systems: The Overhead Comparison

the practitioner had to justify her hydraulic choice to management. The comparison with electromechanical systems was nuanced:

Overhead Equipment Comparison

Frequently, the optimum system is selected on the basis of overhead equipment already available.

Performance Comparison

The Reliability Shift

Although much depends on the specific design, the edge in reliability — even for high-power, fast-response needs — is shifting toward electromechanical systems. There are simply more things that can go wrong in hydraulic/pneumatic systems, as indicated by the shift to more electrical systems in aircraft.

Electromechanical capabilities have been sharply extended through:

• Rare-earth motor magnets with much higher energies than earlier designs
• Semiconductor power components delivering much higher output power at lower prices

Electromechanical control systems are now suited to applications of more than 100 hp with bandwidths up to 40 Hz and sometimes up to 100 Hz.

Open-Loop and Closed-Loop Hydraulic Operation

Discontinuous (Bang-Bang) Operation

Discontinuous operation — sometimes called bang-bang or on-off — works well, is widely used in low- to medium-accuracy systems, and is easy to maintain.

Limitations:

• Accuracy is limited in closed-loop mode
• If the response to error is set too high, the system will oscillate between on-off modes
• The oscillation is noisy, introduces system transients, and may cause rapid wear
• Sudden opening and closing of high-pressure valves introduces transient pulses in fluid flow causing high stresses in components

These problems are addressed by pressure-limiting relief valves and other protective units.

Proportional Control Systems

Where the highest accuracy is required — perhaps in two directions, with aiding or opposing forces or torques — a more sophisticated proportional control closed-loop system is preferred.

In proportional hydraulic control, the amplifier and electric servomotor of electromechanical systems is replaced by an electronically controlled servo-valve.

How the servo-valve works:

• A linear motor positions the spool that determines the flow path for the hydraulic fluid
• In some designs, the linear motor is driven by a solenoid against a bias spring
• In other arrangements, a bidirectional motor permits flow depending on polarity and amplitude of the voltage supplied

Such designs achieve smooth operation and minimum nonlinearities, delivering the maximum accuracy required by the best machine tool applications.

Two-Stage Valves for High Power

Where very high power must be controlled, a two-stage valve is used — the output from the first stage drives the second-stage valve. This allows large-power control from a low-power input.

  Low-Power    ┌─────────────┐    High-Power    ┌──────────────┐
  Input ──────►│ First-Stage  ├──────────────►│ Second-Stage  ├──► To Load
  (Solenoid/   │   Valve      │                │    Valve      │
   Motor)      └─────────────┘                └──────────────┘
                                                      ▲
                                              High Pressure
                                              From Pump

Electronic Controls: Analog vs. Digital — The Defining Decision

An error-sensing electronic amplifier drives the solenoid motor in proportional systems, providing automatic output correction in the closed loop. The input is an ideal place to introduce electrical control features, adding greatly to system versatility.

The electronic amplifier provides driving power using pulse-width modulation as required for minimum heating. The output can respond to signals in the low-microvolt range.

Analog vs. Digital Control

A major decision in any motion control system is whether to use analog or digital control. Here's the comparison:

Why Digital Wins for Modern Applications

Because nonlinearities and small incremental motions are easy to implement digitally, digital systems are capable of very smooth acceleration — avoiding damaging shocks and induced leaks, enhancing reliability so that seals and hose connections last longer.

The accuracy of digital control systems depends on transducer availability — and a full range of suitable devices is now available.

Despite continuing improvements in analog systems, digital control of hydraulic systems is favored in large plants.

Pneumatic Systems: When Compressibility Is Acceptable

Hydraulic systems transmit power through essentially incompressible fluid. Pneumatic systems use highly compressible gas. This single distinction drives every performance difference between the two.

Performance Characteristics

Advantages of Pneumatic Systems

Despite their performance limitations, pneumatic systems offer numerous desirable features:

• Reduced fire hazards compared with most preferred hydraulic fluids
• Air can be vented to atmosphere — only a flow line needed (reducing system complexity, cost, and weight)
• Lighter lines, couplings, and fittings than hydraulic counterparts
• Lighter fluid medium than hydraulic fluid
• Easier to clean, assemble, and maintain
• Fluid viscosity and temperature variations are virtually negligible

Challenges Unique to Pneumatics

• Lubrication must be carefully designed in
• More power required to achieve a desired pressure with compressible gas
• Explosion risk if storage tank is damaged under high pressure — storage must have substantial safety margins
• Low stiffness results in resonances between compressible gas and system inertias at lower frequencies
• Speed-of-sound delays in connecting lines contribute to response delay and closed-loop stabilization difficulty

Pneumatic Control Architecture

In contrast with hydraulic systems (where speed may be controlled by varying pump output), pneumatic system control is almost exclusively by valves that control flow from a pneumatic accumulator or pressure source.

• Pressure is maintained between limits by an intermittently operated pump
• Low-pressure outlet ports must be large enough to accommodate the high volume of the expanded gas
• Pneumatic analogs to electrical networks can be constructed to simplify stabilization at the exact point of delays
• Such pneumatic stabilizing means are commercially available and are essential elements of closed-loop pneumatic control systems

💡 Visual Strategy: A simplified schematic for closed-loop position control applied to an air cylinder, showing the position sensor, valve, cylinder, and feedback path, would be ideal here.

Electromechanical vs. Hydraulic vs. Pneumatic: The Master Comparison

Here's the decision matrix the practitioner built — and the one you should build for every motion control project:

Improvement method and result

Six months after the 340-bracket disaster, the practitioner's motion control system at Harmon Precision was unrecognizable. Here's what changed:

1. Feedback transducers were upgraded to brushless synchros in a multispeed arrangement, reducing angular error by a factor of 64
2. Torsional vibration was eliminated with notch filters tuned to the specific mechanical resonance frequencies — verified by frequency-response testing
3. Temperature compensation was added to every analog transducer, with computer-based lookup tables for recalibration
4. Digital controllers replaced the analog units, enabling backlash compensation, self-calibration, and automatic record-keeping
5. Mechanical stiffness was increased by replacing bolted couplings with interference-fit connections and preloading all bearings

The result: zero scrap attributable to motion control errors in the following 18 months of production. The system positioned within 0.0002 inches under full dynamic load — every cycle, every shift, every season.

The investment paid for itself in eleven weeks.

Your Next Step: The Motion Control Audit

Whether you're designing a new system or maintaining an existing one, the principles in this guide give you a complete framework for every decision. But knowledge without action is just theory.

Here's what you do now:

1. Pull the specification sheet for every feedback transducer in your most critical motion control system
2. Verify that the transducer accuracy exceeds the required system accuracy by a comfortable margin
3. Check the torsional resonant frequency using $f_n = \frac{1}{2\pi}\sqrt{\frac{K}{J_L}}$ and confirm it falls above the system bandwidth
4. Review the damping method in use — is it still optimal for the current load conditions?
5. Assess whether digital control could replace your analog systems for improved self-calibration and backlash compensation

The most expensive motion control failures don't announce themselves. They drift, slowly, quietly — just like the practitioner's transducer — until the reject bin tells you what the sensor wouldn't.

Don't wait for 340 scrapped brackets to find out.

What is the single biggest motion control challenge you're facing in your facility right now — and which section of this guide addresses it? That's where your audit begins.