The use of potentiometers in Incremental Encoder Design

Recently I decided to catalog the competitive Incremental Encoders that have populated the shelves surrounding my desk.  In doing so, I was surprised to find that many of our competitors use potentiometers in their designs.

I can understand why they need to use potentiometers.  In most designs the potentiometers are used to balance the raw analog signals produced by the Incremental Encoder sensor.   A potentiometer is the perfect component for this, you fire up the Incremental Encoder during test, lay a scope probe on it and dial the value specified by Engineering.  For most encoder designs, the only other option is to guess at some resistor values hoping that you don’t have to solder and unsolder resistors too many times until you hone in the correct signal, as that would be a very time consuming process.

While I can understand the use of Potentiometers, the reason that I am a bit shocked by their ubiquity in competitor’s designs is that potentiometers are inherently a much less reliable component.  A resistor is all one solid piece, but a potentiometer (which is a variable resistor) has a resistive track and a movable wiper that slides along to vary the resistance value. Moving parts are inherently less reliable than a non-moveable part.

Here are a few of the PCBs from various manufacturers ,  the potentiometers are circled in Red:

 

This next one is my favorite.  Thirteen potentiometers!

I am proud to say that Quantum Devices Encoders do not use potentiometers in their Incremental Encoder designs.  The reason we can avoid potentiometers is because of our patented phased array sensor that provides perfectly balanced complementary signals right from the sensor.

Other incremental encoder sensors suffer from having their active areas in different locations along the length of the sensor. Since the light source spreads light unevenly over the sensor, some active areas receive more light than others creating signal imbalances.

In Quantum Devices Incremental Encoders the photosensitive areas of each channel are interlaced with each another,  so all active areas receive the same amount of light.  This eliminates the need to balance any signals, which in turn eliminates the need for potentiometers in the design.

Below is an drawing of the phased array sensor Red indicates Channel A active areas, Blue indicates channel B active areas.

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Finding the Index Pulse of an Incremental Encoder

The index pulse, often also called “Z” or “Marker pulse”, of an optical incremental encoder is a once per revolution digital pulse that is used for homing or count verification of incremental signals.

In the QD145 and QD200 series of encoders the index pulse fires when the mark on the top cover of the optical incremental encoder and the mark on the encoders shaft are aligned.

This mark also indicates the rising edge of the U channel for commutated optical incremental encoders.  Knowing the location of this edge is useful for the initial rough timing of Brushless DC motors.

Quantum Devices is a leading manufacturer of Optical Incremental Encoders.

Jim can be reached at 608.924.3000 or by e-mail at jmiller@quantumdev.com

Brushless Motors vs Brush Motors, what’s the difference?

What’s the difference between a Brushless Motor and a Brush Motor?

Well, the brushes of course.

Yeah, but what does that mean?

The principle behind the internal working of both a brushless DC motor and a brushed DC motor are essentially the same.  When the motor windings become energized, a temporary magnetic field is created that repels(and/or attracts) against permanent magnets.  This force is converted into shaft rotation, which allows the motor to do work.  As the shaft rotates, electric current is routed to different sets of windings, maintaining electromotive repulsion/attraction,  forcing the rotor to continually turn.

Construction differences

Brushes inside  electric motors are used to deliver current to the motor windings through commutator contacts.  Brushless motors have none of these current carrying commutators.  The field inside a brushless  motor is switched via an amplifier triggered by a commutating device, such as an optical encoder.

Windings are on the rotor (Rotating part of motor) for brush motors and on the stator (stationary part of motor) for brushless motors.

Brush Motor: windings on rotor, magnets on stator

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Finding the Index on an Incremental Encoder with a DMM

Sometimes you don’t have the right tools to do the job.

Lets say you needed to identify where the index pulse was firing on your incremental encoder, but you left your oscilloscope in your other jacket pocket, and now all you have on hand is a DMM.

Well fear not, finding the index with a multimeter is possible although a bit tedious.

The index fires once per revolution and at higher line counts this makes it VERY easy to miss.  Since there is some delay in a multimeter’s display time, you will need to rotate the encoder very slowly to catch a change in voltage level.

The Blue box has a nine-volt battery inside that I regulated down to 5Vdc for the encoder power.  I have pulled out connections to ground (Black wire) and the index channel (Orange wire). When the index fires, the voltage will go from zero to five volts.

