ENCODERS

May 21, 2016


Once a month a group of guys and I get together for lunch.  Great friends needing to solve the world’s problems.  (Here lately, it’s taken much longer than the one and one-half hours we spend during our meeting.)  One of our friends, call him Joe, just underwent surgery for prostate cancer.  This is called a Prostatectomy and is done every day.  His description of the “event” was fascinating.  To begin with, the surgeon was about twenty (20) feet from the operating table. Yes, that’s correct; the entire surgery was accomplished via robotic systems. OK, why is this procedure more desirable than the “standard” procedure”?   The robotic-assisted approach is less invasive, reduces bleeding and offers large 3-D views of the operating fields. The mechanical arms for the robotic system are controlled by the surgeon and provide greater precision than the human hand.  This allows the surgeon more control when separating nerves and muscles from the prostate. This benefits patients by lowering the risk of side effects, such as erectile dysfunction and incontinence, while also completely removing cancer tissue.  The equipment looks very similar, if not identical to the one given in the JPEG below.  Let’s take a look.

Prostate Surgery and Robotic Systems

As you can see, the electromechanical devices are remarkably sophisticated and represent significant advantages in medical technology.  The equipment you are seeing above is called the “patient side cart”. It looks as follows:

Surgical Side Cart

During a robotic prostatectomy, the patient side cart is positioned next to the operating table.  The system you see above is a da Vinci robotic arm arranged to provide entry points into the human body and prostate.  EndoWrist instruments, and the da Vinci Insite Vision System, are mounted onto the robot’s electromechanical arms representing the surgeon’s left and right hands. They provide the functionality to perform complex tissue manipulation through the entry points, or ports.  EndoWrist instruments include forceps, scissors, electrocautery, scalpels and other surgical tools. If the surgeon needs to change an Endowrist instrument, common during robotic prostatectomy, the instrument is withdrawn from the surgical system using controls at the console. Typically, an operating room nurse standing near the patient physically removes the EndoWrist instruments and replaces them with new instruments.

There are certainly other types of surgery performed today using robotic systems.  Several of these are as follows:

One electromechanical device that helps to make this remarkable procedure possible is called an encoder.  Let’s define an encoder.

An encoder is a sensor of mechanical motion that generates digital signals in response to motion. As an electro-mechanical device, an encoder is able to provide motion control system users with information concerning position, velocity and direction. There are two different types of encoders: linear and rotary. A linear encoder responds to motion along a path, while a rotary encoder responds to rotational motion. An encoder is generally categorized by the means of its output. An incremental encoder generates a train of pulses which can be used to determine position and speed. An absolute encoder generates unique bit configurations to track positions directly.

As you might expect, knowing the exact position of a medical device used during surgery is absolutely critical to the outcome.  The surgeon MUST know the angular position of the device at all times to ensure no errors are made.  Nerves, tendons and muscles must be left intact.  This information is provided by encoders and encoder data systems.

ENCODER TYPES:

Linear and rotary encoders are broken down into two main types: the absolute encoder and the incremental encoder. The construction of these two types of encoders is quite similar; however they differ in physical properties and the interpretation of movement.

Incremental rotary encoders utilize a transparent disk which contains opaque sections that are equally spaced to determine movement. A light emitting diode is used to pass through the glass disk and is detected by a photo detector. This causes the encoder to generate a train of equally spaced pulses as it rotates. The output of incremental rotary encoders is measured in pulses per revolution which is used to keep track of position or determine speed.  This type of encoder is required with the medical system given above.

Absolute encoders utilize stationary mask in between the photodetector and the encoder disk as shown below. The output signal generated from an absolute encoder is in digital bits which correspond to a unique position. The bit configuration is produced by the light which is received by the photodetector when the disk rotates. The light configuration received is translated into gray code. As a result, each position has its own unique bit configuration.

Typical construction for a rotary encoder is given as follows:

Rotary Encoders

Please note the following features:

  • Electrical connection to the right of the encoder body.
  • Encoder shaft that couples to the medical device.
  • Electrical specifications indicating the device is driven by a five (5) volt +/- 5% source.

Encoder Specifics

You can see from the above illustrated parts breakdown that a rotary encoder is quite technical in design.

SYSTEM ACCURACY:

System accuracy is critical, especially during surgery. Let’s look.

An encoder’s performance is typically stated as resolution, rather than accuracy of measurement. The encoder may be able to resolve movement into precise bits very accurately, but the accuracy of each bit is limited by the quality of the machine motion being monitored. For example, if there are deflections of machine elements under load, or if there is a drive screw with 0.1 inch of play, using a 1000 count-per-turn encoder with an output reading to 0.001 inch will not improve the 0.1 inch tolerance on the measurement. The encoder only reports position; it cannot improve on the basic accuracy of the shaft motion from which the position is sensed.  As you can see, the best encoders, hopefully those used in a surgical device, can deliver accuracy to 0.10 inch.  Remarkable accuracy for a robotic device and absolutely necessary.

CONCLUSIONS: 

TECHNOLOGY DELIVERS.  Ours lives are much better served with advancing technology and certainly technology applied to the medical profession. This is the reason engineers and technologists endure the rigor necessary to achieve talents that ultimately will be directed to solving problems and advancing technology you have seen from the post above.

As always, I welcome your comments.  bobjengr@comcast.net

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