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Simple tips improve servo operation

Simple tips improve servo operation

Many things are not always what they seem. Servos are one of those. Here are some ideas for servo systems that may help improve productivity, and delay the arrival of a gray hair or two.

Servoamplifiers control component pickup and placement in Mega-1 robot. Linear motors control X and Y positioning with 0.0005-in. repeatibility throughout 52-in. by 37-in. workcell; provide 1.7-G acceleration and 100-ips speed.

Overlooking simple details during servo installation or specification may degrade performance instead of improving it. For example, a ground may not always be a ground. And a poor ground connection often leads to erratic system operation or even servo self destruction. In addition, neglecting to consider application requirements when specifying a power supply can produce sluggish machine acceleration, reducing productivity.

There are many other examples; and to help you get all the performance you deserve, here are some of the most troublesome situations and how to prevent the unexpected.

Motion controller-to-servoamplifier connections

Frequently, several feet separate the motion controller, which produces the motion commands, from the servoamplifier. Looking insignificant on a typical drawing, Figure 1, such separation often produces a different ground voltage at the motion controller from that at the amplifier. Without proper protection, this voltage difference — called “common mode voltage” (Vcm) — interferes with the analog signal fed by the controller to the amplifier. This, in turn, produces erratic servo operation.

It is nearly impossible to eliminate Vcm, but limiting its magnitude to an acceptable level is doable. First, both the motion controller and the servoamplifier should be grounded together with a good conductor. A machine frame — even if it is a big hunk of cast iron — isn’t a good conductor. Paint and corrosion at junction points preclude good conduction, to say nothing of the resistance of cast iron compared to copper. A better system uses a shielded cable grounded at both ends to the chassis of each unit, Figure 2.

One other aspect is the type of input in the servoamplifier. Some have single-ended inputs as shown in Figure 1. Others offer differential inputs where the unit operation depends on the voltage between two input terminals, Figure 2, rather than on the voltage between one terminal and chassis ground, which is highly susceptible to any Vcm.

However, even a servo with a differential input needs sufficient common mode rejection (CMR), because there will usually be some Vcm. It may range from a few millivolts to tens of volts. Therefore, designers add circuits within electronic units to attenuate the effects of Vcm. A unit’s ability to reject Vcm is its CMR, usually expressed in dB. Generally, for systems with proper grounding, CMRs of 60 dB or more is sufficient. Power supply connections The dc supply rectifies ac to produce constant-voltage dc. A PWM servoamplifier chops (modulates) this constant voltage to deliver adjustable voltage to the servomotor. However, the chopping rate (also called switching frequency) for modern servos usually lies between 15 and 100 kHz. These fast chopping frequencies produce fast rates of current rise (di/dt).

In turn, these fast current rates combined with the inductance of leads longer than about 16 in. may produce enough of an inductive kick (voltage spike) to create amplifier operating problems. To solve these problems, add a 500 mF decoupling capacitor at the amplifier terminals, Figure 3. This capacitor should be connected as close as possible to the servoamplifier power semiconductors, so connect the capacitor across the amplifier terminals, not across the power supply terminals.

Moreover, it is imperative to follow the servo manufacturer’s instructions on grounding practices.

Multiaxis systems

To minimize one amplifier’s interference with another, run separate power cables to each amplifier, Figure 4a. Reason: Servoamplifiers draw appreciable accelerating currents — typically 20 to 50 A. Plus, these currents are a series of fast rising pulses (previously discussed) that may create mutual interference between amplifiers (crosstalk). This crosstalk, in turn, often impairs system performance.

Therefore, daisy-chaining the amplifiers to a common dc bus, Figure 4b, exposes downstream amplifiers to the accumulated power line-voltage disturbances created by amplifiers upstream in the chain.

Machine grounds

To save some wire, it may be tempting to use a machine frame as the return current path for dc servomotors, especially small ones. However, a machine frame has substantial ohmic resistance, especially where paint or other coatings add to the resistance at the points of connection. This high-resistance return increases crosstalk difficulties between amplifiers, which causes erratic machine operation. And this practice may produce some safety questions.

