PRODUCT SPOTLIGHT: CAPACITIVE COMMUTATION ENCODERS AMT31 SERIES

Encoders Provide Directional Information

Detection of shaft direction is very useful and even critical to some applications. In a radio, the rotational direction of the volume knob tells the receiving circuit whether to increase or decrease the volume with each square wave. In automation equipment, the rotational direction is detected and other operations are initiated when a pre-set number of pulses for that direction has been achieved. An elaborate and sophisticated set of movements can be executed automatically to perform tasks like placing components on a pc board, welding seams in an automobile body, moving the flaps of a jumbo jet or just about anything that involves a set of precise motions.

• • In this example, Channel A leads B, i.e., Channel A outputs a signal before Channel B. This indicates the shaft is rotating counter-clockwise.

• In this example, Channel B leads A. This indicates the shaft is rotating clockwise.

Encoders Provide Position Information

Each pulse from Channel A or B increases the counter in a users system by one when the encoder is turning counter-clockwise and reduces by one for each pulse from the encoder when it is turning clockwise. The pulse count can be converted into distance based on the relationship between the shaft the encoder is coupled to and the mechanics that convert rotary encoder motion to linear travel.

Encoders Provide Speed Information

Encoders can detect speed when output pulses are counted in a specified time span. The number of pulses in one revolution must also be known. In the equation below, S represents speed in revolutions per minute (RPM), C represents the number of pulses counted, PPR represents the encoder’s number of pulses per revolution, and T represents the time interval in seconds during which the pulses were counted. The second equation shows that if 60 pulses are counted in a time interval of 10 seconds using a 250 PPR encoder, the shaft speed is 1.44 RPM.

The equation for calculating speed is:

S = (C/PPR) / (T/60)Therefore if 60 pulses were counted in 10 seconds from a 250 PPR encoder, the speed can be calculated as:

S = (60/250) / (10/60) = (0.24) / (0.1667) = 1.44 RPMAll of the counting, timing and calculations can be done electronically in real time and used to monitor or control speed.

Encoders Provide Distance Information

Encoders can detect distance traveled based on the number of pulses counted. In most applications, rotary motion is converted to linear travel by mechanical components like pulleys, drive gears and friction wheels. In this illustration of a cutting table, if the diameter of the friction wheel and the PPR of the encoder are known, linear travel can be calculated. Pulse count to achieve desired linear travel can be calculated in a similar fashion to the diagram above for devices that use ball screws, gears or pulleys to convert rotary motion to linear travel.

In the equation, C is the number of encoder pulses counted, L is the desired cut length in inches, D is the friction wheel diameter in inches, and PPR is the total pulses per revolution of the encoder. The second equation is based on a desired cut length of 12″. Assuming the friction wheel diameter is 8″ and encoder PPR is 2000 we can calculate that 955 pulses must be counted to achieve a cut length of 12″.

What is Commutation?

Commutation is the process of applying current to a motor so that a rotating magnetic field may be established. In the illustration, current is commutated to windings in the stator. The stator is on the perimeter of the motor and does not move. The permanent magnets in the rotor are attracted and/or repelled by the magnetic fields in the stator created as a result of precisely timed application of current through the commutation process.

The illustration above is very simple. Notice that poles in the stator are alternately energized with a polarity that magnetically attracts the rotor to move in a clockwise direction. In the real world, both attraction and repulsion are used to increase speed, power and efficiency.

Electric Motor Basics

Motors with brushes and commutator are inexpensive but require a lot more maintenance than brushless motors. The brushes erode and fail, create friction, impede current and decrease efficiency. The advent of motor controllers has driven a strong and steady market shift toward brushless motors. Brushless motors require another method of current commutation. Hall devices are cheap but not very accurate, and are usually used with a position encoder increasing cost and BOM count. Resolvers are very accurate and durable but very expensive. Encoders with commutation output have become dominant because they are very accurate, already required in many cases for position feedback, and the cost is competitive.

There are four typical methods of commutating current in a motor:

• Brushes/mechanical commutator Inefficient, high-maintenance
• Hall device Inexpensive but inaccurate, ±1~3°
• Encoder with commutation output Very accurate and cost competitive
• Resolver Extreme accuracy, extreme expense

Brushed vs Brushless

	Permanent Magnet	Brushless DC
Voltage	DC	AC, DC (Control)
Speed	1,000-5,000	1,000-5,000
Horsepower	Medium	High
Efficiency	60%-70%	65%-80%
Life	Medium	Very High
Maintainance	Medium	Very High
Noise	Medium	Very Quiet
Speed Regulation	Fair	Excellent
Starting Torque	Very High	Very High

How Brush Commutators Function

Different from BLDC (brushless dc motors), brush motors use only two connections instead of the three phases that BLDC motors use. In the case of a brush motor, only positive or negative dc voltage is applied. The voltage level, and polarity (positive or negative) dictate speed and direction.

In Fig. 1 the ends of the armature are mainly repelled by both permanent magnet poles and attracted to the opposite magnet poles they are moving toward.

In Fig. 2 the ends of the armature are equally repelled and attracted by both magnets. If the rotor does not have enough inertia to rotate beyond this point the motor is in danger of locking. For this reason 2 pole motors are not very popular and not often used.

In Fig. 3 the ends of the armature are mainly attracted by both magnets and barely repelled by either magnet. The polarity of the magnets in the rotor will change when the split in the commutator ring passes beyond the brushes. At that point the current will be reversed changing the south pole of the rotor to north, and the north pole of the rotor to south.