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NicaDrone OpenGrab EPM v3

Overview

The OpenGrab EPM v3 is an electropermanent magnet, combining the advantages of electro and permanent magnets. The device creates a very strong magnetic contact with a ferrous target. It supports UAVCAN, RC PWM and push button operation. OpenGrab EPM v3 has been developed by NicaDrone in cooperation with Zubax Robotics.

Theory of operation

⚡ The device poses an electric shock hazard. Do not touch exposed parts of the circuit while the magnet is operating. ⚡

The NXP LPC11C24 MCU drives a mosfet connected to a transformer in a flyback configuration to charge the main PET capacitors up to 475 V.

A thyristor bridge is used to discharge the capacitor in either direction through the winding inside the AlNiCo material. This results in a short, 20 us 300 A pulse creating a 100 kAm field in the AlNiCo material. This causes the magnetic domains in the AlNiCo magnets to align in a particular orientation to form a magnetic circuit with a ferrous target. More detailed explanation of the operating principle is available on Wikipedia.

An ON command results in the charging and discharging the capacitors 3 times to achieve full magnetization.

An OFF command results in charging and discharging the capacitors several times with changing direction and decreasing amplitude, effectively degaussing the AlNiCo material.

Applications

  • Cargo lifting in UAV and robotic applications.
  • Robot workholding.
  • Education, demonstration of magnetic properties.

Features

  • Steady state power under 50 mW
  • Short cycle time
  • Variety of interfaces:
    • RC PWM
    • UAVCAN
    • Push button
  • Open source firmware and hardware
  • 5 V supply voltage, can be powered via RC PWM connector or via UAVCAN port

Mechanical properties

The bottom surface of the magnet should be kept clean, because dirt or metal shavings can be crushed into the surface when the magnet is turning on, causing an insulation breakdown.

The diagrams below document the mechanical arrangement and dimensions (click to enlarge):

Version 3R5C and newer

Version 3R4B and older

Characteristics

Symbol Parameter Minimum Typical Maximum Unit
Tcycle(ON) Time to complete one cycle 0.75 s
Tcycle(OFF) Time to complete one cycle 1.2 s
Fmax Max holding force 200 300 N
Vsupply Operating voltage 4.75 5.0 6.5 V
Isteady Steady state current draw 10 mA
Ipeak Peak current draw during cycle execution 1000 mA
m Mass of the device 65 g
toperating Operating temperature -40 +70 °C
RHoperating Operating humidity (non-condensing) 0 75 %

Human-machine interface

Push button

Pressing this button for at least 200 milliseconds will toggle the EPM.

External button

An external button can be connected to Pin 6 (GND) and 3 (TXD) of the UART header (J9). Pulling pin 3 (TXD) low momentarily toggles the EPM. External button is supported in firmware builds starting from March 2017.

LED indication

Status LED

This LED indicator shows the status of the device derived from the continuous self-diagnostics, according to the UAVCAN node status code:

Health Blinking ON/OFF duration, milliseconds
OK 50/950
WARNING 50/500
ERROR or CRITICAL 50/100

CAN LED

This LED indicator shows the CAN bus traffic.

Each blink indicates that there was a CAN frame that was successfully transmitted or successfully received during the last few milliseconds. Under high bus load, this LED indicator is expected to glow constantly.

Note that CAN frames filtered out by the hardware acceptance filters will not cause the LED indicator to blink.

RC PWM interface

Connect an RC receiver or some other hardware capable of producing RC PWM signal (e.g. Pixhawk) to the RC PWM connector.

The device divides the PWM pulse duration into three ranges:

  • Neutral - while the signal is in this range, the device ignores it.
  • OFF - while the signal is in this range, the device will be continuously performing the turn-off sequence.
  • ON - while the signal is in this range, the device will be continuously performing the turn-on sequence.
Symbol Parameter Minimum Typical Maximum Unit
TRCPWM(ON) RC PWM pulse duration to turn ON 1.75 2.5 ms
TRCPWM(OFF) RC PWM pulse duration to turn OFF 0.5 1.25 ms
fRCPWM RC PWM input frequency 1 50 50 Hz
VRCPWM(low) Low-level RC PWM input voltage 0.3 Vsupply V
VRCPWM(high) High-level RC PWM input voltage 0.7 Vsupply V

UAVCAN interface

This section describes the properties specific for this product only. For general info about the UAVCAN interface, please refer to the UAVCAN interface documentation page.

Mode and status codes

OpenGrab EPM v3 employs the following UAVCAN-defined operating modes:

UAVCAN operating mode Conditions
INITIALIZING The UAVCAN interface is undergoing initialization. This does not interfere with other functions of the device.
OPERATIONAL UAVCAN interface and the device itself are fully operational.

The following table describes the meanings of the standard UAVCAN health codes.

