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Encoder Motors

Encoder motors are DC motors with a feed-back loop. The feedback loop tracks the motor's position so that it can be closely controlled by a microprocessor. Encoder motors are widely used where mechanisms must change position continually and accurately despite a varying load or uncertain acceleration conditions. For instance, encoders are widely used in dot matrix printers, machine tools and robot arms.

DC Motors

A DC motor has several parts:

The "stator" is usually one or two powerful permanent magnets mounted in the motor shell, which is held static. Large motors have sometimes used DC actuator coils to achieve a strong enough field, but most recent motors use magents made with some rare-earth such as cobalt - samarium.

The motor shaft runs through the centre and carries the "armature" - a set of electomagnets with windings connecting to a commutator.

The commutator is a set of copper contacts arranged in a ring. The commutator turns with the armature in between two "brushes" arranged to bring electric current into the armature, creating a magnetic field in the armature winding.

Brushes could literally be wire-brushes or springs. In almost all ecent motors they are shaped from graphite (carbon) and spring loaded to press against the copper plates of the commutator. Graphite is "slippery" material, so the copper commutator turns relatively easily, although the load it imposes is often quite significant if you turn an armature by hand. Carbon in solid form is a good conductor. Carbon dust shed from the contacts is a bad conductor and tends to gassify to CO2 rather than build up as a contaminant.

The commutator, armature and static field are arranged so that the armature is magnetically attracted or repelled as it turns. As the commutator segments turn the contacts engage the brushes alternately, switching the current in the armature. The switching coils keep changing the magnetic field in the armature so that it keeps on rotating.

Uses

DC motors are commonly used as starter motors, windscreen wipers and window regulators and door locking mechanisms in cars. They also appear in all sorts of small appliances like cassette and video recorders and self winding cameras, in model cars and trains and all kinds of toys. DC motors with a winding rated at about 12 to 24 volts and a power of 10 to a few hundred watts are commonplace. Most of these motors are designed to spin at about 1,000 to 3,000 rpm driving a gear chain, and to be loosely controlled by the actions of a switch - very often they can be reversed by the switch reversing the polarity. DC motors with a winding rated at a thousand volts and power of many kilowatts drive trams and trains. The idea of the electric motor has been widely used.

The Encoder

Encoder motors have an additional part, the encoder itself. Most encoders are optical, but there are magnetic encoders as well - and DC servo loop control is related. Magnetic devices can prove more reliable than optical equivalents in dirty environments such as machine tools but may be less accurate and more expensive to implement.

Optical encoders are normally based on a slotted wheel mounted on the motor shaft, usually at the rear of the motor enclosure. The wheel is usually thin steel plate and the slots are square holes punched around the wheels circumference.

A pair of optical sensors is mounted at one point on the wheel's circumference. As the motor shaft and the wheel turn the slots break the light-beam and the current from the optical sensors turns on and off. Digital logic circuitry counts the number of pulses from the sensors and can therefore keep precise track of the motor shaft position.

A pair of sensors is normally used. As the wheel turns it will break one light-beam before the other, and by noting which sensor leads the counter can tell whether pulses represent the wheel turning in one direction or another.

There will normally be another sensor somehere in the mecahnical sub-system to give the start-up home position. When the power goes off the mechanical sub-system could be in any position. When power comes on the encoder count just shows that the mechanical assembly is moving - but gives no idea where it actually is. When an encoder susbystem is powered up it will seek it's home position first, then set the position counter to zero.

Support Circuits

Encoder motor circuits are a hybrid technology.

There is some logic circuitry, this typically provides
 

motor target position - the position the computer next wants the mechanical device placed in

motor actual position - the up-down sensing device and counter.

a difference signal - this may simply be equal - stop the motor, greater - motor powered one direction, less - motor powered other direction. More complex circuits may give an arithmetic difference which can be converted into a motor power signal - big differences will give the motor a high power.

There is also some "analogue" power circuitry. This takes the form of a "bridge" circuit with the motor across it's output. The bridge has two pairs of transistors, one connected to each pole on the motor. With one pair of transistors on the motor rotates one way, with the other connected it turns the other.

The logic is arranged so that it is impossible for transitors from both pairs to be on at the same time - this would create a short-circuit and blow the power supply fuse.

At first glance a more substantial analogue control circuit might seem to be needed. This would take  the arithmetic difference between the target and the actual position counter and vary the motor power accordingly. In this way the motor will run at maximum speed when the difference is large, slowing down gradually and going to zero power precisely when the target has been met.

