Canadarm

9 min readJul 8, 2021

This article is presented by ISA Manipal in collaboration with Parikshit student satellite team, Manipal Institute of Technology, Manipal.

What is Canadarm?

The Canadarm also called the Shuttle Remote Manipulator System (SRMS) is basically a robotic manipulator which is 15 meters in length consisting of various components. It weighs 410 kg. It had 6 degrees of freedom (Canadarm (2) has 7). It has a maximum payload of 266,000 kilograms in space and can operate at speeds of 60 cm/s. It can place its payload within 5 cm of the required target. Canadarm played a significant role in helping set up the International Space Station. After 30 years of active service, the Canadarm was retired and replaced by Canadarm 2, which is currently attached to the ISS and helps in assisting the capsules that deliver cargo. It is also used to help astronauts and assists them during spacewalks repairs and upgrades on the outside of the ISS.

Materials used:

The manipulator is made up of titanium, stainless steel and ultra-high modulus graphite-epoxy. These materials meet the strength requirements. In order to protect the arm from harsh thermal conditions, a multi-layer insulation system consisting of alternate layers of goldized Kapton, Dacron scrim cloth and a fiberglass outer covering. For the electronics to withstand extremelycold temperatures, thermostatically triggered electric heaters are provided within the body of the manipulator.

Working Mechanism:

The Canadarm has 6 joints. Two in the shoulder, one in the elbow and three in the wrist of the robotic arm. They are all made up of an element known as Joint One degree of freedom (JOD). JODs are gear boxes that move and work the same as a normal human arm. The gear boxes are motor driven. The joint present in the elbow helps the arm to pitch while the joints present in the wrist of the robotic arm allows it to pitch, yaw and roll. The speed of the motor can be controlled, so as the maneuverability of the arm. The motors also have breaks and are capable of moving in different directions. Each joint is driven by a servo mechanism. The output of this mechanism is provided by a brushless DC motor and finally a high resolution gearbox is used to transmit the output. Normally brakes are provided in the joints and are applied when the joints are un-commanded.

End Effector:

As the manipulator would be operating in space under zero gravity conditions, the end effector cannot have traditional jaw-like grippers to capture stationary or free flying payloads. The end effector of the Canadarm provides a large capture envelope of 20.3 cm in diameter and 10 cm in depth. The structure and mechanism of the end effector is capable of providing a soft docking and rigidizing. The end effector accomplishes this by a 2-stage mechanism. It closes 3 cables in the form of a snare around a knobbed pin which is bolted onto a payload. It then draws it to the device until close contact is established. This then applies a load of approximately 499 kg onto the knobbed pin. Hence, allowing the astronauts to maneuver the payload.

How is the Canadarm controlled?

The main controls of the Canadarm are in the cupola module of the ISS. Cupola is the module that lets the astronauts have a 360-degree view of the outside of the ISS, including the view of the Canadarm.

Inside the cupola, there are:

● 4 displays- which show various data that is crucial to controlling the arm, including the view from the on-board cameras.

● One joystick- That is used to manipulate the rotational properties.

● One additional control to manipulate the translational position of the arm.

● There is also an additional panel that can be used to toggle different cameras and modes for the Canadarm.

The Canadarm can also be controlled remotely from the NASA and Canadian space agency mission centers as and when needed.

As an important additional benefit, this mode of operation allows astronauts to work on the shuttle without having to wear a spacesuit. The arm is also designed as a support platform that allows astronauts to repair satellites while wearing extravehicular activity (EVA) spacesuits.

To help increase the redundancy, every joint has 2 motors, along with two electronic circuitry at each point. Being in space, outside the gravity’s influence, the entire ISS is prone to radiation from various sources. Although this is rare, this radiation activity has the capacity of corrupting the flight system hardware. To counter this, special components that are ‘red hardened’ are used. Multiple components are also used to prevent data corruption.

A few advancements of the Canadarm 2 is that they have 3 extra sensors to facilitate a safer working environment in space. They are:

a. Force moment sensors that provide a sense of touch

b. Cameras and other sensors that enable automatic vision feature for capturing modules.

c. There are multiple sensors that are used to implement a collision avoidance system.

Arm Control System:

The movement can be controlled by the controls linked to the general-purpose computer (GPC). The hand controllers used by the astronauts are used to tell the computer what the astronauts would like the arm to do. The software then uses the predefined functions and kinematic equations to figure out which joints to move, by how much and how fast to move them. Every change is updated to the GPC every 80 milliseconds. The computer then recalculates the above factors (joints to move, the speed and how much) and sends the updated commands based on the astronaut’s requests.

The arm health is also monitored every 80 milliseconds, and if a failure occurs in the algorithm, the entire arm ceases its motion to enable rectification. If the task at hand is important, there is also an option at the flight control deck, to control each joint individual.