UAVCAN health code Conditions
OK Everything is OK; the device is functioning properly.
WARNING Not used.
ERROR See below.
CRITICAL Not used.

Possible reasons for the health code being ERROR:

  • Invalid input voltage.
  • The high-voltage flyback charger circuit is damaged.

Also, the device reports extended status information via uavcan.protocol.NodeStatus.vendor_specific_status_code. The higher byte is used to store the current voltage on the buffer capacitor, the units are 2 V per LSB. The lower byte is used to store implementation-specific status flags.

Linux users: You can use uavcan_status_monitor to see the status code of each node on the bus.

Services

This device does not call any services.

The following service servers are implemented:

Data type Note
uavcan.protocol.GetNodeInfo Name: com.zubax.opengrab_epm_v3

Messages

Input:

Data type Note
uavcan.equipment.hardpoint.Command Controls the magnet, see below.
uavcan.protocol.dynamic_node_id.Allocation Used to allocate node ID if dynamic node ID allocation is enabled.

Output:

Data type Note
uavcan.protocol.NodeStatus Described above.
uavcan.equipment.hardpoint.Status Status of the magnet, see below.
uavcan.protocol.dynamic_node_id.Allocation Used to allocate node ID if dynamic node ID allocation is enabled.

uavcan.equipment.hardpoint.Command

This message allows to control the magnet via UAVCAN. The fields are interpreted as follows:

hardpoint_id

If the field does not equal the hardpoint ID of the current device, the message will be ignored.

command
  1. If this field is zero and the magnet is turned on: the magnet will turn off.

  2. If this field is non-zero and the magnet is turned off: the magnet will execute the number of turn on cycles specified in the field, but not less than 3 and not more than 10.

  3. If this field is non-zero, the magnet is turned on, and the field has changed its value: see #2.

  4. In all other cases the command will be ignored.

uavcan.equipment.hardpoint.Status

This message carries the status of the magnet.

hardpoint_id

Hardpoint ID of the current magnet.

payload_weight

Always set to NaN.

payload_weight_variance

Always set to positive infinity.

status

Indicates whether the magnet is turned on or off:

  • 1 - the magnet is turned on.
  • 0 - the magnet is turned off.

CAN bus characteristics

The device will configure CAN bus bit rate automatically after powering on. The automatic configuration is done by means of listening to the bus in silent mode, alternating between the pre-defined set of supported CAN bit rates (defined in the table) until first valid CAN frame is received. Unconfigured CAN bus does not interfere with other functions of the device.

Symbol Parameter Minimum Typical Maximum Unit
fCAN CAN bit rate (autodetect) 100
125
250
500
1000
Kbps
VCAN(out)dif-dom CAN dominant differential output voltage 1.5 0 3 V
VCAN(out)dif-rec CAN recessive differential output voltage -50 0 50 mV
ICAN(out)dom CAN dominant output current 40 70 120 mA
ICAN(out)dom CAN recessive output current -5 5 mA
RCAN(in)diff CAN differential input resistance 19 30 52
tCAN(out)to-dom CAN dominant time-out time 0.3 1 12 ms

DIP switch

The device is equipped with 4-position DIP switch that allows to configure Hardpoint ID and enable or disable UAVCAN dynamic node ID allocation:

DIP switch # Purpose
1 Hardpoint ID bit 0
2 Hardpoint ID bit 1
3 Hardpoint ID bit 2
4 0 - use dynamic node ID allocation; 1 - use fixed node ID

Hardpoint ID

Hardpoint ID is defined in binary by the configuration of the lowest 3 switches. The table below clarifies the binary encoding:

Hardpoint ID DIP #3 DIP #2 DIP #1
0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1

Node ID selection

If the DIP switch #4 is set to OFF, the device will perform dynamic node ID allocation once the CAN bus bit rate detection is done. This implies that the device will not be available via UAVCAN interface unless the UAVCAN network contains a functioning dynamic node ID allocation server. Please refer to the UAVCAN specification for more info.

If the DIP switch #4 is set to ON, the device’s node ID will be fixed at (Hardpoint ID + 100). For example, if Hardpoint ID is set to 5, the fixed node ID will be configured as 105. In this case the device does not require dynamic node ID allocation server, and therefore it will be accessible via UAVCAN immediately once the CAN bus is configured.

UART interface

The EPM reports error and status messages over this interface. It can also be used to update the firmware - please refer to the source repository for instructions (link below).

Parameters of the serial interface:

Parameter Value
Baud rate 115200
Word size 8
Parity None
Stop bits 1
New line sequence \r\n (CR-LF)
Symbol Parameter Minimum Typical Maximum Unit
VUART(in-low) Low-level UART input voltage 0.3 Vsupply V
VUART(in-high) High-level UART input voltage 0.7 Vsupply V
VUART(out-low) Low-level UART output voltage 0.4 V
VUART(out-high) High-level UART output voltage Vsupply - 0.4 V