Jim is an Applications Engineer with Quantum Devices Inc. A leading manufacturer of Optical Encoders.

Understanding Incremental Encoder signals

Which incremental encoder wires should I use?

Channels A & B (Incremental Channels)

Use only A (or only B) for an RPM or counting applications where the rotation is either unidirectional, or where you don’t need to know direction.

Use A and B together to know direction. After two low pulses the next high pulse indicates direction.  This is due to the phasing offset between A and B of 90 electrical degrees, placing the signals in what is known as quadrature.

These signals can also be used to set up an up/down counter

Index pulse, also known as Z, marker, or I

Index pulse is a pulse that occurs once per rotation. It’s duration is nominally one A (or B) electrical cycle, but can be gated to reduce the pulse width.

The Index (Z) pulse can be used to verify correct pulse count

The Incremental Encoder Index pulse is commonly used for precision homing.  As an example, a lead screw may bring a carriage back to a limit switch.  It is the nature of limit switches to close at relatively imprecise points. This only gives a coarse homing point. The machine can then rotate the lead screw until the Z pulse goes high.

For a 5000 line count encoder this would mean locating position to within 1/5000 of a rotation or a precision of .072 Mechanical Degrees.  This number would then be multiplied against lead screw travel.

Commutation (UVW) signals are used to commutate a brushless DC motor. I always like to compare these signals to that of a distributor in a car. The commutation (sometimes called “Hall”) signals tell the motor windings when to fire

You would need to have encoder commutation signals if the motor you are mounting the encoder to has a pole count and there is no other device doing the work of commutation.  It is important to note that commutation signals need to be aligned or “timed” to the motor.


Single ended VS differential

These terms refer not to the waveforms of signals, but instead to the way the signals are wired.

Single ended wiring uses one signal wire per channel and all signals are referenced to a common ground.

TTL and Open Collector are types of single ended wiring.

Differential wiring uses two wires per channel that are referenced to each other.  The signals on these wires are always 180 electrical degrees out of phase, or exact opposites.  This wiring is useful for higher noise immunity, at the cost of having more electrical connections.

Differential wiring is often employed in longer wire runs as any noise picked up on the wiring is common mode rejected.

RS-422 is an example of differential wiring.

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Build an Incremental Encoder Quick Tester

How to build a portable incremental encoder tester.

Every once in while it’s nice to have a hand-held device that can be used to see quickly if encoder signals are present.  I designed and built a quick-tester that allows for fast interface to an encoder without having to drag out a power supply and oscilloscope.

The Quick-tester is an optical encoder tester that is powered by a 9V battery. An internal 5V regulator drops the voltage to 5V, as two of the connections are power connections for the optical encoder.

Current limiting resistors are wired in to red LEDs.   The LED’s illuminate when the optical encoder signals are High.  By rotating the base by hand we can see if a channel is dead or even improperly phased.

Here is the schematic for the Optical Encoder Quick-Tester

Things the Quick Tester can tell you:

Which channels are working.

The LED’s should rapidly blink on and off while the optical encoder hub is slowly rotated. Keep in mind that turning the hub too fast will make the LED’s switch on and off quickly and cause them to appear to be continuously illuminated.

Proper phasing

When channel A is illuminated, channel A- should be dark and vice versa. If both lights are on or off at the same time, there is a problem with the encoder.

If A leads B, or B leads A

Rotate the encoder shaft slowly until both A and B are low (off), then look for the next high channel, that channel is the leading channel for that direction of rotation.

Location of the index pulse

It can be easy to fly right past an index pulse, particularly on higher line count encoders, but the Quick tester can tell you precisely when the index pulse occurs. This can be handy for mechanically timing the optical encoder to a real world position.

Other things to note:

If you have an optical encoder with  open collector outputs, the Quick-tester’s LED’s will not illuminate unless you use pull up resistors between each channel and the positive supply of the Quick Tester.

If all lights are on, you have probably lost the signal ground connection to the Quick tester.

The Quick tester is set up to look at six channels at a time, so it is likely you will be looking at only incremental channel or only commutation channels.

For more information on encoders, go to http://www.quantumdev.com.

Jim can be reached at jmiller@quantumdev.com

Understanding Options on the QDH20 Industrial Rotary Encoder.

The QDH20 is an IP66 sealed encoder made for the rugged duty of an industrial application.  Due to the huge array of configuration options, this optical encoder has over 200,000  possible ways that it can be configured.