In practice the power control circuit can often be omitted - the motor simply runs at full power, overshoots its target, reverses, overshoots again and quickly settles into the correct position as the voltages on the coils alternate more rapidly than the armature can respond. In some systems this behaviour known as "hunting" can be rather exagerated and needs dapming down. Many encoder motors drive a mass like a printhead or cutting tool directly through a rubber belt and the mass exerts enough damping. If there is a power control circuit it may be rudimentary giving just a couple of different levels.

Uses for Encoders.

A common use of encoder motors has been as the horizontal drive in fast dot matrix and inkjet printers. The printhead sits on a carriage which slides back and forth on a rail. The carriage is driven by a rubber / nylon composite toothed belt directly connected to a cog on the end of the motor armature.

When the carriage sets out from it's left position it begins to accelerate, but the pulses from the encoder are also fed to the character generation logic so that the pins fire in the correct position. A substantially built print head weighing a couple of hundred grammes can be swiped back and forth 3 or 4 times per second across a 300mm page - and the motor can keep this up continually if necessary. Dot matrix heads tend to print a swath just one line high (3+mm) per pass. Inkjet printer swath can be 10mm or more so they tend to be faster.

Printer designers often opt for the lower cost option of a stepper motor in cheaper printers and in the slower moving line-feed mechanism. A few fast dot matrix and band printers have used encoders in both carriage and line-feed circuits.

Dot matrix printers are still widely used in industry wherever multi-part stationery is required - but are no longer mass-market products.

Encoders do not have to be built onto the motor, they can detect what is happening at the work-face rather than at the motor shaft. This approach is sometimes taken with inkjet printers, where the (heavy) print-head is being driven back and forth via a rubber belt. Hewlett Packard in particular have favoured putting a plastic strip marked with black strips across the carriage width. The advantage of this is that the encoder is seeing precisely what is happening at the point where the printhead delivers ink to paper. Otherwise the flexibility of the belt (and indeed of a plastic chassis) tends to lead to "margin drift" - printing with the carriage moving left is in a slightly different position to when it is moving right.

Robotics and machine -tools are the main growth field for encoder motors. Traditionally small arms and actuators have tended to be driven by stepper motors. The DC encoder tends to offer superior accuracy and speed of action. Laser cutters and milling machines are typical applications.
 
 

Cost

The DC motor element of an encoder is often a relatively low-cost device. Lots of little motors based on  permanent magnets with carbon brushes are made for toys, models and domestic appliances. Larger motors have ball-bearings and multiple poles, but these are not always regarded as essential. Rather than needing a precision engineered motor the encoder subsystem gets it's equired accuracy from the encoder mechanism. The encoder mechanism itself is not particularly expensive - just a commonplace opto-sensor and a slotted wheel housed in a light and dust-proof enclosure.

One limitation on encoder motors is that they and their support circuitry tend to be made for a specific purpose rather than mass produced. The motor is mass produced, but encoders are built for the task, and there are 1001 varieties of support electronics. Economies of scale may not be as good as those for stepper motor circuits.
 

Effectiveness

A brushless motor or a stepper motor could be used instead of the DC motor. In general designers aim to minimise the number of moving parts in designs - and the commutator of a DC motor is a hot-bed of sparks, voltage spikes and ground up carbon - so it looks like a very undesireable device. Replacing the commutator is not particularly easy, however. Controlling several coils needs several little bridge circuits to switch their currents, and these may have to be located on the stator, not the armature. The armature would then have to carry the permanent magnets - which would be smaller. In practice the brushes in a DC motor commutator do eventually wear out and the motor needs replacing or refurbishing - but the life of the motor is typically 10-20 years.
 

DC motors tend to compare quite well with stepper motors in terms of power to weight ratio, power per dollar, and electrical efficiency. Small DC motors might achieve efficiencies around  70-80% in converting electrical power to mechanical effect when they are turning in one direction continually. When the motor is constantly re-positioning efficiency will be lower because the field is continually changing without the armature gaining any momentum.

On the other hand not much power is lost in the drive circuits - they are mostly switching from full off to full on so they don't waste much power as heat. In principle fully electronic control could be more efficient but in practice there is not much need for great efficiency if the mechanical devices driven are relatively small. The motors run warm - but neither they nor their drive circuits are wasting an exccesive amount of power as heat.

Stepper motors

Stepper Motors with a feed-back loop have the potential to be more efficient than a DC motor. In practice they need two or three bridge and wave shaping circuits to perform better and the overhead of extra circuitry negates any benefit in most applications.

AC Motors

Large domestic appliances which run at constant speeds like washing machines, vacuum cleaners, and blower heaters often use AC motors - these ely on the alternating nature of the electricity supply to do the commutator job and change the field for the armature continually. There is a problem with these motors, they are generally designed to run at just one or two speeds and can't turn efficiently at any other. Starting these motors usually elies on some kind of magnetic bias.