Elevator Encoder
Elevator encoders provide feedback on position, count, speed, and direction that are communicated to a control device in a motion control system.
Controller 18 compares the velocity and motion information provided by signals from encoder 36 to a commanded velocity and motion for elevator 14 such that hoist motor 12 is driven efficiently and accurately.
Absolute Encoders
Absolute encoders are a type of rotary encoder that directly outputs the exact position of the shaft it is measuring. They are typically used for systems that require extreme precision, complex programming and safety. These types of encoders are commonly found in applications such as remote surgery, robotic arms and other industrial systems that need extremely accurate positional information.
Unlike incremental encoders, which are essentially pulses, absolute encoders use a disc with markings that rotates with the shaft to measure changes in position. The disc’s unique patterns provide a unique code that is interpreted by the sensor. The disc’s design is more intricate than that of an incremental encoder, and it can be engraved with patterns that are specific to the machine or assembly line where it will be used.
In addition to their optical sensing capabilities, absolute encoders also can be used with a magnetic sensor that detects the rotation of the shaft. These sensors are often available in compact designs that do not require additional support circuitry.
This allows them to be incorporated into machines without requiring the need for reference marks, and can keep track of the position even when power is lost. This makes them an excellent choice for machines that need to retain their positional data through power cycle events, such as elevators.
Another advantage of absolute encoders is that they do not need to be reset or turned on each time the system starts up. This eliminates the need for a homing cycle that requires the machine to return to its initial start position in order to get started.
Additionally, they do not need to be backed up with an external battery in case of power loss, like the incremental type does. This helps save costs, since power losses can lead to inaccurate encoding.
Both incremental and absolute encoders are useful in a variety of applications, but they have distinct advantages and disadvantages. Compared to incremental encoders, absolute encoders are better at retaining positional data through power cycle events and are more precise when it comes to determining the direction of a rotating shaft. However, they can be more expensive to buy and maintain, and their resolutions tend to be lower.
Incremental Encoders
Incremental encoders measure position by converting the movement of a rotating shaft into a pulse signal that can be counted. These types of encoders are typically used in closed loop, speed control loops or as speed feedback devices.
They are usually made to withstand harsh environments, such as industrial machinery and motor drives. Depending on the application, they can be made to operate at high or low speeds and with either low or high resolutions.
Typical incremental encoders have 2 output signals, called A and B. These signals are set up with a 90 degree offset to detect the shaft rotation. The A signal is rising 90 eldeg before the B signal, when the shaft rotates clockwise, and the other way around.
Some encoders also output a elevator encoder digital signal called an index (or Z) signal. This signal is asserted once per revolution on a dedicated channel (Z channel).
The index signal can be used for homing. When the mechanical system is in its reference orientation, this index signal will be asserted, and it will cause the encoder interface to jam the corresponding position value into its position counter.
Another common use for a rotary incremental encoder is to calculate the shaft’s rotational angle, relative to a reference orientation. This can be done by using a proximity sensor that outputs a signal when the mechanical system is in its home (reference) orientation.
In addition to generating the A and B signals, most encoders have a Z-phase output which outputs data once per shaft rotation on a dedicated channel (Z channel). The Z signal can be used for a number of different functions including verifying the A and B signals or tracking the number of rotations.
The symmetry and phase of incremental encoders are defined by the manufacturer, and they are commonly specified in terms of pulse width and phase difference ranges. These values can be grained to improve accuracy.
An incremental encoder reports position changes nearly instantaneously, elevator encoder which allows it to monitor the movements of a high speed mechanism in near real-time. Because of their ability to report position changes so quickly, they are often used in applications that require precise measurement and control of position and velocity.
Linear Encoders
Linear encoders are used to detect linear motion of an object, such as an elevator car. They are available in a variety of styles and configurations and can provide position data for applications such as machine tools, measuring instruments, conveyors, automated storage/retrieval systems, and elevators.
These devices can be either enclosed or open and can withstand harsh, dirty, and hostile environments. Enclosed encoders typically comprise an aluminum extrusion enclosing a glass or metal scale, which is protected by flexible lip seals. These enclosures can improve accuracy, but are susceptible to measurement hysteresis and friction.
In contrast, open linear encoders feature a non-enclosed reading head, which is guided by the scale. They offer the highest level of accuracy and lowest measurement hysteresis. They are ideally suited for high-end machine-tool applications and also for general automation and industrial controls.
Typical scales for these encoders include multitrack, vernier, digital code, and pseudo-random codes. Some encoders are capable of determining their position without having to move or needing to use a reference scale, and can communicate using serial communication protocols.
Some encoders can provide a reference mark or index signal for use on power-up or following a loss of power, which is a pulse which can be one to four units-of-resolution wide and provides a datum position along the scale. This may consist of a single feature on the scale, an autocorrelator pattern (typically a Barker code) or a chirp pattern.
Another type of linear encoder is a capacitive encoder, which is ideal for measuring a range of small objects. This device uses a sensor to measure the capacitance between the reader and the scale, which is a direct readout of position. This is the most common form of encoder and is used in calipers, coordinate-measuring machines, and other linear measuring applications.
These encoders are available in a variety of resolutions, including micrometer-scales and millimeter-scales. They are a cost-effective alternative to magnetic and optical encoders. They are suitable for most motor shafts but not those with diameters larger than 8mm. They can provide an output in either analog or digital format and are often programmable.
Motor Encoders
Elevators require precise motion feedback to operate safely and efficiently. Encoders mounted on motor shafts help elevator cars start gently, stop at just the right height, rapidly open and close their doors, and smoothly speed off again.
Encoders are also used to detect when elevator cars are going over speed and to trigger a safety mechanism. This includes an assembly known as an elevator governor that runs over the sheaves and connects to a brake that prevents cars from moving.
In addition, these encoders are important for detecting when a car goes out of control and needs to be disabled or tripped by a controller. To do this, controller 18 samples the fault bit periodically (e.g., every 10 ms).
When controller 18 determines that a fault bit has been set, it disables inverter 22 and engages brake 34 to prevent unintended movement of elevator car 26. This reduces the likelihood of a collision and increases the safety of passengers in the elevator car.
HEIDENHAIN encoders are also designed specifically for cable-less elevators, helping to ensure smooth and gentle operation at slow speeds, acceleration after a stop, and braking before a stop. Their high direct read resolutions allow them to provide accurate, repeatable position values.
If the encoder is not secured properly to the motor shaft, it may slip and cause damage or failure of the elevator car or its parts. This can be prevented by verifying that all set screws are tight and the encoder is securely mounted squarely on the motor shaft.
Some encoders are made with a screw that fits inside the end of the motor shaft. If this screw is too long and bottoms out on the threads of the motor shaft, it can create a poor connection that will cause slipping.
If the encoder is not positioned correctly on the motor shaft, it can result in errors in stator field commutation and a mismatch in the incrementing and decrementing conventions for position value increments and decrements. This can lead to the motor drawing a higher current and may also require the incremental A-B speed channel phasing to be changed physically at the interface or with a software parameter setting based on the drive type.