If everything fails, there is a final safety feature is a jettison capability to ensure that the payload bay doors are always closed

Arm Instrumentation:

The current instrumentation present on the arm includes

● A joint angle optical encoder that gives an accurate measure of joint position. They are mounted on the gearbox output shaft of each joint. The encoders used are 16 bit encoders to get precise values.

● A joint tachometer which provides joint rate data (sends data in the form of 12 bits) and strain gauges to measure the bending and torsion. They are located on the motor output shaft. And the strain gauges are mounted near the shoulder pitch and wrist pitch joints.

● There are also 6 closed circuit television cameras (CCTV) to primarily facilitate payload handling.

The CCTVs provide 9x zoom, a pan capability of 170 degrees on both sides, and a tilt capability of 170 degrees on both sides as well

The tachometer and the encoders record data at a rate of 12.5 Hz to upload it to the control deck. The strain gauge, on the other hand, has a transmission frequency of 25Hz

The arm is also said to have a Force-Torque Sensor sandwiched between the wrist roll joint and the end effector, and is useful to provide the torque and force data while the arm is in use. This data can then be utilized for future analysis

· The altitude control system of the spas contains 3 linear accelerometers which are Sundstrand QA 1200-AA 08 style accelerometers and have an operating range of 0.1g to 0.0001g

The 6 Degrees of Freedom:

The inflight ‘6 Degrees of Freedom’ control system of the Canadarm had two parts in it:

● Control by Onboard computers

● 6 Degree of Freedom Control by the two in hand controllers.

1. Control by Onboard computers:

The controller onboard the computer is operated by a technology named “Fly by Wire” (FBW).

An FBW control system is one that places a computer between the operator and the actuators in the arm’s joints. Fly-by-Wire (FBW) is the common term for flight control systems that employ computers to process the pilot or autopilot’s flight control inputs and send corresponding electrical signals to the flight control surface actuators.

The advantage of this technology is that such a control system is relatively light in weight, can be re-programmed to meet different requirements and now, not as costly as other alternatives. As previously noted, there was a backup mode of operation that allowed the Canadarms’ joints to be operated individually. As a matter of interest, these FBW control systems are now common in modern aircraft design and are also being introduced in automobiles for acceleration, braking and steering.

2. The working principle of a “Fly by wire” control system:

The FBW control system is a method of controlling an actuator without any human involvement and decisions following the input of a control step. Eg: Moving the joystick to the right for a right turn. After this input the onboard system figures out the kind of input to feed the actuators with to get the desired output.
The system guides its own self by taking inputs in the from of control variables from the onboard sensors in a feedback loop to achieve real time correction and inputs for the next cycle of the control flow, A similar controller is employed onboard aircrafts which have gyroscopes for sensing the angular velocity and the onboard algorithm outputs the torque the actuators have to give in order to correct the orientation.

3. 6 Degree of Freedom Control by in-hand controllers:

The two in-hand controllers were hand-operated by the astronauts from the view they had through the large windows in the space shuttle and the cameras on the elbow and wrist of the arm. These motions were pre-programmed on the arm’s onboard computer already.

The canadarm was the first space robotics of any size. An extremely clever breakthrough was the control by a joystick. The tip was controlled by the joystick, and all the joints were coordinated to make the same happen. The six degrees of freedom, similar to a human arm are as follows:

● Two joints in the shoulder

● One joint in the elbow

● Three joints in the wrist

● Elbow rotation, limited to 160 degrees

The cameras placed on the elbow and wrist assisted the astronauts on the space shuttle to control the robotic arm

Thermal Protection System:

Since the arm is fixedly installed in the cargo compartment of the space shuttle, methods must be developed to control the sub-zero temperature on the shadow side of the arm and the extreme heat on the sunlit side. The solution is to cover the entire arm with a multi-layer thermal insulation blanket to provide passive thermal control. This material is composed of alternating layers of products manufactured by DuPont. It has unique electrical, thermal and chemical properties and can reflect heat and cold. In addition, the electric heater is connected to key mechanical and electronic components and turns on and off in sequence to maintain a stable operating temperature.

How does docking work?

The grappling fixture that is employed is known as the Latching End Effector (LEE). This consists of three high tension wires that are controlled by 3 separate disks in such a way that they form a triangle once they are in the ‘docking’ mode. There are multiple sensors and motors involved here that make sure that the docking part is done as smoothly as possible.

Safety Strategies:

There were multiple fail-safe procedures that were decided on and coded into the software to ensure the safety of the astronauts on board. The most important ones out of them are:

● Flexible modes for the arm

● Dynamic fatigue protection

● Collision avoidance

● If everything were to fail, there was also the option to control each individual joint of the arm separately to get the arm in a safe position to carry out repairs

All the parts of the arm are modular to facilitate easy repairs while in space.