While this is certainly an advantage to an end user that may be seeking just the right encoder, the sheer number of choices can make configuring a part number a confusing proposition.  I am going to try to shed some light on the QDH20’s available choices in order to make things a bit more clear.


Mounting Options:

This refers to the style of the mounting “face” of the encoder.  There are three basic types: Flange, Servo and Flex mount.  These size of each of these types are referred to by motor sizing terminology. A “Size 20” mount is approximately two inches and a “Size 25” is about 2.5 inches.

The flange mount is used when you are bolting directly to a surface. The flange surface is typically square with bolt holes in each of the four corners.

I am using CAD drawings for my examples as they are more readily available and easier for me to work with than photos.

The servo mount is typically round and there is a built in retaining groove around the mounting edge. While it can be mounted using this edge, there are also three tapped mounting holes for the size 25 and a set of three and as set of four tapped mounting holes for the size 20, drilled into the face of the Servo mount.

The flex mount  has a spring steel mounting that allows for misalignment between the encoder and whatever it is mounted to. This is used with a “hollow shaft” option where the QDH20 encoder is receiving a shaft.

Mounting options may include a “pilot” or boss that is used to help align the encoder to a mounting surface. These are either female, which are recessed into the encoder housing, or male, which protrude out from the encoder housing.

Quick mounting summary:
For QDH20 Mounting options you can have a hollow shaft which can only be a flex mount, or a standard shafted encoder which can be either a flange mount (square) or a servo mount (round).

Both of these configurations may have either a male or female pilot.

Hollow shaft QDH20 encoders always have a flex mount and do not have a pilot.


Connector Housing Options:

The housing of a QDH20 encoder has two main considerations; the style of electrical connection to the encoder, and the way these electrical connections exit the encoder.

Electrical connections for the QDH20 encoder can be either a MS (Military Style) connector, or a flying leads type connection, referred to as a “wire gland”

The MS Connectors have three sizes 10 Pin, 7 Pin and 6 Pin.  The number of output signals needed determines these connector sizes. For  one channel single ended applications  a six pin connector will work just fine, but for an application where all channels ( AB &Z ) and their complements are needed a ten pin MS Connector is a must.

Note that while it is possible to make an encoder that has  a single output with a ten-pin connector, items like these are not part of the QDH20 standard offering. If a special QDH20 configuration is needed, Quantum Devices’ crack engineering team will typically be able to make it happen.

Both styles of electrical connections can exit the encoder either axially, or along the same axis as the encoder’s shaft, or radially, perpendicular to the shaft.   Being able to choose this electrical connection exit method allows for the QDH20 encoder to fit into tight spaces.


Shaft Options:

Shafts options will either be a standard solid shaft or a hollow shaft.  Keep in mind that the solid shafts  go with the servo and flange style mount and the hollow shaft has a flex style mount.


Resolution options:

The QDH20 encoder currently can have resolutions (pulses per rotation) of up to 5000 direct read. (not interpolated).

The current QDH20 resolution options are:
200, 250, 256, 500, 512, 600, 1000, 1024, 1250, 2000, 2048, 2500, 3600, 4096 and 5000

These resolutions are all “direct read”, which means that the signals are taken right from the encoder disk, as opposed to interpolation which creates more pulses than are represented by the disk pattern.  Direct read is always preferred if you can get it as there is one less error factor (Interpolation error) to worry about.

Output Options:
The  electrical output that the QDH20 encoder can provide are either 5 to 26 Volt differential or single ended via the  OL7272 line driver, or an Open Collector style.

The OL7272 has thermal and electrical overload protections built into it.   The encoder is designed so that whatever voltage you put into the power rails you get out, minus a small voltage drop, on the signal wires.

Open collector style encoders require pull up resistors to make the signals work.


Channel Options:

This is used to select the style and number of output channels you need.   This is the option that drives the style of connector the QDH20 will have.


Output Waveform Options:

This option refers to the phasing relationships between the encoder channels.  Some encoder manufacturers have established traditional waveforms styles that their industrial encoders fall into.

The QDH20 Waveform options 01 and 02 match exactly with certain BEI style encoders. QDH20 waveform options 03 and 04  match up with certain EPC style encoders.

While the many ways  to configure a QDH20 encoder can be confusing, hopefully looking at each option has made it a bit simpler to understand.