Moreover, high currents flowing through a mechanical structure, may produce a longer-term and less obvious problem — galvanic action. This can seriously degrade mechanical joints.

The answer, of course, is to install a separate –Vdc return wire from each motor to its amplifier.

Power supply droop

By their very nature, dc power supplies are unregulated, so the output changes with the input ac voltage and the instantaneous load. Therefore, it is imperative to select a power supply that can supply full rated voltage to the amplifier when the ac voltage to the power supply is 15% below nominal rating (a common industry standard for allowable low voltage) and the servo is delivering rated peak torque at full speed. Typically, a servoamplifier’s peak current is twice its maximum continuous output. Granted, this is a worstcase situation, but it is usually within the component ratings.

A good rule-of-thumb is to go one step further and specify a power supply capable of delivering 10% more than rated voltage under these worst-case

conditions. Many engineers have learned from experience that this 10% “headroom” is a wise investment.

The duty-cycle syndrome

Of all the “gotchas” engineers encounter in the servo-selection process, none is more frequent than the duty-cycle syndrome. It is commonplace to underestimate the worst-case speed-torque values in applications with wide load variations.

For example, a printed-circuit-board (PCB) drilling system operates with many rapid, high-current accelerating and braking periods. Operation at what might be considered “normal” running current (which, in this example, is really no-load current) occurs only when the drilling head is withdrawing while inserting a new PCB in the fixture.

Because power semiconductors have much shorter thermal time constants than do motors, such applications require selecting servo power supplies and amplifiers with continuous ratings near or equal to the peak rating of that application. Without this seemingly conservative selection, the solid-state units are susceptible to early failure.

Coil winders, centrifuges, and similar high-inertia loads are slow to attain full speed, hence they draw high accelerating currents for long periods. Here again, selection of the servo must be guided by the rigorous requirements of the unusual duty cycle — where accelerating torques establish the continuous rating.

Regenerative braking

The method for stopping loads powered by servo systems is often as important as the technique used for motoring. Because a servomotor is an obedient servant, it can operate as a motor to accelerate and run a load, and it can turn into a generator during decelerating, letting the load supply the energy. It is then the job of the servosystem to somehow dissipate the energy from the motor-turned-generator.

Battery powered systems — golf carts, fork-lift trucks, and automatic guided vehicles — frequently return the energy of the vehicle’s motion to recharge the vehicle’s battery. This method extends equipment operating life between charges.

For servos operating from conventional ac plant power, here are the most frequently used methods for absorbing or dissipating the energy:

• Return the energy to a capacitor or resistor connected across the dc bus between the power supply and the amplifier, Figure 5. In industrial applications, this is the most common. The complexity and costs depend on the specific application. But you must be aware that regenerative energy can prove to be hazardous. If the amount of returned energy is more than the capacitor or resistor can handle, the system can turn into a smoke generator.

There’s an even more perilous consequence: excessive kinetic energy can overcharge the capacitor and activate the servoamplifier’s over-voltage shutdown circuit. The motor and load are then no longer under servoamplifier control, but are free to coast until friction absorbs the kinetic energy. Or more likely, until the coasting mechanism slams into mechanical stops, possibly damaging equipment and causing injury. Therefore, it is imperative to provide adequate capacity for the capacitor or resistor.

A trick-of-the-trade obviates the need for sophisticated circuitry. Drive the mechanical load with a low-voltage servomotor. Power the motor from a low-voltage dc supply, and control the motor with a high-voltage servoamplifier. Also, make sure the reservoir capacitor and rectifier diodes have the same high-voltage rating as the servoamplifier.

For example, operate a 50-V servomotor from the normal 55 to 60-Vdc supply, but use a servoamplifier and capacitor rated for, say 180V.

• Using a special regenerative power supply — a costly unit — return the energy to the ac plant power system. For applications with continuous or frequent decelerating requirements — unwind stands and centrifuges, for example — this method may be justified.

Barry Friedman is vice president at Copley Controls Corp., Westwood, Mass.

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